TI1 ADS122U04IRTER 24-bit, 4-channel, 2-ksps, delta-sigma adc with uart interface Datasheet

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ADS122U04
SBAS752 – MAY 2017
1 Features
3 Description
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The ADS122U04 is a precision, 24-bit, analog-todigital converter (ADC) that offers many integrated
features to reduce system cost and component count
in applications measuring small sensor signals. The
device features two differential or four single-ended
inputs through a flexible input multiplexer (MUX), a
low-noise, programmable gain amplifier (PGA), two
programmable excitation current sources, a voltage
reference, an oscillator, and a precision temperature
sensor.
1
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Current Consumption as Low as 315 μA (typ)
Wide Supply Range: 2.3 V to 5.5 V
Programmable Gain: 1 to 128
Programmable Data Rates: Up to 2 kSPS
Up to 20-Bits Effective Resolution
Simultaneous 50-Hz and 60-Hz Rejection at
20 SPS with Single-Cycle Settling Digital Filter
Two Differential or Four Single-Ended Inputs
Dual-Matched Programmable Current Sources:
10 μA to 1.5 mA
Internal 2.048-V Reference: 5 ppm/°C (typ) Drift
Internal 2% Accurate Oscillator
Internal Temperature Sensor:
0.5°C (typ) Accuracy
Three General-Purpose Inputs/Outputs
2-Wire UART Compatible Interface (8-N-1 Format)
With Baud Rates up to 120 kBaud and AutoBaud-Rate Detection
Package: 3.0-mm × 3.0-mm × 0.75-mm WQFN
2 Applications
•
•
Temperature Sensor Measurements:
– Thermistors
– Thermocouples
– Resistance Temperature Detectors (RTDs):
2-, 3-, or 4-Wire Types
Resistive Bridge Sensor Measurements:
– Pressure Sensors
– Strain Gauges
– Weigh Scales
The device can perform conversions at data rates up
to 2000 samples-per-second (SPS) with single-cycle
settling. At 20 SPS, the digital filter offers
simultaneous 50-Hz and 60-Hz rejection for noisy
industrial applications. The internal PGA offers gains
up to 128. This PGA makes the ADS122U04 ideally
suited for applications measuring small sensor
signals, such as resistance temperature detectors
(RTDs), thermocouples, thermistors, and resistive
bridge sensors.
The ADS122U04 features a 2-wire, UART-compatible
interface. In applications that require galvanic
isolation
this
universal
asynchronous
receiver/transmitter (UART) interface minimizes the
number of digital isolation channels, thus saving cost,
board space, and power.
The ADS122U04 is offered in a leadless 16-pin
WQFN or a 16-pin TSSOP package and is specified
over a temperature range of –40°C to +125°C.
Device Information(1)
PART NUMBER
PACKAGE
ADS122U04
BODY SIZE (NOM)
WQFN (16)
3.00 mm × 3.00 mm
TSSOP (16)
5.00 mm × 4.40 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
K-Type Thermocouple Measurement
3.3 V
3.3 V
0.1 F
0.1 F
REFP
3.3 V
10 A to
1.5 mA
REFN
AVDD
DVDD
2.048-V
Reference
AIN0
Reference Mux
ADS122U04
TX
AIN1
RX
Thermocouple
PGA
Mux
24-Bit
û ADC
Digital Filter
and
UART Interface
Isothermal Block
AIN2
GPIO0
GPIO1
GPIO2/ DRDY
RESET
AIN3
Precision
Temperature
Sensor
AVSS
Low Drift
Oscillator
DGND
Copyright © 2017, Texas Instruments Incorporated
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. ADVANCE INFORMATION for pre-production products; subject to
change without notice.
ADVANCE INFORMATION
ADS122U04 24-Bit, 4-Channel, 2-kSPS, Delta-Sigma ADC With UART Interface
ADS122U04
SBAS752 – MAY 2017
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Device Family Comparison Table ........................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
5
7.1
7.2
7.3
7.4
7.5
7.6
7.7
5
5
5
6
6
9
9
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
UART Timing Requirements .....................................
UART Switching Characteristics ...............................
8
Parameter Measurement Information ................ 11
9
Detailed Description ............................................ 14
9.5 Programming........................................................... 29
9.6 Register Map........................................................... 35
10 Application and Implementation........................ 41
10.1 Application Information.......................................... 41
10.2 Typical Applications .............................................. 46
11 Power Supply Recommendations ..................... 56
11.1 Power-Supply Sequencing.................................... 56
11.2 Power-Supply Ramp Rate .................................... 56
11.3 Power-Supply Decoupling..................................... 56
12 Layout................................................................... 57
12.1 Layout Guidelines ................................................. 57
12.2 Layout Example .................................................... 58
13 Device and Documentation Support ................. 59
13.1
13.2
13.3
13.4
13.5
13.6
8.1 Noise Performance ................................................. 11
ADVANCE INFORMATION
9.1
9.2
9.3
9.4
Overview .................................................................
Functional Block Diagram .......................................
Feature Description.................................................
Device Functional Modes........................................
14
14
15
26
Documentation Support ........................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
59
59
59
59
59
59
14 Mechanical, Packaging, and Orderable
Information ........................................................... 59
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
2
DATE
REVISION
NOTES
May 2017
*
Initial release.
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5 Device Family Comparison Table
RESOLUTION
(Bits)
INTERFACE
MAX DATA
RATE
(kSPS)
EXCITATION
CURRENT
SOURCES
GPIOS
TEMPERATURE
SENSOR
BIPOLAR
SUPPLY
CLOCK
LOW-SIDE
SWITCH
ADS1120
16
SPI
2
Yes
0
Yes
Yes
Internal,
external
Yes
ADS1220
24
SPI
2
Yes
0
Yes
Yes
Internal,
external
Yes
ADS112U04
16
UART
2
Yes
3
Yes
Yes
Internal
No
ADS122U04
24
UART
2
Yes
3
Yes
Yes
Internal
No
ADVANCE INFORMATION
DEVICE
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6 Pin Configuration and Functions
TX
13
3
10
AVDD
4
9
AIN0
AIN2
ADVANCE INFORMATION
GPIO1
1
16
RX
GPIO0
2
15
TX
RESET
3
14
GPIO2/DRDY
DGND
4
13
DVDD
AVSS
5
12
AVDD
AIN3
6
11
AIN0
AIN2
7
10
AIN1
REFN
8
9
REFP
8
AIN1
AIN3
RX
DVDD
7
AVSS
14
11
Thermal pad
REFP
2
GPIO1
GPIO2/DRDY
6
DGND
PW Package
16-Pin TSSOP
Top View
12
REFN
1
5
RESET
15
16
GPIO0
RTE Package
16-Pin WQFN
Top View
Not to scale
Not to scale
Pin Functions
PIN
NO.
RTE
PW
ANALOG OR DIGITAL
INPUT/OUTPUT
AIN0
9
11
Analog input
Analog input 0
AIN1
8
10
Analog input
Analog input 1
AIN2
5
7
Analog input
Analog input 2
Analog input 3
NAME
DESCRIPTION (1)
AIN3
4
6
Analog input
AVDD
10
12
Analog supply
Positive analog power supply. Connect a 100-nF (or larger) capacitor to AVSS.
AVSS
3
5
Analog supply
Negative analog power supply
DGND
2
4
Digital supply
Digital ground
DVDD
11
13
Digital supply
Positive digital power supply. Connect a 100-nF (or larger) capacitor to DGND.
GPIO0
16
2
Digital input/output
General-purpose input/output 0
GPIO1
15
1
Digital input/output
General-purpose input/output 1
GPIO2/DRDY
12
14
Digital input/output
General-purpose input/output 2 or data ready; active low.
REFN
6
8
Analog input
Negative reference input
REFP
7
9
Analog input
Positive reference input
RESET
1
3
Digital input
Reset; active low
RX
14
16
Digital input
Receive
TX
13
15
Digital output
Transmit
Pad
—
—
Thermal pad
(1)
4
Thermal power pad. Do not connect or only connect to AVSS.
See the Unused Inputs and Outputs section for details on how to connect unused pins.
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7 Specifications
7.1 Absolute Maximum Ratings (1)
Power-supply voltage
MIN
MAX
AVDD to AVSS
–0.3
7
DVDD to DGND
–0.3
7
AVSS to DGND
–2.8
0.3
UNIT
V
Analog input voltage
AIN0, AIN1, AIN2, AIN3, REFP, REFN
AVSS – 0.3
AVDD + 0.3
Digital input voltage
TX, RX, GPIO0, GPIO1, GPIO2/DRDY, RESET
DGND – 0.3
DVDD + 0.3
V
Input current
Continuous, any pin except power-supply pins
–10
10
mA
Junction, TJ
Temperature
(1)
V
150
Storage, Tstg
–60
°C
150
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
7.2 ESD Ratings
V(ESD)
(1)
(2)
Electrostatic discharge
±2000
Charged-device model (CDM), per JEDEC specification JESD22-C101 (2)
±500
UNIT
ADVANCE INFORMATION
VALUE
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
7.3 Recommended Operating Conditions
over operating ambient temperature range (unless otherwise noted)
MIN
NOM
MAX
UNIT
POWER SUPPLY
Unipolar analog power supply
Bipolar analog power supply
Digital power supply
AVDD to AVSS
2.3
AVSS to DGND
–0.1
5.5
0
0.1
V
AVDD to DGND
2.3
2.5
2.75
AVSS to DGND
–2.75
–2.5
–2.3
DVDD to DGND
2.3
5.5
PGA disabled, gain = 1 to 4
AVSS – 0.1
AVDD + 0.1
PGA enabled, gain = 1 to 4
AVSS + 0.2
AVDD – 0.2
AVSS + 0.2 +
|VINMAX|·(Gain – 4) / 8
AVDD – 0.2 –
|VINMAX|·(Gain – 4) / 8
–VREF / Gain
VREF / Gain
V
V
V
ANALOG INPUTS (1)
V(AINx)
Absolute input voltage (2)
PGA enabled, gain = 8 to 128
VIN
Differential input voltage
VIN = VAINP – VAINN (3)
V
VOLTAGE REFERENCE INPUTS
VREF
Differential reference input voltage
V(REFN)
Absolute negative reference voltage
V(REFP)
Absolute positive reference voltage
VREF = V(REFP) – V(REFN)
AVDD – AVSS
V
AVSS – 0.1
0.75
2.5
V(REFP) – 0.75
V
V(REFN) + 0.75
AVDD + 0.1
V
DGND
DVDD
V
–40
125
°C
DIGITAL INPUTS
Input voltage
RX, GPIO0, GPIO1,
GPIO2/DRDY, RESET
TEMPERATURE RANGE
TA
(1)
(2)
(3)
Operating ambient temperature
AINP and AINN denote the positive and negative inputs of the PGA. AINx denotes one of the four available analog inputs.
PGA disabled means the low-noise PGA is powered down and bypassed. Gains of 1, 2, and 4 are still possible in this case.
See the Low-Noise Programmable Gain Stage section for more information.
VINMAX denotes the maximum differential input voltage, VIN, that is expected in the application. |VINMAX| can be smaller than VREF / Gain.
Excluding the effects of offset and gain error.
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7.4 Thermal Information
ADS122U04
THERMAL METRIC (1)
WQFN (RTE)
TSSOP (PW)
16 PINS
16 PINS
UNIT
RθJA
Junction-to-ambient thermal resistance
57.7
90.3
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
29.0
31.7
°C/W
RθJB
Junction-to-board thermal resistance
19.9
41.8
°C/W
ψJT
Junction-to-top characterization parameter
0.3
1.8
°C/W
ψJB
Junction-to-board characterization parameter
19.8
41.2
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
11.8
N/A
°C/W
(1)
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
7.5 Electrical Characteristics
minimum and maximum specifications apply from TA = –40°C to +125°C; typical specifications are at TA = 25°C;
all specifications are at AVDD = 2.3 V to 5.5 V, AVSS = 0 V, DVDD = 3.3 V, PGA enabled, all data rates, and internal
reference enabled (unless otherwise noted)
ADVANCE INFORMATION
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
ANALOG INPUTS
Absolute input current
Absolute input current drift
Differential input current
Differential input current drift
PGA disabled, gain = 1 to 4
TBD
Gain = 1 to 128,
V(AINx)MIN ≤ V(AINx) ≤ V(AINx)MAX
TBD
PGA disabled, gain = 1 to 4
TBD
Gain = 1 to 128,
V(AINx)MIN ≤ V(AINx) ≤ V(AINx)MAX
TBD
PGA disabled, gain = 1 to 4,
VCM = AVDD / 2, –VREF / Gain ≤ VIN ≤ VREF / Gain
TBD
nA/V
Gain = 1 to 128,
VCM = AVDD / 2, –VREF / Gain ≤ VIN ≤ VREF / Gain
TBD
nA
PGA disabled, gain = 1 to 4,
VCM = AVDD / 2, –VREF / Gain ≤ VIN ≤ VREF / Gain
TBD
Gain = 1 to 128,
VCM = AVDD / 2, –VREF / Gain ≤ VIN ≤ VREF / Gain
TBD
nA
pA/°C
pA/°C
SYSTEM PERFORMANCE
Resolution (no missing codes)
DR
Data rate
24
Normal mode
Turbo mode
Normal mode, gain = 128, DR = 20 SPS
INL
Integral nonlinearity
AVDD = 3.3 V, gain = 1 to 128, VCM = 0.5 AVDD,
external VREF, normal mode, best fit
VIO
Input offset voltage
TBD
–15
PGA disabled, gain = 1 to 4, differential inputs
Gain = 1, differential inputs, TA = 25°C
Gain error
6
ppmFSR
±4
30
µV
µV/°C
±4
0.25
Gain = 1 to 128, TA = –40°C to +85°C
0.08
0.3
Gain = 1 to 128
0.25
TBD
Match between any two inputs (at the same gain
setting TBD?)
±20
PGA disabled, gain = 1 to 4
Gain = 1 to 128, TA = 25°C
Gain = 1 to 128, TA = –40°C to +85°C
Gain = 1 to 128
(1)
15
PGA disabled, gain = 1 to 4
PGA disabled, gain = 1 to 4
Gain drift
±6
nVRMS
±4
–30
Gain = 2 to 128, differential inputs
Offset match
SPS
40, 90, 180, 350, 660, 1200, 2000
Noise (input-referred) (1)
Offset drift
Bits
20, 45, 90, 175, 330, 600, 1000
µV
±0.015%
–0.1%
±0.015%
0.1%
1
TBD
TBD
1
4
ppm/°C
See the Noise Performance section for more information.
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Electrical Characteristics (continued)
minimum and maximum specifications apply from TA = –40°C to +125°C; typical specifications are at TA = 25°C;
all specifications are at AVDD = 2.3 V to 5.5 V, AVSS = 0 V, DVDD = 3.3 V, PGA enabled, all data rates, and internal
reference enabled (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
50 Hz ±1 Hz, DR = 20 SPS
78
88
60 Hz ±1 Hz, DR = 20 SPS
80
88
MAX
UNIT
SYSTEM PERFORMANCE (continued)
NMRR
Normal-mode rejection ratio
At dc, gain = 1
fCM = 50 Hz, DR = 20 SPS
CMRR
Common-mode rejection ratio
fCM = 50 Hz, DR = 2000 SPS
Power-supply rejection ratio
105
TBD
95
115
TBD
TBD
fCM = 60 Hz, DR = 2000 SPS
95
115
AVDD at dc (VCM = 0.5 AVDD, gain = 1 TBD)
80
105
TBD
TBD
100
115
fCM = 60 Hz, DR = 20 SPS
PSRR
90
TBD
AVDD at 50 Hz or 60 Hz
(VCM = 0.5 AVDD, gain = 1 TBD)
DVDD at dc (VCM = 0.5 AVDD, gain = 1 TBD)
dB
dB
dB
VREF
Reference voltage
Accuracy
Temperature drift
Long-term drift
2.048
TA = 25°C
–0.15%
TA = –40°C to +85°C
ADVANCE INFORMATION
INTERNAL VOLTAGE REFERENCE
V
TBD
0.15%
TBD
TBD
5
30
ppm/°C
1000 hours
110
ppm
REFP = VREF, REFN = AVSS
±10
nA
VOLTAGE REFERENCE INPUTS
Reference input current
INTERNAL OSCILLATOR
fCLK
Frequency
Accuracy
Normal mode
1.024
Turbo mode
2.048
Normal mode
–2%
MHz
±1%
2%
EXCITATION CURRENT SOURCES (IDACs) (AVDD = 3.3 V to 5.5 V)
Current settings
10, 50, 100, 250, 500, 1000, 1500
Compliance voltage
All current settings, 0.1% (TBD) deviation
Accuracy
All current settings, each IDAC
Current matching
Between IDACs (not valid for 10-µA setting)
Temperature drift
Each IDAC (not valid for 10-µA setting)
50
Temperature drift matching
Between IDACs (not valid for 10-µA setting)
10
µA
AVDD – 0.9
–6%
±1%
6%
±0.3%
TBD
V
ppm/°C
TBD
ppm/°C
BURN-OUT CURRENT SOURCES (BOCS)
Magnitude
Sink and source
10
Accuracy
µA
TBD%
TEMPERATURE SENSOR
Conversion resolution
14
Temperature resolution
Accuracy
Bits
0.03125
TA = 0°C to +75°C
TA = –40°C to +125°C
°C
–0.5
±0.25
0.5
–1
±0.5
1
0.0625
0.25
Accuracy vs analog supply
voltage
°C
°C/V
DIGITAL INPUTS/OUTPUTS
VIL
Logic input level, low
VIH
Logic input level, high
VOL
Logic output level, low
IOL = 1 mA
VOH
Logic output level, high
IOH = 1 mA
Input current
DGND ≤ VDigital Input ≤ DVDD
DGND
0.3 DVDD
V
0.7 DVDD
DVDD
V
0.2 DVDD
V
1
µA
0.8 DVDD
–1
V
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Electrical Characteristics (continued)
minimum and maximum specifications apply from TA = –40°C to +125°C; typical specifications are at TA = 25°C;
all specifications are at AVDD = 2.3 V to 5.5 V, AVSS = 0 V, DVDD = 3.3 V, PGA enabled, all data rates, and internal
reference enabled (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
Power-down mode
0.1
3
Normal mode, PGA disabled, gain = 1 to 4
240
Normal mode, gain = 1 to 16
340
Normal mode, gain = 32
425
Normal mode, gain = 64, 128
510
Turbo mode, PGA disabled, gain = 1 to 4
360
Turbo mode, gain = 1 to 16
540
Turbo mode, gain = 32
715
Turbo mode, gain = 64, 128
890
UNIT
ANALOG SUPPLY CURRENT (AVDD = 3.3 V, VIN = 0 V, IDACs Turned Off)
IAVDD
Analog supply current
490
µA
ADDITIONAL ANALOG SUPPLY CURRENTS PER FUNCTION (AVDD = 3.3 V)
IAVDD
Analog supply current
External reference selected
ADVANCE INFORMATION
IDAC overhead (excludes the actual IDAC current)
70
µA
190
DIGITAL SUPPLY CURRENT (DVDD = 3.3 V, All Data Rates, SPI Not Active)
IDVDD
Digital supply current
Power-down mode
0.3
5
Normal mode
75
110
Turbo mode
95
µA
POWER DISSIPATION (AVDD = DVDD = 3.3 V, All Data Rates, VIN = 0 V, SPI Not Active)
PD
8
Power dissipation
Normal mode, gain = 1 to 16
1.4
Turbo mode, gain = 1 to 16
2.1
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7.6 UART Timing Requirements
over operating ambient temperature range and DVDD = 2.3 V to 5.5 V (unless otherwise noted)
MIN
MAX
UNIT
120
kBaud
10-pF load
15
% of tBAUD
10-pF load
15
% of tBAUD
1/tBAUD
Bus baud rate
tr(RX)
Rise time
tf(RX)
Fall time
tJITTER
Edge timing variance
tw(RSL)
Pulse duration, RESET low
td(RSRX)
Delay time, start of communication after RESET rising edge
(or RESET command decoded (1))
–1%
Timeout (2)
(1)
(2)
TYP
2
1%
250
ns
TBD
ns
Normal mode
32760
Turbo mode
65520
tMOD
The UART baud rate affects the command latch timing; see the Command Latching section for more details.
See the Timeout section for more information.
tMOD = 1 / fMOD. Modulator frequency fMOD = 256 kHz (normal mode) and 512 kHz (turbo mode).
7.7 UART Switching Characteristics
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT (1)
TBD
tCLK
tpd(RDDR)
Propagation delay time, RDATA command
decoded to DRDY rising edge (2)
Manual data read mode
tpd(RDTX)
Propagation delay time, RDATA command
decoded to TX falling edge (2)
Manual data read mode
2
tBAUD
tpd(DRTX)
Propagation delay time, DRDY rising edge to
TX falling edge (2)
Automatic data read mode
2
tBAUD
tw(DRH)
Pulse duration, DRDY high
tw(DRL)
Pulse duration, DRDY low
tp(RREG)
Propagation delay time, RREG command
decoded to TX falling edge (2)
tp(GPIO)
Propagation delay time, WREG command
decoded to GPIOx output valid (2)
(1)
(2)
Automatic data read mode
2
tMOD
4
tCLK
2
TBD
ADVANCE INFORMATION
over operating ambient temperature range and DVDD = 2.3 V to 5.5 V (unless otherwise noted)
tBAUD
TBD
ns
tCLK = 1 / fCLK. Oscillator frequency fCLK = 1.024 MHz (normal mode) and 2.048 MHz (turbo mode).
tMOD = 1 / fMOD. Modulator frequency fMOD = 256 kHz (normal mode) and 512 kHz (turbo mode).
The UART baud rate affects the command latch timing; see the Command Latching section for more details.
ttBAUDt
tr(RX)
tJITTER
tf(RX)
RX/TX
VIH
VIL
ttBAUDt
tJITTER
Figure 1. UART Timing Requirements
tw(RSL)
RESET
ttd(RSRX)t
RX
Synchronization word
RESET command
Synchronization word
New command
Figure 2. RESET Pin and RESET Command Timing Requirements
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tw(DRH)
DRDY
tpd(RDDR)
RX
Synchronization word
RDATA command
tpd(RDTX)
TX
Data 1
Figure 3. Manual Data Read Mode DRDY Switching Characteristics
ADVANCE INFORMATION
DRDY
tw(DRL)
TX
Data 1
tpd(DRTX)
Figure 4. Automatic Data Read Mode DRDY Switching Characteristics
TX
Register data
tp(RREG)
RX
Synchronization word
RREG command
Figure 5. Register Read Switching Characteristics
GPIOx
tp(GPIO)
RX
Synchronization word
WREG command
Register configuration
Figure 6. GPIO Switching Characteristics
10
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8 Parameter Measurement Information
8.1 Noise Performance
Delta-sigma (ΔΣ) analog-to-digital converters (ADCs) are based on the principle of oversampling. The input
signal of a ΔΣ ADC is sampled at a high frequency (modulator frequency) and subsequently filtered and
decimated in the digital domain to yield a conversion result at the respective output data rate. The ratio between
modulator frequency and output data rate is called oversampling ratio (OSR). By increasing the OSR, and thus
reducing the output data rate, the noise performance of the ADC can be optimized. In other words, the inputreferred noise drops when reducing the output data rate because more samples of the internal modulator are
averaged to yield one conversion result. Increasing the gain also reduces the input-referred noise, which is
particularly useful when measuring low-level signals.
The input-referred noise (Table 1, Table 3, Table 5, and Table 7) only changes marginally when using an
external low-noise reference, such as the REF5020. To calculate effective resolution numbers and noise-free
resolution when using a reference voltage other than 2.048 V, use Equation 1 and Equation 2:
Effective Resolution = ln [2 · VREF / (Gain · VRMS-Noise)] / ln(2)
Noise-Free Resolution = ln [2 · VREF / (Gain · VPP-Noise)] / ln(2)
(1)
(2)
Table 1. Noise in μVRMS (μVPP)
at AVDD = 3.3 V, AVSS = 0 V, Normal Mode, and Internal Reference = 2.048 V
DATA
RATE
(SPS)
GAIN (PGA Enabled)
1
2
4
8
16
32
64
128
20
3.71 (13.67)
1.54 (5.37)
1.15 (4.15)
0.80 (3.36)
0.35 (1.16)
0.23 (0.73)
0.10 (0.35)
0.09 (0.41)
45
7.36 (29.54)
2.93 (13.06)
1.71 (9.28)
0.88 (4.06)
0.50 (2.26)
0.29 (1.49)
0.19 (0.82)
0.12 (0.51)
90
10.55 (47.36)
4.50 (20.75)
2.43 (11.35)
1.51 (6.65)
0.65 (3.62)
0.42 (2.14)
0.27 (1.22)
0.18 (0.85)
175
11.90 (63.72)
6.45 (34.06)
3.26 (17.76)
1.82 (11.20)
1.01 (5.13)
0.57 (3.09)
0.34 (2.14)
0.26 (1.60)
330
19.19 (106.93)
9.38 (50.78)
4.25 (26.25)
2.68 (14.13)
1.45 (7.52)
0.79 (4.66)
0.50 (2.69)
0.34 (1.99)
600
24.78 (151.61)
13.35 (72.27)
6.68 (39.43)
3.66 (19.26)
2.10 (12.77)
1.14 (6.87)
0.70 (4.76)
0.55 (3.34)
1000
37.53 (227.29)
18.87 (122.68)
9.53 (58.53)
5.37 (31.52)
2.95 (18.08)
1.65 (10.71)
1.03 (6.52)
0.70 (4.01)
Table 2. Effective Resolution from RMS Noise (Noise-Free Resolution From Peak-to-Peak Noise)
at AVDD = 3.3 V, AVSS = 0 V, Normal Mode, and Internal Reference = 2.048 V
DATA
RATE
(SPS)
GAIN (PGA Enabled)
1
2
4
8
16
32
64
128
20
20.08 (18.19)
20.34 (18.54)
19.76 (17.91)
19.28 (17.22)
19.48 (17.75)
19.10 (17.42)
19.33 (17.49)
18.49 (16.26)
45
19.09 (17.08)
19.42 (17.26)
19.19 (16.75)
19.15 (16.94)
18.95 (16.79)
18.74 (16.39)
18.38 (16.25)
18.00 (15.49)
90
18.57 (16.40)
18.80 (16.59)
18.68 (16.46)
18.37 (16.23)
18.60 (16.11)
18.20 (15.87)
17.87 (15.67)
17.44 (15.20)
175
18.39 (15.97)
18.28 (15.88)
18.26 (15.82)
18.10 (15.48)
17.96 (15.61)
17.78 (15.34)
17.53 (14.87)
16.91 (14.29)
330
17.70 (15.23)
17.74 (15.30)
17.88 (15.25)
17.54 (15.15)
17.43 (15.05)
17.30 (14.74)
16.96 (14.54)
16.50 (13.97)
600
17.33 (14.72)
17.23 (14.79)
17.23 (14.66)
17.09 (14.70)
16.89 (14.29)
16.77 (14.18)
16.48 (13.72)
15.83 (13.23)
1000
16.74 (14.14)
16.73 (14.03)
16.71 (14.09)
16.54 (13.99)
16.41 (13.79)
16.25 (13.54)
15.92 (13.26)
15.49 (12.96)
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Table 1 to Table 8 summarize the device noise performance. Data are representative of typical noise
performance at TA = 25°C using the internal 2.048-V reference. Data shown are the result of averaging readings
from a single device over a time period of approximately 0.75 seconds and are measured with the inputs
internally shorted together. Table 1, Table 3, Table 5, and Table 7 list the input-referred noise in units of μVRMS
for the conditions shown. Values in µVPP are shown in parenthesis. Table 2, Table 4, Table 6, and Table 8 list
the corresponding data in effective resolution calculated from μVRMS values using Equation 1. Noise-free
resolution calculated from peak-to-peak noise values using Equation 2 are shown in parenthesis.
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Table 3. Noise in μVRMS (μVPP) With PGA Disabled
at AVDD = 3.3 V, AVSS = 0 V, Normal Mode, and Internal Reference = 2.048 V
GAIN (PGA Disabled)
DATA RATE
(SPS)
1
2
4
20
3.89 (13.43)
1.85 (6.84)
1.26 (3.91)
45
6.97 (31.98)
2.94 (12.94)
1.41 (5.62)
90
8.50 (42.48)
4.49 (18.92)
2.07 (9.95)
175
12.99 (65.92)
6.24 (35.40)
3.04 (18.92)
330
18.18 (94.24)
8.12 (50.17)
4.71 (28.75)
600
25.29 (138.67)
12.77 (78.13)
6.27 (39.79)
1000
38.04 (260.50)
18.40 (120.97)
9.48 (63.72)
Table 4. Effective Resolution From RMS Noise (Noise-Free Resolution From Peak-to-Peak Noise) With
PGA Disabled
at AVDD = 3.3 V, AVSS = 0 V, Normal Mode, and Internal Reference = 2.048 V
GAIN (PGA Disabled)
ADVANCE INFORMATION
DATA RATE
(SPS)
1
2
4
20
20.01 (18.22)
20.08 (18.19)
19.63 (18.00)
45
19.61 (16.97)
19.41 (17.27)
19.47 (17.48)
90
18.88 (16.56)
18.80 (16.72)
18.91 (16.65)
175
18.27 (15.92)
18.32 (15.82)
18.36 (15.72)
330
17.78 (15.41)
17.94 (15.32)
17.73 (15.12)
600
17.31 (14.85)
17.29 (14.68)
17.32 (14.65)
1000
16.72 (13.94)
16.76 (14.05)
16.72 (13.97)
Table 5. Noise in μVRMS (μVPP)
at AVDD = 3.3 V, AVSS = 0 V, Turbo Mode, and Internal Reference = 2.048 V
DATA
RATE
(SPS)
GAIN (PGA Enabled)
1
2
4
8
16
32
64
128
40
4.56 (24.17)
2.40 (11.35)
1.22 (4.94)
0.71 (2.84)
0.35 (1.60)
0.19 (0.85)
0.16 (0.71)
0.09 (0.55)
90
5.74 (25.88)
2.97 (14.40)
1.47 (5.80)
1.13 (5.52)
0.50 (2.67)
0.32 (1.32)
0.23 (1.13)
0.15 (0.69)
180
8.49 (46.88)
4.66 (21.36)
2.30 (12.88)
1.24 (7.23)
0.72 (4.82)
0.42 (2.57)
0.28 (1.47)
0.24 (1.34)
350
13.42 (84.72)
5.86 (40.04)
3.39 (19.04)
1.88 (10.13)
1.05 (6.15)
0.64 (3.59)
0.43 (2.29)
0.28 (1.39)
660
17.09 (120.36)
9.34 (47.36)
4.81 (27.83)
2.97 (17.36)
1.54 (10.21)
0.82 (4.43)
0.58 (3.67)
0.41 (2.93)
1200
25.71 (162.35)
12.31 (85.94)
6.81 (44.01)
3.72 (21.55)
2.09 (15.14)
1.23 (7.58)
0.80 (5.31)
0.57 (3.51)
2000
36.23 (265.14)
18.24 (127.32)
9.24 (65.43)
5.49 (37.02)
2.89 (18.89)
1.77 (12.00)
1.13 (7.60)
0.82 (5.81)
Table 6. Effective Resolution From RMS Noise (Noise-Free Resolution From Peak-to-Peak Noise)
at AVDD = 3.3 V, AVSS = 0 V, Turbo Mode, and Internal Reference = 2.048 V
12
DATA
RATE
(SPS)
GAIN (PGA Enabled)
1
2
4
8
16
32
64
128
40
19.78 (17.37)
19.71 (17.46)
19.68 (17.66)
19.45 (17.46)
19.47 (17.29)
19.37 (17.21)
18.65 (16.46)
18.40 (15.83)
90
19.45 (17.27)
19.39 (17.12)
19.41 (17.43)
18.79 (16.50)
18.97 (16.55)
18.62 (16.57)
18.11 (15.80)
17.75 (15.49)
180
18.88 (16.42)
18.75 (16.55)
18.76 (16.28)
18.65 (16.11)
18.43 (15.70)
18.23 (15.60)
17.79 (15.41)
17.05 (14.54)
350
18.22 (15.56)
18.42 (15.64)
18.21 (15.71)
18.05 (15.62)
17.89 (15.35)
17.62 (15.12)
17.20 (14.77)
16.78 (14.49)
660
17.87 (15.05)
17.74 (15.40)
17.70 (15.17)
17.39 (14.85)
17.34 (14.61)
17.25 (14.82)
16.75 (14.09)
16.25 (13.42)
1200
17.28 (14.62)
17.34 (14.54)
17.20 (14.51)
17.07 (14.54)
16.90 (14.05)
16.67 (14.04)
16.28 (13.56)
15.77 (13.15)
2000
16.79 (13.92)
16.78 (13.97)
16.76 (13.93)
16.51 (13.76)
16.44 (13.73)
16.14 (13.38)
15.79 (13.04)
15.25 (12.43)
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Table 7. Noise in μVRMS (μVPP) With PGA Disabled
at AVDD = 3.3 V, AVSS = 0 V, Turbo Mode, and Internal Reference = 2.048 V
GAIN (PGA Disabled)
DATA RATE
(SPS)
1
2
4
40
4.22 (22.46)
2.30 (10.74)
0.93 (3.91)
90
6.57 (31.01)
3.53 (14.28)
1.59 (6.84)
180
8.41 (55.66)
4.30 (22.09)
2.31 (14.59)
350
12.68 (75.20)
6.02 (34.18)
3.22 (17.64)
660
17.81 (111.08)
9.06 (56.76)
4.24 (27.47)
1200
25.43 (176.03)
12.70 (89.23)
6.28 (40.95)
2000
36.11 (250.98)
17.30 (131.35)
8.77 (68.18)
Table 8. Effective Resolution From RMS Noise (Noise-Free Resolution From Peak-to-Peak Noise) With
PGA Disabled
at AVDD = 3.3 V, AVSS = 0 V, Turbo Mode, and Internal Reference = 2.048 V
1
2
4
40
19.89 (17.48)
19.76 (17.54)
20.07 (18.00)
90
19.25 (17.01)
19.15 (17.13)
19.29 (17.19)
180
18.89 (16.17)
18.86 (16.50)
18.76 (16.10)
350
18.30 (15.73)
18.38 (15.87)
18.28 (15.83)
660
17.81 (15.17)
17.79 (15.14)
17.88 (15.19)
1200
17.30 (14.51)
17.30 (14.49)
17.31 (14.61)
2000
16.79 (13.99)
16.85 (13.93)
16.83 (13.87)
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ADVANCE INFORMATION
GAIN (PGA Disabled)
DATA RATE
(SPS)
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9 Detailed Description
9.1 Overview
The ADS122U04 is a small, low-power, 24-bit, ΔΣ ADC that offers many integrated features to reduce system
cost and component count in applications measuring small sensor signals.
In addition to the ΔΣ ADC core and single-cycle settling digital filter, the device offers a low-noise, high input
impedance, programmable gain amplifier (PGA), an internal voltage reference, and a clock oscillator. The device
also integrates a highly linear and accurate temperature sensor as well as two matched programmable current
sources (IDACs) for sensor excitation. All of these features are intended to reduce the required external circuitry
in typical sensor applications and improve overall system performance. The device is fully configured through five
registers and controlled by six commands through a universal asynchronous receiver/transmitter (UART)compatible interface. The Functional Block Diagram section shows the device functional block diagram.
The ADS122U04 ADC measures a differential signal, VIN, which is the difference in voltage between nodes AINP
and AINN. The converter core consists of a differential, switched-capacitor, ΔΣ modulator followed by a digital
filter. The digital filter receives a high-speed bitstream from the modulator and outputs a code proportional to the
input voltage. This architecture results in a very strong attenuation of any common-mode signal.
ADVANCE INFORMATION
The device has two available conversion modes: single-shot conversion and continuous conversion mode. In
single-shot conversion mode, the ADC performs one conversion of the input signal upon request and stores the
value in an internal data buffer. The device then enters a low-power state to save power. Single-shot conversion
mode is intended to provide significant power savings in systems that require only periodic conversions, or when
there are long idle periods between conversions. In continuous conversion mode, the ADC automatically begins
a conversion of the input signal as soon as the previous conversion is completed. New data are available at the
programmed data rate. Data can be read at any time without concern of data corruption and always reflect the
most recently completed conversion.
9.2 Functional Block Diagram
REFP
10 A to
1.5 mA
REFN
DVDD
AVDD
2.048-V
Reference
AIN0
Reference Mux
ADS122U04
TX
RX
AIN1
PGA
Mux
24-bit
û ADC
Digital Filter and
UART Interface
AIN2
GPIO0
GPIO1
DRDY
RESET
AIN3
Precision
Temperature
Sensor
Low Drift
Oscillator
AVSS
DGND
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9.3 Feature Description
9.3.1 Multiplexer
Figure 7 shows that the device contains a very flexible input multiplexer. Either four single-ended signals, two
differential signals, or a combination of two single-ended signals and one differential signal can be measured.
The multiplexer is configured by four bits (MUX[3:0]) in the configuration register. When single-ended signals are
measured, the negative ADC input (AINN) is internally connected to AVSS by a switch within the multiplexer. For
system-monitoring purposes, the analog supply [(AVDD – AVSS) / 4] or the currently selected external reference
voltage [(VREFP – VREFN) / 4] can be selected as inputs to the ADC. The multiplexer also offers the possibility to
route any of the two programmable current sources to any analog input (AINx) or to the dedicated reference pins
(REFP, REFN).
System Monitors
(VREFP ± VREFN) / 4
AVDD
AVDD
IDAC1
AVDD
AVSS
AVDD
AVSS
ADVANCE INFORMATION
(AVDD ± AVSS) / 4
IDAC2
(AVDD + AVSS) / 2
AIN0
AVDD
AIN1
Burnout Current Source (10 µA)
AVDD
AVSS
AVDD
AVSS
AIN2
AINP
PGA
To ADC
AINN
AIN3
AVDD
AVSS
Burnout Current Source (10 µA)
REFP
AVDD
AVSS
AVSS
AVSS
REFN
Figure 7. Analog Input Multiplexer
Electrostatic discharge (ESD) diodes to AVDD and AVSS protect the inputs. The absolute voltage on any input
must stay within the range provided by Equation 3 to prevent the ESD diodes from turning on:
AVSS – 0.3 V < V(AINx) < AVDD + 0.3 V
(3)
If the voltages on the input pins have any potential to violate these conditions, external Schottky clamp diodes or
series resistors may be required to limit the input current to safe values (see the Absolute Maximum Ratings
table). Overdriving an unused input on the device can affect conversions taking place on other input pins.
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Feature Description (continued)
9.3.2 Low-Noise Programmable Gain Stage
The device features programmable gains of 1, 2, 4, 8, 16, 32, 64, and 128. Three bits (GAIN[2:0]) in the
configuration register are used to configure the gain. Gains are achieved in two stages. The first stage is a lownoise, low-drift, high input impedance, programmable gain amplifier (PGA). The second gain stage is
implemented by a switched-capacitor circuit at the input to the ΔΣ modulator. Table 9 shows how each gain is
implemented.
Table 9. Gain Implementation
GAIN SETTING
PGA GAIN
SWITCHED-CAPACITOR GAIN
1
1
1
2
1
2
4
1
4
ADVANCE INFORMATION
8
2
4
16
4
4
32
8
4
64
16
4
128
32
4
The PGA consists of two chopper-stabilized amplifiers (A1 and A2) and a resistor feedback network that sets the
PGA gain. The input is equipped with an electromagnetic interference (EMI) filter. Figure 8 shows a simplified
diagram of the PGA.
200
AINP
+
A1
25 pF
-
RF
OUTP
VOUT = PGA Gain·VIN
RG
VIN
RF
OUTN
A2
200
AINN
+
25 pF
Figure 8. Simplified PGA Diagram
VIN denotes the differential input voltage VIN = VAINP – VAINN. Use Equation 4 to calculate the gain of the PGA.
Gain is changed inside the device using a variable resistor, RG.
PGA Gain = 1 + 2 · RF / RG
(4)
The switched-capacitor gain is changed using variable capacitors at the input to the ΔΣ modulator. Gains 1, 2,
and 4 are implemented by using only the switched-capacitor circuit, which allows these gains to be used even
when the PGA is bypassed; see the Bypassing the PGA section for more information about bypassing the PGA.
Equation 5 shows that the differential full-scale input voltage range (FSR) of the PGA is defined by the gain
setting and the reference voltage used:
FSR = ±VREF / Gain
16
(5)
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Table 10 shows the corresponding full-scale ranges when using the internal 2.048-V reference.
Table 10. Full-Scale Range
GAIN SETTING
FSR
1
±2.048 V
2
±1.024 V
4
±0.512 V
8
±0.256 V
16
±0.128 V
32
±0.064 V
64
±0.032 V
128
±0.016 V
As with many amplifiers, the PGA has an absolute input voltage range requirement that cannot be exceeded.
The maximum and minimum absolute input voltages are limited by the voltage swing capability of the PGA
output. The specified minimum and maximum absolute input voltages (VAINP and VAINN) depend on the PGA gain,
the maximum differential input voltage (VINMAX), and the tolerance of the analog power-supply voltages (AVDD
and AVSS). Because gain on the ADS122U04 is implemented by both the PGA and a switched-capacitor gain
circuit, there are two formulas that define the absolute input voltages. Use Equation 6 when the device gain is
configured to less than or equal to 4. Use Equation 7 when the device gain is greater than 4. Use the maximum
differential input voltage expected in the application for VINMAX.
AVSS + 0.2 V ≤ VAINP, VAINN ≤ AVDD – 0.2 V
AVSS + 0.2 V + |VINMAX| · (Gain – 4) / 8 ≤ VAINP, VAINN ≤ AVDD – 0.2 V – |VINMAX| · (Gain – 4) / 8
(6)
(7)
Figure 9 graphically shows the relationship between the PGA input voltages to the PGA output voltages for gains
larger than 4. The PGA output voltages (VOUTP, VOUTN) depend on the PGA gain and the differential input voltage
magnitudes. For linear operation, the PGA output voltages must not exceed AVDD – 0.2 V or AVSS + 0.2 V.
Figure 9 depicts an example of a positive differential input voltage that results in a positive differential output
voltage.
PGA Input
PGA Output
AVDD
AVDD ± 0.2 V
VOUTP = VAINP + VIN Â (Gain ± 4) / 8
VAINP
VIN = VAINP ± VAINN
VAINN
VOUTN = VAINN ± VIN Â (Gain ± 4) / 8
AVSS + 0.2 V
AVSS
Figure 9. PGA Input/Output Voltage Relationship
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9.3.2.1 PGA Input Voltage Requirements
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9.3.2.2 Bypassing the PGA
At gains of 1, 2, and 4, the device can be configured to disable and bypass the low-noise PGA by setting the
PGA_BYPASS bit in the configuration register. Disabling the PGA lowers the overall power consumption and
also removes the restrictions of Equation 6 and Equation 7 for the absolute input voltage range, VCM. The usable
absolute input voltage range is (AVSS – 0.1 V ≤ VAINP, VAINN ≤ AVDD + 0.1 V) when the PGA is disabled.
In order to measure single-ended signals that are referenced to AVSS (AINP = VIN, AINN = AVSS), the PGA must
be bypassed. Configure the device for single-ended measurements by either connecting one of the analog inputs
to AVSS externally or by using the internal AVSS connection of the multiplexer (MUX[3:0] settings 1000 through
1011). When configuring the internal multiplexer for settings where AINN = AVSS (MUX[3:0] = 1000 through
1011), the PGA is automatically bypassed and disabled irrespective of the PGA_BYPASS setting and gain is
limited to 1, 2, and 4. In case gain is set to greater than 4, the device limits gain to 4.
When the PGA is disabled, the device uses a buffered switched-capacitor stage to obtain gains 1, 2, and 4. An
internal buffer in front of the switched-capacitor stage ensures that the effect on the input loading resulting from
the capacitor charging and discharging is minimal. See the Electrical Characteristics section for the typical values
of absolute input currents (current flowing into or out of each input) and differential input currents (difference in
absolute current between the positive and negative input) when the PGA is disabled.
ADVANCE INFORMATION
For signal sources with high output impedance, external buffering may still be necessary. Active buffers can
introduce noise as well as offset and gain errors. Consider all of these factors in high-accuracy applications.
9.3.3 Modulator and Internal Oscillator
A ΔΣ modulator is used in the ADS122U04 to convert the analog input voltage into a pulse code modulated
(PCM) data stream. The modulator runs at a modulator clock frequency of fMOD = fCLK / 4, where fCLK is provided
by the internal oscillator. The oscillator frequency, and therefore also the modulator frequency, depend on the
selected operating mode. Table 11 shows the oscillator and modulator frequencies for the different operating
modes.
Table 11. Oscillator and Modulator Clock Frequencies for Different Operating Modes
18
OPERATING MODE
fCLK
fMOD
Normal mode
1.024 MHz
256 kHz
Turbo mode
2.048 MHz
512 kHz
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9.3.4 Digital Filter
0
0
-40
-40
Magnitude (dB)
-80
-120
-160
-80
-120
-160
-200
0
20
40
60
80 100 120
Frequency (Hz)
140
160
180
-200
46
200
Figure 10. Filter Response
(Normal Mode, DR = 20 SPS)
50
52
54
56
58
Frequency (Hz)
60
62
64
D001
Figure 11. Detailed View of the Filter Response
(Normal Mode, DR = 20 SPS)
0
0
-20
-20
Magnitude (dB)
Magnitude (dB)
48
D002
ADVANCE INFORMATION
Magnitude (dB)
The device uses a linear-phase finite impulse response (FIR) digital filter that performs both filtering and
decimation of the digital data stream coming from the modulator. The digital filter is automatically adjusted for the
different data rates and always settles within a single cycle. The frequency responses of the digital filter are
illustrated in Figure 10 to Figure 18 for different output data rates. The filter notches and output data rate scale
proportionally with the clock frequency. The internal oscillator can vary over temperature as specified in the
Electrical Characteristics table. The data rate or conversion time, respectively, and consequently also the filter
notches vary proportionally.
-40
-60
-40
-60
-80
-80
0
20
40
60
80 100 120
Frequency (Hz)
140
160
180
200
0
100
D003
Figure 12. Filter Response
(Normal Mode, DR = 45 SPS)
200
300
400 500 600
Frequency (Hz)
700
800
900 1000
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D004
Figure 13. Filter Response
(Normal Mode, DR = 90 SPS)
19
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0
0
-20
-20
Magnitude (dB)
Magnitude (dB)
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-40
-60
-40
-60
-80
-80
0
100
200
300
400 500 600
Frequency (Hz)
700
800
900 1000
0
200
400
Figure 14. Filter Response
(Normal Mode, DR = 175 SPS)
800 1000 1200 1400 1600 1800 2000
Frequency (Hz)
D006
Figure 15. Filter Response
(Normal Mode, DR = 330 SPS)
0
0
-20
-20
Magnitude (dB)
ADVANCE INFORMATION
Magnitude (dB)
600
D005
-40
-40
-60
-60
-80
-80
0
500
1000
1500 2000 2500
Frequency (Hz)
3000
3500
4000
0
1
2
3
4
5
6
Frequency (kHz)
D007
Figure 16. Filter Response
(Normal Mode, DR = 600 SPS)
7
8
9
10
D008
Figure 17. Filter Response
(Normal Mode, DR = 1 kSPS)
0
Magnitude (dB)
-20
-40
-60
-80
0
1
2
3
4
5
6
Frequency (kHz)
7
8
9
10
D009
Figure 18. Filter Response
(Turbo Mode, DR = 2 kSPS)
20
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9.3.5 Conversion Times
Table 12 shows the actual conversion times for each data rate setting. The values provided are in terms of tCLK
cycles and in milliseconds.
Continuous conversion mode data rates are timed from one DRDY falling edge to the next DRDY falling edge.
The first conversion starts 28.5 · tCLK (normal mode) or 105 · tCLK (turbo mode) after the START/SYNC command
is latched.
Single-shot conversion mode data rates are timed from when the START/SYNC command is latched to the
DRDY falling edge and rounded to the next tCLK. The exact time that a command is latched in relation to the
rising edge of the stop bit depends on the baud rate; see the Command Latching section for details about
command latch timing.
Table 12. Conversion Times
NOMINAL
DATA RATE
(SPS)
–3-dB
BANDWIDTH
(Hz)
CONTINUOUS CONVERSION MODE (1)
SINGLE-SHOT CONVERSION MODE
ACTUAL
CONVERSION TIME
(tCLK) (2)
ACTUAL
CONVERSION TIME
(ms)
ACTUAL
CONVERSION TIME
(tCLK) (2)
ACTUAL
CONVERSION TIME
(ms)
20
13.1
51192
49.99
51213
50.01
45
20.0
22780
22.5
22805
22.27
90
39.6
11532
11.26
11557
11.29
175
77.8
5916
5.78
5941
5.80
330
150.1
3116
3.04
3141
3.07
600
279.0
1724
1.68
1749
1.71
1000
483.8
1036
1.01
1061
1.04
40
26.2
51192
25.00
51217
25.01
TURBO MODE
(1)
(2)
90
39.9
22780
11.12
22809
11.14
180
79.2
11532
5.63
11561
5.65
350
155.6
5916
2.89
5945
2.90
660
300.3
3116
1.52
3145
1.54
1200
558.1
1724
0.84
1753
0.86
2000
967.6
1036
0.51
1065
0.52
The first conversion starts 28.5 · tCLK (normal mode) or 105 · tCLK (turbo mode) after the START/SYNC command is latched. The times
listed in this table do not include that time.
tCLK = 1 / fCLK. fCLK = 1.024 MHz in normal mode and 2.048 MHz in turbo mode.
Although the conversion time at the 20-SPS setting is not exactly 1 / 20 Hz = 50 ms, this discrepancy does not
affect the 50-Hz or 60-Hz rejection. The conversion time and filter notches vary by the amount specified in the
table for oscillator accuracy.
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9.3.6 Voltage Reference
The device offers an integrated, low-drift, 2.048-V reference. For applications that require a different reference
voltage value or a ratiometric measurement approach, the device offers a differential reference input pair (REFP
and REFN). In addition, the analog supply (AVDD – AVSS) can be used as a reference.
The reference source is selected by two bits (VREF[1:0]) in the configuration register. By default, the internal
reference is selected. The internal voltage reference requires less than 25 µs to fully settle after power-up, when
coming out of power-down mode, or when switching from an external reference source to the internal reference.
The differential reference input allows freedom in the reference common-mode voltage. The reference inputs are
internally buffered to increase input impedance. Therefore, additional reference buffers are usually not required
when using an external reference. When used in ratiometric applications, the reference inputs do not load the
external circuitry; however, the analog supply current increases when using an external reference because the
reference buffers are enabled.
In most cases the conversion result is directly proportional to the stability of the reference source. Any noise and
drift of the voltage reference is reflected in the conversion result.
9.3.7 Excitation Current Sources
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The device provides two matched programmable excitation current sources (IDACs) for resistance temperature
detector (RTD) applications. The output current of the current sources can be programmed to 10 µA, 50 µA,
100 µA, 250 µA, 500 µA, 1000 µA, or 1500 µA using the respective bits (IDAC[2:0]) in the configuration register.
Each current source can be connected to any of the analog inputs (AINx) as well as to the dedicated reference
inputs (REFP and REFN). Both current sources can also be connected to the same pin. Routing of the IDACs is
configured by bits (I1MUX[2:0], I2MUX[2:0]) in the configuration register. Care must be taken not to exceed the
compliance voltage of the IDACs. In other words, limit the voltage on the pin where the IDAC is routed to
≤ (AVDD – 0.9 V), otherwise the specified accuracy of the IDAC current is not met. For three-wire RTD
applications, the matched current sources can be used to cancel errors caused by sensor lead resistance (see
the 3-Wire RTD Measurement section for more details).
The IDACs require up to 200 µs to start up after the IDAC current is programmed to the respective value using
the IDAC[2:0] bits. Set the IDAC current to the respective value using the IDAC[2:0] bits and then select the
routing for each IDAC (I1MUX[2:0], I2MUX[2:0]) thereafter.
In single-shot conversion mode, the IDACs remain active between any two conversions if the IDAC[2:0] bits are
set to a value other than 000. However, the IDACs are powered down whenever the POWERDOWN command is
issued.
Keep in mind that the analog supply current increases when enabling the IDACs (that is, when the IDAC[2:0] bits
are set to a value other than 000). The IDAC circuit needs this bias current to operate even when the IDACs are
not routed to any pin (I1MUX[2:0] = I2MUX[2:0] = 000). In addition, the selected output current is drawn from the
analog supply when I1MUX[2:0] or I2MUX[2:0] are set to a value other than 000.
22
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9.3.8 Sensor Detection
To help detect a possible sensor malfunction, the device provides internal 10-µA, burn-out current sources.
When enabled by setting the respective bit (BCS) in the configuration register, one current source provides
current to the positive analog input (AINP) currently selected and the other current source sinks current from the
selected negative analog input (AINN).
In case of an open circuit in the sensor, these burn-out current sources pull the positive input towards AVDD and
the negative input towards AVSS, resulting in a full-scale reading. A full-scale reading can also indicate that the
sensor is overloaded or that the reference voltage is absent. A near-zero reading can indicate a shorted sensor.
The absolute value of the burn-out current sources typically varies by ±10% and the internal multiplexer adds a
small series resistance. Therefore, distinguishing a shorted sensor condition from a normal reading can be
difficult, especially if an RC filter is used at the inputs. In other words, even if the sensor is shorted, the voltage
drop across the external filter resistance and the residual resistance of the multiplexer causes the output to read
a value higher than zero.
Keep in mind that ADC readings of a functional sensor may be corrupted when the burn-out current sources are
enabled. Disable the burn-out current sources when preforming the precision measurement, and only enable
these sources to test for sensor fault conditions.
The device provides some means for monitoring the analog power supply and the external voltage reference. To
select a monitoring voltage, the internal multiplexer (MUX[3:0]) must be configured accordingly in the
configuration register. The device automatically bypasses the PGA and sets the gain to 1, irrespective of the
configuration register settings when the monitoring feature is used. The system monitor function only provides a
coarse result and is not meant to be a precision measurement.
When measuring the analog power supply (MUX[3:0] = 1101), the resulting conversion is approximately (AVDD –
AVSS) / 4. The device uses the internal 2.048-V reference for the measurement regardless of what reference
source is selected in the configuration register (VREF[1:0]).
When monitoring the external reference voltage source (MUX[3:0] = 1100), the result is approximately (V(REFP) –
V(REFN)) / 4. The device automatically uses the internal reference for the measurement.
9.3.10 Offset Calibration
The internal multiplexer offers the option to short both PGA inputs (AINP and AINN) to mid-supply (AVDD +
AVSS) / 2. This option can be used to measure and calibrate the device offset voltage by storing the result of the
shorted input voltage reading in a microcontroller and consequently subtracting the result from each following
reading. Take multiple readings with the inputs shorted and average the result to reduce the effect of noise.
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9.3.9 System Monitor
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9.3.11 Temperature Sensor
The ADS122U04 offers an integrated precision temperature sensor. The temperature sensor mode is enabled by
setting bit TS = 1 in the configuration register. When in temperature sensor mode, the settings of configuration
register 0 have no effect and the device uses the internal reference for measurement, regardless of the selected
voltage reference source. Temperature readings follow the same process as the analog inputs for starting and
reading conversion results. Temperature data are represented as a 14-bit effective result that is left-justified
within the 24-bit conversion result. Data are output starting with the least significant bit (LSB). When reading the
three data bytes, the last 14 bits (MSBs) are used to indicate the temperature measurement result. The LSBs of
the data output do not indicate temperature. Only the 14 MSBs are relevant. One 14-bit LSB equals 0.03125°C.
Negative numbers are represented in binary twos complement format. Table 13 shows the mapping between
temperature and digital codes.
Table 13. 14-Bit Temperature Data Format
TEMPERATURE (°C)
DIGITAL OUTPUT (BINARY)
HEX
ADVANCE INFORMATION
128
01 0000 0000 0000
1000
127.96875
00 1111 1111 1111
0FFF
100
00 1100 1000 0000
0C80
75
00 1001 0110 0000
0960
50
00 0110 0100 0000
0640
25
00 0011 0010 0000
0320
0.25
00 0000 0000 1000
0008
0.03125
00 0000 0000 0001
0001
0
00 0000 0000 0000
0000
–0.25
11 1111 1111 1000
3FF8
–25
11 1100 1110 0000
3CE0
–40
11 1011 0000 0000
3B00
9.3.11.1 Converting From Temperature to Digital Codes
9.3.11.1.1 For Positive Temperatures (For Example, 50°C):
Twos complement is not performed on positive numbers. Therefore, simply convert the number to binary code in
a 14-bit, left-justified format with the MSB = 0 to denote the positive sign.
Example: 50°C / (0.03125°C per count) = 1600 = 0640h = 00 0110 0100 0000
9.3.11.1.2 For Negative Temperatures (For Example, –25°C):
Generate the twos complement of a negative number by complementing the absolute binary number and adding
1. Then, denote the negative sign with the MSB = 1.
Example: |–25°C| / (0.03125°C per count) = 800 = 0320h = 00 0011 0010 0000
Twos complement format: 11 1100 1101 1111 + 1 = 11 1100 1110 0000
9.3.11.2 Converting From Digital Codes to Temperature
To convert from digital codes to temperature, first check whether the MSB is a 0 or a 1. If the MSB is a 0, simply
multiply the decimal code by 0.03125°C to obtain the result. If the MSB is a 1, subtract 1 from the result and
complement all bits. Then, multiply the result by –0.03125°C.
Example: The device reads back 0960h: 0960h has an MSB = 0.
0960h · 0.03125°C = 2400 · 0.03125°C = 75°C
Example: The device reads back 3CE0h: 3CE0h has an MSB = 1.
Subtract 1 and complement the result: 3CE0h → 0320h
0320h · (–0.03125°C) = 800 · (–0.03125°C) = –25°C
24
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9.3.12 Conversion Data Counter
The ADS122U04 offers an optional data counter word to help the host determine if the conversion data are new.
The DCNT bit in the configuration register enables the conversion data counter. The data counter appears as an
8-bit word that precedes the conversion data each time a conversion result is read. The reset value of the
counter is 00h. The word increments each time the ADC completes a conversion. The counter rolls over to 00h
after reaching FFh.
When the host reads a conversion result, the host can determine if the data being read are new by comparing
the counter value with the counter value obtained with the last data read. If the counter values are the same,
then this result indicates that no new conversion data are available from the ADC. The counter can also help the
host determine if a conversion result was missed.
Reset the conversion data counter by clearing the DCNT bit to 0 and then setting DCNT back to 1. A device
reset also resets the conversion data counter.
There are two methods for ensuring data integrity for data output on the ADS122U04. Output data can be
register contents or conversion results. The data integrity modes are configured using the CRC bits in the
configuration register. When CRC[1:0] = 01, a bitwise-inverted version of the data is output immediately following
the most significant byte (MSB) of the data.
When CRC[1:0] = 10, a 16-bit CRC word is output immediately following the MSB of the data. In CRC mode, the
checksum bytes are the 16-bit remainder of the bitwise exclusive-OR (XOR) of the data bytes with a CRC
polynomial. The CRC is based on the CRC-16-CCITT polynomial: x16 + x12 + x5 + 1 with an initial value of FFh.
The 17 binary coefficients of the polynomial are: 1 0001 0000 0010 0001. To calculate the CRC, divide (XOR
operation) the data bytes (excluding the CRC) with the polynomial and compare the calculated CRC values to
the ADC CRC value. If the values do not match, a data transmission error has occurred. In the event of a data
transmission error, read the data again.
The following list shows a general procedure to compute the CRC value:
1. Left-shift the initial data value by 16 bits, with zeros padded to the right.
2. Align the MSB of the CRC polynomial to the left-most, logic-one value of the data.
3. Perform an XOR operation on the data value with the aligned CRC polynomial. The XOR operation creates a
new, shorter-length value. The bits of the data values that are not in alignment with the CRC polynomial drop
down and append to the right of the new XOR result.
4. When the XOR result is less than 1 0000 0000 0000 0000, the procedure ends, yielding the 16-bit CRC
value. Otherwise, continue with the XOR operation shown in step 2 using the current data value. The number
of loop iterations depends on the value of the initial data.
9.3.14 General-Purpose Digital Inputs/Outputs
The ADS122U04 offers three dedicated general-purpose input/output (GPIO) pins. Use the GPIOnDIR (where n
= 0, 1, 2) bits in the configuration register to configure the pin as either an input or an output. The GPIOnDAT
bits in the configuration register contain the input or output GPIO data. If a GPIO pin is configured as an input,
the respective GPIOnDAT bit reads the status of the pin; if the GPIO pin is configured as an output, write the
output status to the respective GPIOnDAT bit.
GPIO2 shares a pin with the DRDY signal. When the pin is configured as an output by the GPIO2DIR bit, the
GPIO2SEL bit in the configuration register selects the function of the GPIO2/DRDY pin. If the GPIO2SEL bit is
cleared, GPIO2 is routed to the pin. If the bit is set, the pin is driven with the DRDY signal.
See the Configuration Registers section for more information regarding the configuration of the GPIO pins.
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9.4 Device Functional Modes
Figure 19 shows a flow chart of the different operating modes and how the device transitions from one mode to
another.
Power-On Reset or
RESET pin high or
RESET command(1)
Reset device to
default settings
Low-power state
No
ADVANCE INFORMATION
No
START/SYNC
Command?
POWERDOWN
Command?
Yes
Yes
Conversion
Mode
Power-down Mode(3)
Yes
Start new
conversion
START/SYNC
Command?
No
No
0 = Single-Shot
conversion mode
Conversion
mode selection(2)
1 = Continuous
conversion mode
POWERDOWN
Command?
Yes
(1)
Any reset (power-on, command, or pin) immediately resets the device.
(2)
The conversion mode is selected with the CM bit in the configuration register.
(3)
The POWERDOWN command allows any ongoing conversion to complete before placing the device in power-down
mode.
Figure 19. Operating Flow Chart
9.4.1 Power-Up and Reset
The ADS122U04 is reset in one of three ways: either by a power-on reset, by the RESET pin, or by a RESET
command.
When a reset occurs, the configuration registers reset to the default values and the device enters a low-power
state. The device then waits for the START/SYNC command to enter conversion mode; see the UART Timing
Requirements section for reset timing information.
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Device Functional Modes (continued)
9.4.1.1 Power-On Reset
When the device powers up, a reset is performed. The reset process takes approximately 50 µs. After this
power-up reset time, all internal circuitry (including the voltage reference) are stable and communication with the
device is possible. As part of the reset process, the device sets all bits in the configuration registers to the
respective default settings. By default, the device is set to single-shot conversion mode. After power-up, the
device enters a low-power state. The power-up behavior is intended to prevent systems with tight power-supply
requirements from encountering a current surge during power-up.
9.4.1.2 RESET Pin
Reset the ADC by taking the RESET pin low for a minimum of tw(RSL) and then returning the pin high. After the
rising edge of the RESET pin, a delay time of td(RSTX) is required before sending the first serial interface
command or starting a conversion; see the UART Timing Requirements section for reset timing information.
Reset the ADC by using the RESET command (06h or 07h). After the RESET command is latched, a delay time
of td(RSTX) is required before sending the first serial interface command or starting a conversion; see the UART
Timing Requirements section for reset timing information. The exact time that a command is latched in relation to
the rising edge of the stop bit depends on the baud rate; see the Command Latching section for details about
command latch timing.
9.4.2 Conversion Modes
The device operates in one of two conversion modes that are selected by the CM bit in the configuration register.
These conversion modes are single-shot conversion and continuous conversion mode. A START/SYNC
command must be issued each time the CM bit is changed.
9.4.2.1 Single-Shot Conversion Mode
In single-shot conversion mode, the device only performs a conversion when a START/SYNC command is
issued. The device consequently performs one single conversion and returns to a low-power state afterwards.
The internal oscillator and all analog circuitry (except for the excitation current sources) are turned off while the
device waits in this low-power state until the next conversion is started. Writing to any configuration register
besides register 04h when a conversion is ongoing functions as a new START/SYNC command that stops the
current conversion and restarts a single new conversion. Each conversion is fully settled (assuming the analog
input signal settles to the final value before the conversion starts) because the device digital filter settles within a
single cycle.
9.4.2.2 Continuous Conversion Mode
In continuous conversion mode, the device continuously performs conversions. When a conversion completes,
the device places the result in the output buffer and immediately begins another conversion.
In order to start continuous conversion mode, the CM bit must be set to 1 followed by a START/SYNC command.
The first conversion starts 28.5 · tCLK (normal mode) or 105 · tCLK (turbo mode) after the START/SYNC command
is latched. The exact time that a command is latched in relation to the rising edge of the stop bit depends on the
baud rate; see the Command Latching section for details about command latch timing. Writing to any
configuration register besides register 04h during an ongoing conversion restarts the current conversion. Send a
START/SYNC command immediately after the CM bit is set to 1.
Stop continuous conversions by sending the POWERDOWN command.
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Device Functional Modes (continued)
9.4.3 Operating Modes
In addition to the different conversion modes, the device can also be operated in different operating modes that
can be selected to trade-off power consumption, noise performance, and output data rate. These modes are:
normal mode, turbo mode, and power-down mode.
9.4.3.1 Normal Mode
Normal mode is the default mode of operation after power-up. In this mode, the internal modulator of the ΔΣ ADC
runs at a modulator clock frequency of fMOD = fCLK / 4 = 256 kHz, where the system clock (fCLK) is provided by the
internal oscillator. Normal mode offers output data rate options ranging from 20 SPS to 1 kSPS. The data rate is
selected by the DR[2:0] bits in the configuration register.
9.4.3.2 Turbo Mode
ADVANCE INFORMATION
Applications that require higher data rates up to 2 kSPS can operate the device in turbo mode. In this mode, the
internal modulator runs at a higher frequency of fMOD = fCLK / 4 = 512 kHz. Compared to normal mode, the device
power consumption increases because the modulator runs at a higher frequency. Running the ADS122U04 in
turbo mode at a comparable output data rate as in normal mode yields better noise performance. For example,
the input-referred noise at 90 SPS in turbo mode is lower than the input-referred noise at 90 SPS in normal
mode.
9.4.3.3 Power-Down Mode
When the POWERDOWN command is issued, the device enters power-down mode after completing the current
conversion. In this mode, all analog circuitry (including the voltage reference and both IDACs) are powered down
and the device typically only uses 400 nA of current. When in power-down mode, the device holds the
configuration register settings and responds to commands, but does not perform any data conversions.
Issuing a START/SYNC command wakes up the device and either starts a single conversion or starts continuous
conversion mode, depending on the conversion mode selected by the CM bit.
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9.5 Programming
9.5.1 UART Interface
The serial data and control interface on the ADS122U04 is universal asynchronous receiver transmitter (UART)
compatible. Commands from the host are received by the device through the RX pin. Data are transmitted from
the device to the host through the TX pin. The ADS122U04 actively synchronizes to the baud rate of the host
each time the host transmits a command. The interface is half duplex; meaning only either the host or the device
can communicate at any given time.
9.5.1.1 Receive (RX)
The UART receive pin (RX) is used to send data (commands and register data) to the device. The device never
drives the RX pin.
9.5.1.2 Transmit (TX)
The UART transmit pin (TX) is used to read conversion and register data from the device. The TX pin is held at
logic high when not transmitting data.
DRDY indicates when a new conversion result is ready for retrieval. The DRDY signal appears on the
GPIO2/DRDY pin only when GPIO2 is configured as an output and the GPIO2SEL bit in the CONFIG4 register is
set. When DRDY falls low, new conversion data are ready. DRDY transitions back high when the conversion
result is latched for output transmission. In case a conversion result in continuous conversion mode is not read
(only applies to manual data read mode), DRDY pulses high for tw(DRH) before the next conversion completes;
see the UART Switching Characteristics section for more details.
9.5.1.4 Protocol
Serial data transfer in the UART is performed in byte increments. For each byte that is sent by either the host or
the device, a start bit (logic low) is transmitted first, followed by eight bits of data in LSB-first format. A stop bit
(logic high) is transmitted at the end of each byte. By using a start and stop bit for each byte, the ADS122U04
can latch each byte and maintain synchronous communication throughout the process.
The ADS122U04 actively synchronizes to the baud rate of the host each time the host transmits a command.
Baud rate synchronization occurs when the host transmits a synchronization word (55h) preceding any command
sent to the ADS122U04. The host must always transmit the synchronization word first followed by the command
byte or bytes. Each byte begins with a start bit and ends with a stop bit, including the synchronization word.
Figure 20 shows the timing sequence for the UART communication. In Figure 20, as an example, there is only
one byte for the command and one byte for the readback data. There may be multiple bytes for a command or
the data that is read from the device. The protocol takes the 8-N-1 format: eight (8) data bits, no (N) parity bit,
and one (1) stop bit.
tfrom the hostt
Synchronization Word
RX
S
1
0
1
0
1
0
Command Word
1
0
P
S
C0
C1
C2
C3
C4
C5
C6
C7
P
tfrom the devicet
S
TX
D0
D1
D2
D3
D4
D5
D6
D7
P
Figure 20. Example ADS122U04 UART Protocol
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9.5.1.3 Data Ready (DRDY)
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Programming (continued)
9.5.1.5 Timeout
The ADS122U04 offers a UART timeout feature that can be used to recover communication when a serial
interface transmission is interrupted. If the host initiates contact with the ADS122U04 but subsequently remains
idle for 32760 · tMOD in normal mode and 65520 · tMOD in turbo mode before completing a command, the
ADS122U04 interface is reset. If the ADS122U04 interface has reset because of a timeout condition, the host
must abort the transaction and restart the communication again by sending the synchronization word first
followed by the command byte or bytes.
9.5.1.6 Data Format
The device provides 24 bits of data in binary twos complement format. Use Equation 8 to calculate the size of
one code (LSB).
1 LSB = (2 · VREF / Gain) / 224 = +FS / 223
(8)
A positive full-scale input [VIN ≥ (+FS – 1 LSB) = (VREF / Gain – 1 LSB)] produces an output code of 7FFFFFh
and a negative full scale input (VIN ≤ –FS = –VREF / Gain) produces an output code of 800000h. The output clips
at these codes for signals that exceed full-scale.
Table 14. Ideal Output Code versus Input Signal
INPUT SIGNAL,
VIN = VAINP – VAINN
IDEAL OUTPUT CODE
≥ FS (223 – 1) / 223
7FFFFFh
23
FS / 2
000001h
0
000000h
–FS / 223
FFFFFFh
≤ –FS
800000h
Figure 21 shows the mapping of the analog input signal to the output codes.
7FFFFFh
¼
7FFFFEh
000001h
000000h
FFFFFFh
¼
Output Code
ADVANCE INFORMATION
Table 14 summarizes the ideal output codes for different input signals.
800001h
800000h
¼
-FS
2
23
FS
¼
-1
-FS
2
0
Input Voltage VIN
23
2
23
FS
2
-1
23
Figure 21. Code Transition Diagram
30
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9.5.2 Commands
As Table 15 shows, the device offers six different commands to control device operation. Four commands are
stand-alone instructions (RESET, START/SYNC, POWERDOWN, and RDATA). The commands to read (RREG)
and write (WREG) configuration register data from and to the device require additional information as part of the
instruction. For clarity, Table 15 shows the command bits MSB first, but data are always transmitted byte-wise
LSB first on the ADS122U04.
Table 15. Command Definitions
COMMAND
DESCRIPTION
COMMAND BYTE (1)
RESET
Reset the device
0000 011x
START/SYNC
Start or restart conversions
0000 100x
POWERDOWN
Enter power-down mode
0000 001x
RDATA
Read data by command
0001 xxxx
RREG
Read register at address rrr
0010 rrrx
WREG
Write register at address rrr
0100 rrrx
Operands: rrr = register address (000 to 100), x = don't care.
9.5.2.1 RESET (0000 011x)
Resets the device to the default states. Wait at least (50 µs + 32 · tCLK) after the RESET command is sent before
sending any other command.
9.5.2.2 START/SYNC (0000 100x)
In single-shot conversion mode, the START/SYNC command is used to start a single conversion, or (when sent
during an ongoing conversion) to reset the digital filter and then restart a single new conversion. When the
device is set to continuous conversion mode, the START/SYNC command must be issued one time to start
converting continuously. Sending the START/SYNC command when converting in continuous conversion mode
resets the digital filter and restarts continuous conversions.
9.5.2.3 POWERDOWN (0000 001x)
The POWERDOWN command places the device into power-down mode. The command shuts down all internal
analog components and turns off both IDACs, but holds all register values. In case the POWERDOWN command
is issued when a conversion is ongoing, the conversion completes before the ADS122U04 enters power-down
mode. As soon as a START/SYNC command is issued, all analog components return to their previous states.
9.5.2.4 RDATA (0001 xxxx)
The RDATA command loads the output shift register with the most recent conversion result right after the
command is received. If a conversion finishes in the middle of the RDATA command byte, the state of the DRDY
pin at the end of the read operation signals whether the old or the new result is loaded. If the old result is loaded,
DRDY stays low, indicating that the new result is not read out. The new conversion result loads when DRDY is
high.
NOTE
UART transmissions take place byte-wise. Bytes are transmitted least significant bit first.
Data words are transmitted least significant byte first.
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9.5.2.5 RREG (0010 rrrx)
The RREG command reads the value of the register at address rrr. If the register pointed to by rrr does not exist,
the read back data are 00h. Figure 22 shows the sequence for reading a register. The synchronization word
must be sent by the host before the RREG command is sent.
RX
10101010
0010 0100
REG 2h DATA
TX
Figure 22. Read Register Sequence
9.5.2.6 WREG (0100 rrrx dddd dddd)
The WREG command writes dddd dddd to the register at address rrr. If the register pointed to by rrr does not
exist, the WREG command is ignored. Figure 23 shows the sequence for writing a register. The synchronization
word must be sent by the host before the WREG command is sent. The WREG command forces the digital filter
to reset and any ongoing ADC conversion to restart.
RX
10101010
0010 0010
REG 2h DATA
ADVANCE INFORMATION
TX
Figure 23. Write Register Sequence
9.5.2.7 Command Latching
The ADS122U04 interface automatically synchronizes to the baud rate of the host. This means the time required
for commands to be latched by the interface varies with baud rate. Commands are not processed until after
being latched by the ADS122U04.
Commands are latched by the ADS122U04 when the device detects the stop bit. Stop bit detection generally
occurs in the middle of the stop bit where the middle of the stop bit is defined as tBAUD / 2 after the rising edge of
the stop bit. However, this timing is not exact because of the asynchronous nature between the host baud
clocking and the ADS122U04 internal oscillator as well as jitter in the ADS122U04 internal oscillator. The stop bit
detection timing error can be as large as 4 · tCLK in normal mode and 8 · tCLK in turbo mode.
9.5.3 Reading Data
There are two ways to read data from the ADS122U04: manual data read mode and automatic data read mode.
In manual data read mode, the host retrieves data by issuing the RDATA command. In automatic data read
mode, the ADS122U04 automatically outputs conversion data on the TX pin as soon as a conversion completes.
32
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9.5.3.1 Manual Data Read Mode
In manual data read mode, data are read by issuing the RDATA command. The ADS122U04 responds to the
RDATA command with the latest conversion data. There are three ways to monitor for new conversion data.
One way is to monitor for the falling edge of the DRDY signal. To configure the GPIO2/DRDY pin to output the
DRDY signal, the pin must be configured as an output by setting the GPIO2DIR bit in the configuration register,
and DRDY must be multiplexed to the pin by setting the GPIO2SEL bit in the configuration register.
Figure 24 shows the timing diagram for collecting data in manual data read mode using DRDY to indicate new
data.
DRDY
RX
55h
RDATA
DATA LSB
TX
DATA
DATA MSB
Another way to monitor for a new conversion result is to periodically read the configuration register for the DRDY
bit. If set, the DRDY bit indicates that a new conversion result is ready for retrieval. The host can subsequently
issue an RDATA command to retrieve the data. The rate at which the host polls the ADS122U04 for new data
must be at least as fast as the data rate in continuous conversion mode to prevent the host from missing a
conversion result.
If a new conversion result becomes ready during a UART transmission, the transmission will not be corrupted.
The new data are loaded into the output shift register upon the following RDATA command.
Figure 25 shows the timing diagram for collecting data in manual data read mode using the DRDY bit in the
configuration register to indicate new data.
RX
55h
RREG 2h
55h
RDATA
REGISTER 2h
TX
DATA LSB
DATA
DATA MSB
Figure 25. Manual Data Read Mode Using the RREG Command (Continuous Conversion Mode)
The last way to detect if new conversion data are available is through the use of the conversion data counter
word. In this mode, the host periodically requests data from the device using the RDATA command and checks
the conversion data counter word against the conversion data counter word read for the previous data received.
If the counter values are the same, the host can disregard the data because that data has already been
gathered. If the counter has incremented, the host records the data. The rate at which the host polls the
ADS122U04 for new data must be at least as fast as the data rate in continuous conversion mode to prevent the
host from missing a conversion result.
If a new conversion result becomes ready during a UART transmission, the transmission is not corrupted. The
new data are loaded into the output shift register after the following RDATA command.
Figure 26 shows the timing diagram for collecting data in manual data read mode using the conversion data
counter word to indicate new data.
RX
55h
RDATA
TX
COUNTER
DATA LSB
DATA
DATA MSB
Figure 26. Manual Data Read Mode Using the Conversion Data Counter (Continuous Conversion Mode)
The conversion data counter can be used in conjunction with the previously discussed methods of detecting new
data to ensure that the host did not miss a conversion result.
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Figure 24. Manual Data Read Mode Using DRDY (Continuous Conversion Mode)
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9.5.3.2 Automatic Data Read Mode
In automatic data read mode, the ADS122U04 automatically outputs the latest conversion data on the TX pin
without the host sending an RDATA command. The DRDY signal does not have to be monitored in this mode;
thus making this mode useful for applications that require the number of digital lines to be minimized. Using
automatic data read mode requires the least amount of communication between the host and device when
compared to monitoring the DRDY bit of the configuration register or the conversion data counter in manual data
read mode. The conversion data counter can also be used in this mode to verify that the host has not missed a
conversion result.
Figure 27 shows the timing diagram for collecting data in automatic data read mode.
DRDY
RX
TX
DATA0 LSB
DATA0
DATA0 MSB
DATA1 LSB
DATA1
DATA1 MSB
Figure 27. Automatic Data Read Mode (Continuous Conversion Mode)
ADVANCE INFORMATION
9.5.4 Data Integrity
The optional data integrity checks can be configured using the CRC[1:0] bits in the configuration register. When
one of the data integrity options is enabled, the data integrity check is output on the TX pin immediately following
the conversion data; see the Data Integrity section for detailed description of the data integrity functionality.
Additional words are always two bytes when CRC16 is enabled. The number of additional words in the inverted
data mode varies from three to four depending on whether the conversion data counter is enabled. Figure 28
shows data retrieval in which either the inverted data or CRC are enabled in the configuration register.
RX
55h
RDATA
DATA LSB
TX
DATA
DATA MSB
DATA LSB/CRC0
DATA/CRC1
DATA MSB
Figure 28. Data Output Including Optional Data Integrity Bytes
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9.6 Register Map
9.6.1 Configuration Registers
The device has five 8-bit configuration registers that are accessible through the UART interface using the RREG
and WREG commands. The configuration registers control how the device operates and can be changed at any
time without causing data corruption. After power-up or reset, all registers are set to the default values (which are
all 0). All register values are retained during power-down mode. Table 16 shows the register map of the
configuration registers. Table 17 lists the register access codes for the ADS122U04.
Table 16. Configuration Register Map
REGISTER
(Hex)
BIT 7
BIT 6
00h
BIT 5
BIT 4
BIT 3
MODE
CM
MUX[3:0]
01h
DRDY
03h
DCNT
CRC[1:0]
0
GPIO1DIR
GPIO0DIR
GPIO2SEL
TS
IDAC[2:0]
I2MUX[2:0]
GPIO2DIR
BIT 0
PGA_BYPASS
VREF[1:0]
BCS
I1MUX[2:0]
04h
BIT 1
GAIN[2:0]
DR[2:0]
02h
BIT 2
GPIO2DAT
0
AUTO
GPIO1DAT
GPIO0DAT
Access Type
Code
Description
R
R
Read
R/W
R/W
Read-Write
W
W
Write
-n
ADVANCE INFORMATION
Table 17. ADS122U04 Register Access Type Codes
Value after reset or the default value
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9.6.1.1 Configuration Register 0 (address = 00h) [reset = 00h]
Figure 29. Configuration Register 0
7
6
5
4
3
MUX[3:0]
R/W-0h
2
GAIN[2:0]
R/W-0h
1
0
PGA_BYPASS
R/W-0h
Table 18. Configuration Register 0 Field Descriptions
Bit
Field
Type
Reset
Description
7:4
MUX[3:0]
R/W
0h
Input multiplexer configuration.
These bits configure the input multiplexer.
For settings where AINN = AVSS, the PGA must be disabled (PGA_BYPASS = 1)
and only gains 1, 2, and 4 can be used.
ADVANCE INFORMATION
0000 :
0001 :
0010 :
0011 :
0100 :
0101 :
0110 :
0111 :
1000 :
1001 :
1010 :
1011 :
1100 :
1101 :
1110 :
1111 :
3:1
GAIN[2:0]
R/W
0h
Gain configuration.
These bits configure the device gain.
Gains 1, 2, and 4 can be used without the PGA. In this case, gain is obtained by
a switched-capacitor structure.
000 :
001 :
010 :
011 :
100 :
101 :
110 :
111 :
0
PGA_BYPASS
R/W
0h
AINP = AIN0, AINN = AIN1 (default)
AINP = AIN0, AINN = AIN2
AINP = AIN0, AINN = AIN3
AINP = AIN1, AINN = AIN0
AINP = AIN1, AINN = AIN2
AINP = AIN1, AINN = AIN3
AINP = AIN2, AINN = AIN3
AINP = AIN3, AINN = AIN2
AINP = AIN0, AINN = AVSS
AINP = AIN1, AINN = AVSS
AINP = AIN2, AINN = AVSS
AINP = AIN3, AINN = AVSS
(V(REFP) – V(REFN)) / 4 monitor (PGA bypassed)
(AVDD – AVSS) / 4 monitor (PGA bypassed)
AINP and AINN shorted to (AVDD + AVSS) / 2
Reserved
Gain
Gain
Gain
Gain
Gain
Gain
Gain
Gain
= 1 (default)
=2
=4
=8
= 16
= 32
= 64
= 128
Disables and bypasses the internal low-noise PGA.
Disabling the PGA reduces overall power consumption and allows the commonmode voltage range (VCM) to span from AVSS – 0.1 V to AVDD + 0.1 V.
The PGA can only be disabled for gains 1, 2, and 4.
The PGA is always enabled for gain settings 8 to 128, regardless of the
PGA_BYPASS setting.
0 : PGA enabled (default)
1 : PGA disabled and bypassed
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9.6.1.2 Configuration Register 1 (address = 01h) [reset = 00h]
Figure 30. Configuration Register 1
7
6
DR[2:0]
R/W-0h
5
4
MODE
R/W-0h
3
CM
R/W-0h
2
1
VREF[1:0]
R/W-0h
0
TS
R/W-0h
Table 19. Configuration Register 1 Field Descriptions
Bit
Field
Type
Reset
Description
7:5
DR[2:0]
R/W
0h
Data rate.
These bits control the data rate setting depending on the selected operating
mode. Table 20 lists the bit settings for normal and turbo mode.
4
MODE
R/W
0h
Operating mode.
These bits control the operating mode that the device operates in.
0 : Normal mode (256-kHz modulator clock, default)
1 : Turbo mode (512-kHz modulator clock)
3
CM
R/W
0h
Conversion mode.
This bit sets the conversion mode for the device.
2:1
VREF[1:0]
R/W
0h
Voltage reference selection.
These bits select the voltage reference source that is used for the conversion.
00
01
10
11
0
TS
R/W
0h
ADVANCE INFORMATION
0 : Single-shot conversion mode (default)
1 : Continuous conversion mode
:
:
:
:
Internal 2.048-V reference selected (default)
External reference selected using the REFP and REFN inputs
Analog supply (AVDD – AVSS) used as reference
Analog supply (AVDD – AVSS) used as reference
Temperature sensor mode.
This bit enables the internal temperature sensor and puts the device in
temperature sensor mode.
The settings of configuration register 0 have no effect and the device uses the
internal reference for measurement when temperature sensor mode is enabled.
0 : Disables temperature sensor (default)
1 : Enables temperature sensor
Table 20. DR Bit Settings
NORMAL MODE
TURBO MODE
000 = 20 SPS
000 = 40 SPS
001 = 45 SPS
001 = 90 SPS
010 = 90 SPS
010 = 180 SPS
011 = 175 SPS
011 = 350 SPS
100 = 330 SPS
100 = 660 SPS
101 = 600 SPS
101 = 1200 SPS
110 = 1000 SPS
110 = 2000 SPS
111 = Reserved
111 = Reserved
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9.6.1.3 Configuration Register 2 (address = 02h) [reset = 00h]
Figure 31. Configuration Register 2
7
DRDY
R-0h
6
DCNT
R/W-0h
5
4
3
BCS
R/W-0h
CRC[1:0]
R/W-0h
2
1
IDAC[2:0]
R/W-0h
0
Table 21. Configuration Register 2 Field Descriptions
Bit
Field
Type
Reset
Description
7
DRDY
R
0h
Conversion result ready flag.
This bit flags if a new conversion result is ready. This bit is reset when conversion
data are read.
0 : No new conversion result available (default)
1 : New conversion result ready
6
5:4
ADVANCE INFORMATION
DCNT
R/W
0h
Data counter enable.
The bit enables the conversion data counter.
0 : Conversion counter disabled (default)
1 : Conversion counter enabled
CRC[1:0]
R/W
0h
Data integrity check enable.
These bits enable and select the data integrity checks.
00
01
10
11
3
2:0
:
:
:
:
Disabled (default)
Inverted conversion result data output enabled
CRC16 enabled
Reserved
BCS
R/W
0h
Burn-out current sources.
This bit controls the 10-μA, burn-out current sources. The burn-out current
sources can be used to detect sensor faults such as wire breaks and shorted
sensors.
0 : Current sources off (default)
1 : Current sources on
IDAC[2:0]
R/W
0h
IDAC current setting.
These bits set the current for both IDAC1 and IDAC2 excitation current sources.
000 :
001 :
010 :
011 :
100 :
101 :
110 :
111 :
38
Off (default)
10 µA
50 µA
100 µA
250 µA
500 µA
1000 µA
1500 µA
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9.6.1.4 Configuration Register 3 (address = 03h) [reset = 00h]
Figure 32. Configuration Register 3
7
6
I1MUX[2:0]
R/W-0h
5
4
3
I2MUX[2:0]
R/W-0h
2
1
0
R-0h
0
AUTO
R/W-0h
Table 22. Configuration Register 3 Field Descriptions
Bit
Field
Type
Reset
Description
7:5
I1MUX[2:0]
R/W
0h
IDAC1 routing configuration.
These bits select the channel that IDAC1 is routed to.
000 :
001 :
010 :
011 :
100 :
101 :
110 :
111 :
I2MUX[2:0]
R/W
0h
IDAC2 routing configuration.
These bits select the channel that IDAC2 is routed to.
000 :
001 :
010 :
011 :
100 :
101 :
110 :
111 :
ADVANCE INFORMATION
4:2
IDAC1 disabled (default)
IDAC1 connected to AIN0
IDAC1 connected to AIN1
IDAC1 connected to AIN2
IDAC1 connected to AIN3
IDAC1 connected to REFP
IDAC1 connected to REFN
Reserved
IDAC2 disabled (default)
IDAC2 connected to AIN0
IDAC2 connected to AIN1
IDAC2 connected to AIN2
IDAC2 connected to AIN3
IDAC2 connected to REFP
IDAC2 connected to REFN
Reserved
1
RESERVED
R
0h
Reserved.
Always write 0
0
AUTO
R/W
0h
ADC data output mode.
The bit controls the UART data output mode for the conversion result.
0 : Manual data read (read on demand) mode (default)
1 : Automatic data read mode
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9.6.1.5 Configuration Register 4 (address = 04h) [reset = 00h]
Figure 33. Configuration Register 4
7
0
R-0h
6
GPIO2DIR
R/W-0h
5
GPIO1DIR
R/W-0h
4
GPIO0DIR
R/W-0h
3
GPIO2SEL
R/W-0h
2
GPIO2DAT
R/W-0h
1
GPIO1DAT
R/W-0h
0
GPIO0DAT
R/W-0h
Table 23. Configuration Register 4 Field Descriptions
Bit
Field
Type
Reset
Description
7
RESERVED
R
0h
Reserved.
Always write 0
6
GPIO2DIR
R/W
0h
GPIO2 direction control.
This bit configures GPIO2 as an input or output.
0 : Input (default)
1 : Output
5
GPIO1DIR
R/W
0h
GPIO1 direction control.
This bit configures GPIO1 as an input or output.
0 : Input (default)
1 : Output
ADVANCE INFORMATION
4
GPIO0DIR
R/W
0h
GPIO0 direction control.
This bit configures GPIO0 as an input or output.
0 : Input (default)
1 : Output
3
GPIO2SEL
R/W
0h
GPIO2/DRDY control.
This bit controls which source controls the state of the GPIO2/DRDY pin when
GPIO2 is configured as an output.
0 : GPIO2DAT (default)
1 : DRDY
2
GPIO2DAT
R/W
0h
GPIO2 input/output level.
This bit controls the state of GPIO2 when configured as an output or holds the
value of GPIO2 when configured as an input.
0 : Logic low (default)
1 : Logic high
1
GPIO1DAT
R/W
0h
GPIO1 input/output level.
This bit controls the state of GPIO1 when configured as an output or holds the
value of GPIO1 when configured as an input.
0 : Logic low (default)
1 : Logic high
0
GPIO0DAT
R/W
0h
GPIO0 input/output level.
This bit controls the state of GPIO0 when configured as an output or holds the
value of GPIO0 when configured as an input.
0 : Logic low (default)
1 : Logic high
40
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10 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
10.1 Application Information
The ADS122U04 is a precision, 24-bit, delta-sigma (ΔΣ), analog-to-digital converter (ADC) that offers many
integrated features to ease the measurement of the most common sensor types, including various types of
temperature and bridge sensors. Primary considerations when designing an application with the ADS122U04
include analog input filtering, establishing an appropriate external reference for ratiometric measurements, and
setting the common-mode input voltage for the internal PGA. Connecting and configuring the serial interface
appropriately is another concern. These considerations are discussed in the following sections.
10.1.1 Serial Interface Connections
ADVANCE INFORMATION
Figure 34 shows the principle serial interface connections for the ADS122U04.
47 O
GPIO/IRQ
DVSS
0.1 PF
47 O
47 O
TX
RX
DVDD
Microcontroller with UART Interface
3.3 V
3.3 V
1
GPIO1
RX
16
2
GPIO0
TX
15
3
RESET
GPIO2/ DRDY
14
4
DGND
DVDD
13
3.3 V
Device
5
AVSS
AVDD
12
6
AIN3
AIN0
11
7
AIN2
AIN1
10
8
REFN
REFP
9
3.3 V
0.1 PF
0.1 PF
Figure 34. Serial Interface Connections
Most microcontroller UART peripherals can operate with the ADS122U04. The baud rate is determined by the
host via a synchronization word that must be sent to the ADS122U04 before each command. Details of the
UART communication protocol of the device can be found in the Programming section.
TI recommends placing 47-Ω resistors in series with all digital input and output pins (TX, RX, and GPIO2/DRDY).
This resistance smooths sharp transitions, suppresses overshoot, and offers some overvoltage protection. Care
must be taken to meet all UART timing requirements because the additional resistors interact with the bus
capacitances present on the digital signal lines.
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Application Information (continued)
10.1.2 Analog Input Filtering
Analog input filtering serves two purposes: first, to limit the effect of aliasing during the sampling process, and
second, to reduce external noise from being a part of the measurement.
As with any sampled system, aliasing can occur if proper antialias filtering is not in place. Aliasing occurs when
frequency components are present in the input signal that are higher than half the sampling frequency of the
ADC (also known as the Nyquist frequency). These frequency components are folded back and show up in the
actual frequency band of interest below half the sampling frequency. Inside a ΔΣ ADC, the input signal is
sampled at the modulator frequency fMOD and not at the output data rate. Figure 35 shows that the filter response
of the digital filter repeats at multiples of the sampling frequency (fMOD). Signals or noise up to a frequency where
the filter response repeats are attenuated to a certain amount by the digital filter depending on the filter
architecture. Any frequency components present in the input signal around the modulator frequency or multiples
thereof are not attenuated and alias back into the band of interest, unless attenuated by an external analog filter.
Magnitude
Sensor
Signal
ADVANCE INFORMATION
Unwanted Signals
Unwanted Signals
Output
Data Rate
fMOD/2
f(MOD)
Frequency
f(MOD)
Frequency
f(MOD)
Frequency
Magnitude
Digital Filter
Aliasing of Unwanted
Signals
Output
Data Rate
fMOD/2
Magnitude
External
Antialiasing Filter
Roll-Off
Output
Data Rate
fMOD/2
Figure 35. Effect of Aliasing
42
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Application Information (continued)
Many sensor signals are inherently band limited; for example, the output of a thermocouple has a limited rate of
change. In this case the sensor signal does not alias back into the pass band when using a ΔΣ ADC. However,
any noise pick-up along the sensor wiring or the application circuitry can potentially alias into the pass band.
Power-line-cycle frequency and harmonics are one common noise source. External noise can also be generated
from electromagnetic interference (EMI) or radio frequency interference (RFI) sources, such as nearby motors
and cellular phones. Another noise source typically exists on the printed circuit board (PCB) itself in the form of
clocks and other digital signals. Analog input filtering helps remove unwanted signals from affecting the
measurement result.
A first-order resistor-capacitor (RC) filter is (in most cases) sufficient to either totally eliminate aliasing, or to
reduce the effect of aliasing to a level within the noise floor of the sensor. Ideally, any signal beyond fMOD / 2 is
attenuated to a level below the noise floor of the ADC. The digital filter of the ADS122U04 attenuates signals to a
certain degree, as illustrated in the filter response plots in the Digital Filter section. In addition, noise components
are usually smaller in magnitude than the actual sensor signal. Therefore, using a first-order RC filter with a
cutoff frequency set at the output data rate or 10 times higher is generally a good starting point for a system
design.
10.1.3 External Reference and Ratiometric Measurements
The full-scale range (FSR) of the ADS122U04 is defined by the reference voltage and the PGA gain (FSR =
±VREF / Gain). An external reference can be used instead of the integrated 2.048-V reference to adapt the FSR to
the specific system needs. An external reference must be used if VIN is greater than 2.048 V. For example, an
external 5-V reference and an AVDD = 5 V are required in order to measure a single-ended signal that can swing
between 0 V and 5 V.
The reference inputs of the device also allow the implementation of ratiometric measurements. In a ratiometric
measurement the same excitation source that is used to excite the sensor is also used to establish the reference
for the ADC. As an example, a simple form of a ratiometric measurement uses the same current source to excite
both the resistive sensor element (such as an RTD) and another resistive reference element that is in series with
the element being measured. The voltage that develops across the reference element is used as the reference
source for the ADC. These components cancel out in the ADC transfer function because current noise and drift
are common to both the sensor measurement and the reference. The output code is only a ratio of the sensor
element and the value of the reference resistor. The value of the excitation current source itself is not part of the
ADC transfer function.
10.1.4 Establishing Proper Limits on the Absolute Input Voltage
The ADS122U04 can be used to measure various types of input signal configurations: single-ended, pseudodifferential, and fully differential signals (which can be either unipolar or bipolar). However, configuring the device
properly for the respective signal type is important.
Signals where the negative analog input is fixed and referenced to analog ground (VAINN = 0 V) are commonly
called single-ended signals. If the PGA is disabled and bypassed, the absolute input voltages of the ADS122U04
can be as low as 100 mV below AVSS and as large as 100 mV above AVDD. Therefore, the PGA_BYPASS bit
must be set in order to measure single-ended signals when a unipolar analog supply is used (AVSS = 0 V).
Gains of 1, 2, and 4 are still possible in this configuration. Measuring a 0-mA to 20-mA or 4-mA to 20-mA signal
across a load resistor of 100 Ω referenced to GND is a typical example. The ADS122U04 can directly measure
the signal across the load resistor using a unipolar supply, the internal 2.048-V reference, and gain = 1 when the
PGA is bypassed.
If gains larger than 4 are needed to measure a single-ended signal, the PGA must be enabled. In this case, a
bipolar supply is required for the ADS122U04 to meet the absolute input voltage requirement of the PGA.
Signals where the negative analog input (AINN) is fixed at a voltage other the 0 V are referred to as pseudodifferential signals.
Fully differential signals in contrast are defined as signals having a constant common-mode voltage where the
positive and negative analog inputs swing 180° out-of-phase but have the same amplitude.
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ADVANCE INFORMATION
Internal to the device, prior to the PGA inputs, is an EMI filter; see Figure 8. The cutoff frequency of this filter is
approximately 31.8 MHz, which helps reject high-frequency interferences.
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The ADS122U04 can measure pseudo-differential and fully differential signals with the PGA enabled or
bypassed. However, the PGA must be enabled in order to use gains greater than 4. The absolute input voltages
of the input signal must meet the absolute input voltage restrictions of the PGA (as explained in the PGA Input
Voltage Requirements section) when the PGA is enabled. Setting the common-mode voltage at or near (AVSS +
AVDD) / 2 in most cases satisfies the PGA absolute input voltage requirements.
Signals where both the positive and negative inputs are always ≥ 0 V are called unipolar signals. These signals
can in general be measured with the ADS122U04 using a unipolar analog supply (AVSS = 0 V). As mentioned
previously, the PGA must be bypassed in order to measure single-ended, unipolar signals when using a unipolar
supply.
A signal is called bipolar when either the positive or negative input can swing below 0 V. A bipolar analog supply
(such as AVDD = 2.5 V, AVSS = –2.5 V) is required in order to measure bipolar signals with the ADS122U04. A
typical application task is measuring a single-ended, bipolar, ±10-V signal where AINN is fixed at 0 V and AINP
swings between –10 V and 10 V. The ADS122U04 cannot directly measure this signal because the 10 V
exceeds the analog power-supply limits. However, one possible solution is to use a bipolar analog supply (AVDD
= 2.5 V, AVSS = –2.5 V), gain = 1, and a resistor divider in front of the ADS122U04. The resistor divider must
divide the voltage down to ≤ ±2.048 V in order to measure the voltage using the internal 2.048-V reference.
ADVANCE INFORMATION
10.1.5 Unused Inputs and Outputs
To minimize leakage currents on the analog inputs, leave unused analog and reference inputs floating, or
connect the inputs to mid-supply or to AVDD. Connecting unused analog or reference inputs to AVSS is possible
as well, but can yield higher leakage currents on other analog inputs than the previously mentioned options.
Do not float unused digital inputs; excessive power-supply leakage current can result. Tie all unused digital
inputs to the appropriate levels, DVDD or DGND, even when in power-down mode. Connections for unused
digital inputs are:
• Tie the RESET pin to DVDD if the RESET pin is not used
• Leave the GPIO0 or GPIO1 pins configured in the default states as GPIO inputs and tie GPIO0 or GPIO1,
respectively, to either DVDD or DGND if unused
• Leave the GPIO2/DRDY pin configured in the default state as a GPIO input and tie to either DVDD or DGND
if both the DRDY output and GPIO2 are unused
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Application Information (continued)
10.1.6 Pseudo Code Example
The following list shows a pseudo code sequence with the required steps to set up the device and the
microcontroller that interfaces to the ADC in order to take subsequent readings from the ADS122U04 in
continuous conversion mode. The dedicated GPIO2/DRDY pin is used to indicate availability of new conversion
data. The default configuration register settings are changed to gain = 16, continuous conversion mode. This
example shows data collection using manual data read mode.
Power-up;
Delay to allow power supplies to settle and power-up reset to complete; minimum of 50 µs;
Configure the UART interface of the microcontroller to 8-N-1 format;
Configure the microcontroller GPIO connected to the GPIO2/DRDY pin as a falling edge triggered
interrupt input;
Write the respective register configurations with the WREG command, sending the synchronization word
each time (55h, 40h, 08h, 55h, 42h, 08h, 55h, 48h, 48h);
As an optional sanity check, send the synchronization word then read back all configuration registers
with the RREG command (55h, 2xh);
Send the synchronization word to the device (55h);
Send the START/SYNC command (08h) to start converting in continuous conversion mode;
Loop
{
Wait for GPIO2/DRDY to transition low;
Send the synchronization word (55h);
Send the RDATA command (10h);
Receive 3 bytes of data from TX;
}
Send the synchronization word (55h);
Send the POWERDOWN command (02h) to stop conversions and put the device in power-down mode;
TI recommends running an offset calibration before performing any measurements or when changing the gain of
the PGA. The internal offset of the device can, for example, be measured by shorting the inputs to mid-supply
(MUX[3:1] = 1110). The microcontroller then takes multiple readings from the device with the inputs shorted and
stores the average value in the microcontroller memory. When measuring the sensor signal, the microcontroller
then subtracts the stored offset value from each device reading to obtain an offset compensated result; the offset
can be either positive or negative in value.
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Send the synchronization word to the device (55h);
Send the RESET command (06h) to make sure the device is properly reset after power-up;
Delay for a minimum of 50 µs + 32 · tCLK;
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10.2 Typical Applications
10.2.1 K-Type Thermocouple Measurement (–200°C to +1250°C)
Figure 36 shows the basic connections of a thermocouple measurement system when using the internal highprecision temperature sensor for cold-junction compensation. Apart from the thermocouple itself, the only
external circuitry required are two biasing resistors, a simple low-pass, antialiasing filter, and the power-supply
decoupling capacitors.
3.3 V
3.3 V
0.1 F
0.1 F
3.3 V
REFP
RB2
10 A to
1.5 mA
CCM2
RF2
REFN
AVDD
AIN0
DVDD
2.048-V
Reference
Reference MUX
TI Device
CDIF
RF1
TX
AIN1
RX
AINP
Thermocouple
RB1
CCM1
PGA
MUX
Isothermal Block
ADVANCE INFORMATION
AIN2
24-Bit
û ADC
Digital Filter and
UART
Interface
AINN
GPIO0
GPIO1
GPIO2/ DRDY
RESET
Precision
Temperature
Sensor
AIN3
Low-Drift
Oscillator
AVSS
DGND
Copyright © 2017, Texas Instruments Incorporated
Figure 36. Thermocouple Measurement
10.2.1.1 Design Requirements
Table 24. Design Requirements
(1)
DESIGN PARAMETER
VALUE
Supply voltage
3.3 V
Reference voltage
Internal 2.048-V reference
Update rate
≥10 readings per second
Thermocouple type
K
Temperature measurement range
–200°C to +1250°C
Measurement accuracy at TA = 25°C (1)
±0.2°C
Not accounting for error of the thermocouple and cold-junction temperature measurement;
offset calibration at T(TC) = T(CJ) = 25°C; no gain calibration.
10.2.1.2 Detailed Design Procedure
The biasing resistors RB1 and RB2 are used to set the common-mode voltage of the thermocouple such that the
input voltages do not exceed the absolute input voltage range of the PGA (in this example, to mid-supply AVDD /
2). If the application requires the thermocouple to be biased to GND, either a bipolar supply (for example, AVDD
= 2.5 V and AVSS = –2.5 V) must be used for the device to meet the absolute input voltage requirement of the
PGA, or the PGA must be bypassed. When choosing the values of the biasing resistors, care must be taken so
that the biasing current does not degrade measurement accuracy. The biasing current flows through the
thermocouple and can cause self-heating and additional voltage drops across the thermocouple leads. Typical
values for the biasing resistors range from 1 MΩ to 50 MΩ.
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In addition to biasing the thermocouple, RB1 and RB2 are also useful for detecting an open thermocouple lead.
When one of the thermocouple leads fails open, the biasing resistors pull the analog inputs (AIN0 and AIN1) to
AVDD and AVSS, respectively. The ADC consequently reads a full-scale value, which is outside the normal
measurement range of the thermocouple voltage, to indicate this failure condition.
Although the device digital filter attenuates high-frequency components of noise, performance can be further
improved by providing a first-order, passive RC filter at the inputs. Equation 9 calculates the cutoff frequency that
is created by the differential RC filter formed by RF1, RF2, and the differential capacitor CDIF.
fC = 1 / [2π · (RF1 + RF2) · CDIF]
(9)
Two common-mode filter capacitors (CM1 and CM2) are also added to offer attenuation of high-frequency,
common-mode noise components. Choose a differential capacitor CDIF that is at least an order of magnitude (10
times) larger than the common-mode capacitors (CM1 and CM2) because mismatches in the common-mode
capacitors can convert common-mode noise into differential noise.
The filter component values used in this design are: RF1 = RF2 = 1 kΩ, CDIF = 100 nF, and CCM1 = CCM2 = 10 nF.
The highest measurement resolution is achieved when matching the largest potential input signal to the FSR of
the ADC by choosing the highest possible gain. From the design requirement, the maximum thermocouple
voltage occurs at T(TC) = 1250°C and is V(TC) = 50.644 mV as defined in the tables published by the National
Institute of Standards and Technology (NIST) using a cold-junction temperature of T(CJ) = 0°C. A thermocouple
produces an output voltage that is proportional to the temperature difference between the thermocouple tip and
the cold junction. If the cold junction is at a temperature below 0°C, the thermocouple produces a voltage larger
than 50.644 mV. The isothermal block area is constrained by the operating temperature range of the device.
Therefore, the isothermal block temperature is limited to –40°C. A K-type thermocouple at T(TC) = 1250°C
produces an output voltage of V(TC) = 50.644 mV – (–1.527 mV) = 52.171 mV when referenced to a cold-junction
temperature of T(CJ) = –40°C. The maximum gain that can be applied when using the internal 2.048-V reference
is then calculated as (2.048 V / 52.171 mV) = 39.3. The next smaller PGA gain setting that the device offers is
32.
The device integrates a high-precision temperature sensor that can be used to measure the temperature of the
cold junction. To measure the internal temperature of the ADS122U04, the device must be set to internal
temperature sensor mode by setting the TS bit to 1 in the configuration register. For best performance, careful
board layout is critical to achieve good thermal conductivity between the cold junction and the device package.
However, the device does not perform automatic cold-junction compensation of the thermocouple. This
compensation must be done in the microcontroller that interfaces to the device. The microcontroller requests one
or multiple readings of the thermocouple voltage from the device and then sets the device to internal temperature
sensor mode (TS = 1) to acquire the temperature of the cold junction. An algorithm similar to the following must
be implemented on the microcontroller to compensate for the cold-junction temperature:
1. Measure the thermocouple voltage, V(TC), between AIN0 and AIN1
2. Measure the temperature of the cold junction, T(CJ), using the temperature sensor mode of the ADS122U04
3. Convert the cold-junction temperature into an equivalent thermoelectric voltage, V(CJ), using the tables or
equations provided by NIST
4. Add V(TC) and V(CJ) and translate the summation back into a thermocouple temperature using the NIST tables
or equations again
In some applications, the integrated temperature sensor of the ADS122U04 cannot be used (for example, if the
accuracy is not high enough or if the device cannot be placed close enough to the cold junction). The additional
analog input channels of the device can be used in this case to measure the cold-junction temperature with a
thermistor, RTD, or an analog temperature sensor.
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The filter resistors RF1 and RF2 also serve as current-limiting resistors. These resistors limit the current into the
analog inputs (AIN0 and AIN1) of the device to safe levels if an overvoltage on the inputs occur. Care must be
taken when choosing the filter resistor values because the input currents flowing into and out of the device cause
a voltage drop across the resistors. This voltage drop shows up as an additional offset error at the ADC inputs.
TI therefore recommends limiting the filter resistor values to below 1 kΩ.
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As shown in Equation 10, the rms noise of the ADS122U04 at gain = 32 and DR = 20 SPS (0.23 µVrms) is
divided by the average sensitivity of a K-type thermocouple (41 µV/°C) to obtain an approximation of the
achievable temperature resolution.
Temperature Resolution = 0.23 µV / 41 µV/°C = 0.006°C
(10)
Table 25 shows the register settings for this design.
Table 25. Register Settings
REGISTER
SETTING
00h
0Ah
AINP = AIN0, AINN = AIN1, gain = 32, PGA enabled
DESCRIPTION
01h
08h
DR = 20 SPS, normal mode, continuous conversion mode
02h
00h
Data counter disabled, data integrity disabled, burnout current
sources disabled, IDACs off
03h
00h
No IDACs used, manual data read mode
04h
48h
GPIO2/DRDY pin configured as a DRDY output
10.2.1.3 Application Curves
The design meets the required temperature measurement accuracy given in Table 24. The measurement error
shown in Figure 38 does not include the error of the thermocouple itself nor the measurement error of the coldjunction temperature. Those two error sources are in general larger than 0.2°C and therefore, in many cases,
dominate the overall system measurement accuracy.
0.01
0.2
0.005
0.1
Measurement Error (°C)
Measurement Error (mV)
ADVANCE INFORMATION
Figure 37 and Figure 38 show the measurement results. The measurements are taken at TA = T(CJ) = 25°C. A
system offset calibration is performed at T(TC) = 25°C, which translates to a V(TC) = 0 V when T(CJ) = 25°C. No
gain calibration is implemented. The data in Figure 37 are taken using a precision voltage source as the input
signal instead of a thermocouple. The respective temperature measurement error in Figure 38 is calculated from
the data in Figure 37 using the NIST tables.
0
-0.005
-0.01
-10
0
10
20
30
Thermocouple Voltage (mV)
40
-0.1
-0.2
-200
D002
Figure 37. Voltage Measurement Error vs V(TC)
48
50
0
0
200
400
600
Temperature (°C)
800
1000
1200
D001
Figure 38. Temperature Measurement Error vs T(TC)
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10.2.2 3-Wire RTD Measurement (–200°C to +850°C)
The ADS122U04 integrates all necessary features (such as dual-matched programmable current sources,
buffered reference inputs, and a PGA) to ease the implementation of ratiometric 2-, 3-, and 4-wire RTD
measurements. Figure 39 shows a typical implementation of a ratiometric 3-wire RTD measurement using the
excitation current sources integrated in the device to excite the RTD as well as to implement automatic RTD
lead-resistance compensation.
RREF
IIDAC1 + IIDAC2
3.3 V
RF3
CCM3
0.1 µF
10 A to
1.5 mA
RLEAD3
RLEAD2
RF2
REFN
REFP
3.3 V
CCM4
0.1 µF
DVDD
CCM2
AIN0
2.048-V
Reference
Reference
MUX
TI Device
CDIF1
RLEAD1
RF1
TX
AIN1
AINP
CCM1
AIN2
24-Bit
û ADC
PGA
MUX
Digital Filter
and
UART
Interface
AINN
AIN3
GPIO0
GPIO1
GPIO2/DRDY
(IDAC1)
(IDAC2)
RX
ADVANCE INFORMATION
3-Wire RTD
AVDD
RF4
CDIF2
RESET
Precision
Temperature
Sensor
Low-Drift
Oscillator
AVSS
DGND
Copyright © 2017, Texas Instruments Incorporated
Figure 39. 3-Wire RTD Measurement
10.2.2.1 Design Requirements
Table 26. Design Requirements
DESIGN PARAMETER
(1)
VALUE
Supply voltage
3.3 V
Update rate
20 readings per second
RTD type
3-wire Pt100
Maximum RTD lead resistance
15 Ω
RTD excitation current
500 µA
Temperature measurement range
–200°C to +850°C
Measurement accuracy at TA = 25°C (1)
±0.2°C
Not accounting for error of RTD;
offset calibration is performed with RRTD = 100 Ω; no gain calibration.
10.2.2.2 Detailed Design Procedure
The circuit in Figure 39 employs a ratiometric measurement approach. In other words, the sensor signal (that is,
the voltage across the RTD in this case) and the reference voltage for the ADC are derived from the same
excitation source. Therefore, errors resulting from temperature drift or noise of the excitation source cancel out
because these errors are common to both the sensor signal and the reference.
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In order to implement a ratiometric 3-wire RTD measurement using the device, IDAC1 is routed to one of the
leads of the RTD and IDAC2 is routed to the second RTD lead. Both currents have the same value, which is
programmable by the IDAC[2:0] bits in the configuration register. The design of the device ensures that both
IDAC values are closely matched, even across temperature. The sum of both currents flows through a precision,
low-drift reference resistor, RREF. The voltage, VREF, generated across the reference resistor (as shown in
Equation 11) is used as the ADC reference voltage. Equation 11 reduces to Equation 12 because IIDAC1 = IIDAC2.
VREF = (IIDAC1 + IIDAC2) · RREF
VREF = 2 · IIDAC1 · RREF
(11)
(12)
To simplify the following discussion, the individual lead resistance values of the RTD (RLEADx) are set to zero. As
Equation 13 shows, only IDAC1 excites the RTD to produce a voltage (VRTD) proportional to the temperaturedependable RTD value and the IDAC1 value.
VRTD = RRTD (at temperature) · IIDAC1
(13)
The device internally amplifies the voltage across the RTD using the PGA and compares the resulting voltage
against the reference voltage to produce a digital output code proportional to Equation 14 through Equation 16:
Code ∝ VRTD · Gain / VREF
Code ∝ (RRTD (at temperature) · IIDAC1 · Gain) / (2 · IIDAC1 · RREF)
Code ∝ (RRTD (at temperature) · Gain) / (2 · RREF)
(14)
(15)
(16)
ADVANCE INFORMATION
As shown in Equation 16, the output code only depends on the value of the RTD, the PGA gain, and the
reference resistor (RREF), but not on the IDAC1 value. The absolute accuracy and temperature drift of the
excitation current therefore does not matter. However, because the value of the reference resistor directly affects
the measurement result, choosing a reference resistor with a very low temperature coefficient is important to limit
errors introduced by the temperature drift of RREF.
The second IDAC2 is used to compensate for errors introduced by the voltage drop across the lead resistance of
the RTD. All three leads of a 3-wire RTD typically have the same length and, thus, the same lead resistance.
Also, IDAC1 and IDAC2 have the same value. Taking the lead resistance into account, use Equation 17 to
calculate the differential voltage (VIN) across the ADC inputs (AIN0 and AIN1):
VIN = IIDAC1 · (RRTD + RLEAD1) – IIDAC2 · RLEAD2
(17)
Equation 17 reduces to Equation 18 when RLEAD1 = RLEAD2 and IIDAC1 = IIDAC2:
VIN = IIDAC1 · RRTD
(18)
In other words, the measurement error resulting from the voltage drop across the RTD lead resistance is
compensated, as long as the lead resistance values and the IDAC values are well matched.
A first-order differential and common-mode RC filter (RF1, RF2, CDIF1, CCM1, and CCM2) is placed on the ADC
inputs, as well as on the reference inputs (RF3, RF4, CDIF2, CCM3, and CCM4). The same guidelines for designing
the input filter apply as described in the K-Type Thermocouple Measurement section. Match the corner
frequencies of the input and reference filter for best performance. For more detailed information on matching the
input and reference filter, see the RTD Ratiometric Measurements and Filtering Using the ADS1148 and
ADS1248 application report.
The reference resistor RREF not only serves to generate the reference voltage for the device, but also sets the
voltages at the leads of the RTD to within the specified absolute input voltage range of the PGA.
When designing the circuit, care must also be taken to meet the compliance voltage requirement of the IDACs.
The IDACs require that the maximum voltage drop developed across the current path to AVSS be equal to or
less than AVDD – 0.9 V in order to operate accurately. This requirement means that Equation 19 must be met at
all times.
AVSS + IIDAC1 · (RLEAD1 + RRTD) + (IIDAC1 + IIDAC2) · (RLEAD3 + RREF) ≤ AVDD – 0.9 V
(19)
The device also offers the possibility to route the IDACs to the same inputs used for measurement. If the filter
resistor values RF1 and RF2 in Figure 39 are small enough and well matched, then IDAC1 can be routed to AIN1
and IDAC2 to AIN0. In this manner, even two 3-wire RTDs sharing the same reference resistor can be measured
with a single device.
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As stated in Table 26, this design example discusses the implementation of a 3-wire Pt100 measurement to be
used to measure temperatures ranging from –200°C to +850°C. The excitation current for the Pt100 is chosen as
IIDAC1 = 500 µA, which means a combined current of 1 mA is flowing through the reference resistor, RREF. As
mentioned previously, besides creating the reference voltage for the ADS122U04, the voltage across RREF also
sets the absolute input voltages for the RTD measurement. In general, choose the largest reference voltage
possible that maintains the compliance voltage of the IDACs and meets the absolute input voltage requirement of
the PGA. Setting the common-mode voltage at or near half the analog supply (in this case 3.3 V / 2 = 1.65 V) in
most cases satisfies the absolute input voltage requirements of the PGA. Equation 20 is then used to calculate
the value for RREF:
RREF = VREF / (IIDAC1 + IIDAC2) = 1.65 V / 1 mA = 1.65 kΩ
(20)
The stability of RREF is critical to achieve good measurement accuracy over temperature and time. Choosing a
reference resistor with a temperature coefficient of ±10 ppm/°C or better is advisable. If a 1.65-kΩ value is not
readily available, another value near 1.65 kΩ (such as 1.62 kΩ or 1.69 kΩ) can certainly be used as well.
As a last step, the PGA gain must be selected in order to match the maximum input signal to the FSR of the
ADC. The resistance of a Pt100 increases with temperature. Therefore, the maximum voltage to be measured
(VINMAX) occurs at the positive temperature extreme. At 850°C, a Pt100 has an equivalent resistance of
approximately 391 Ω as per the NIST tables. The voltage across the Pt100 equates to Equation 21:
(21)
The maximum gain that can be applied when using a 1.65-V reference is then calculated as (1.65 V / 195.5 mV)
= 8.4. The next smaller PGA gain setting available in the ADS122U04 is 8. At a gain of 8, the ADS122U04 offers
a FSR value as described in Equation 22:
FSR = ±VREF / Gain = ±1.65 V / 8 = ±206.25 mV
(22)
This range allows for margin with respect to initial accuracy and drift of the IDACs and reference resistor.
After selecting the values for the IDACs, RREF, and PGA gain, make sure to double check that the settings meet
the absolute input voltage requirements of the PGA and the compliance voltage of the IDACs. To determine the
true absolute input voltages at the ADC inputs (AIN0 and AIN1), the lead resistance must be taken into account
as well.
The smallest absolute input voltage occurs on AIN0 at the lowest measurement temperature (–200°C) with
RLEADx = 0 Ω, and is equal to VREF = 1.65 V.
The minimum absolute input voltage must not exceed the limit set in Equation 7 to meet Equation 23:
VAIN0
(MIN)
≥ AVSS + 0.2 V + |VINMAX| · (Gain – 4) / 8 = 0 V + 0.2 V + 97.75 mV = 297.75 mV
(23)
The restriction is satisfied with VAIN0 = 1.65 V.
The largest absolute input voltage (calculated using Equation 24 and Equation 25) occurs on AIN1 at the highest
measurement temperature (850°C).
VAIN1 (MAX) = VREF + (IIDAC1 + IIDAC2) · RLEAD3 + IIDAC1 · (RLEAD1 + RRTD (at 850°C))
VAIN1 (MAX) = 1.65 V + 1 mA · 15 Ω + 500 µA · (15 Ω + 391 Ω) = 1.868 V
VAIN1
(24)
(25)
(MAX)
meets the requirement given by Equation 7 and equates to Equation 26 in this design:
VAINP
(MAX)
≤ AVDD – 0.2 V – |VINMAX| · (Gain – 4) / 8 = 3.3 V – 0.2 V – 97.75 mV = 3.002 V
(26)
The restriction on the compliance voltage (AVDD – 0.9 V = 3.3 V – 0.9 V = 2.4 V) of IDAC1 is met as well.
Table 27 shows the register settings for this design.
Table 27. Register Settings
REGISTER
SETTING
00h
36h
AINP = AIN1, AINN = AIN0, gain = 8, PGA enabled
DESCRIPTION
01h
0Ah
DR = 20 SPS, normal mode, continuous conversion mode, external reference
02h
55h
Data counter disabled, data integrity disabled, burnout current sources disabled, IDAC = 500 µA
03h
70h
IDAC1 = AIN2, IDAC2 = AIN3, manual data read mode
04h
48h
GPIO2/DRDY pin configured as a DRDY output
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ADVANCE INFORMATION
VINMAX = VRTD (at 850°C) = RRTD (at 850°C) · IIDAC1 = 391 Ω · 500 µA = 195.5 mV
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10.2.2.2.1 Design Variations for 2-Wire and 4-Wire RTD Measurements
Implementing a 2- or 4-wire RTD measurement is very similar to the 3-wire RTD measurement illustrated in
Figure 39, except that only one IDAC is required.
Figure 40 shows a typical circuit implementation of a 2-wire RTD measurement. The main difference compared
to a 3-wire RTD measurement is with respect to the lead resistance compensation. The voltage drop across the
lead resistors, RLEAD1 and RLEAD2, in this configuration is directly part of the measurement (as shown in
Equation 27) because there is no means to compensate the lead resistance by use of the second current source.
Any compensation must be done by calibration.
VIN = IIDAC1 · (RLEAD1 + RRTD + RLEAD2)
(27)
RREF
IIDAC1
3.3 V
RF3
CCM3
0.1 µF
10 A to
1.5 mA
RLEAD2
AVDD
3.3 V
RF4
CDIF2
CCM4
REFP
0.1 µF
REFN
DVDD
CCM2
RF2
AIN0
2-Wire RTD
2.048-V
Reference
Reference
MUX
TI Device
CDIF1
RLEAD1
TX
RF1
AIN1
ADVANCE INFORMATION
RX
AINP
CCM1
AIN2
Digital Filter and
UART
Interface
24-Bit
û ADC
PGA
MUX
GPIO0
GPIO1
AINN
GPIO2/ DRDY
RESET
Precision
Temperature
Sensor
AIN3
(IDAC1)
Low-Drift
Oscillator
DGND
AVSS
Copyright © 2017, Texas Instruments Incorporated
Figure 40. 2-Wire RTD Measurement
Figure 41 shows a typical circuit implementation of a 4-wire RTD measurement. Similar to the 2-wire RTD
measurement, only one IDAC is required for exciting and measuring a 4-wire RTD in a ratiometric manner. The
main benefit of using a 4-wire RTD is that the ADC inputs are connected to the RTD in the form of a Kelvin
connection. Apart from the input leakage currents of the ADC, there is no current flow through the lead resistors
RLEAD2 and RLEAD3 and therefore no voltage drop is created across them. The voltage at the ADC inputs
consequently equals the voltage across the RTD and the lead resistance is of no concern.
RREF
IIDAC1
3.3 V
RF3
CCM3
RF4
CDIF2
3.3 V
CCM4
0.1 F
0.1 F
10 A to
1.5 mA
RLEAD4
RLEAD3
RF2
4-Wire RTD
AVDD
REFP
REFN
DVDD
CCM2
AIN0
2.048-V
Reference
Reference MUX
TI Device
CDIF1
RLEAD2
RF1
TX
AIN1
RX
AINP
CCM1
PGA
MUX
RLEAD1
AIN2
24-Bit
û ADC
Digital Filter and
UART
Interface
AINN
GPIO0
GPIO1
GPIO2/DRDY
RESET
AIN3
Precision
Temperature
Sensor
(IDAC1)
Low-Drift
Oscillator
AVSS
DGND
Copyright © 2017, Texas Instruments Incorporated
Figure 41. 4-Wire RTD Measurement
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As shown in Equation 28, the transfer function of a 2- and 4-wire RTD measurement differs compared to the one
of a 3-wire RTD measurement by a factor of 2 because only one IDAC is used and only one IDAC flows through
the reference resistor, RREF.
Code ∝ (RRTD (at Temperature) · Gain) / RREF
(28)
In addition, the input common-mode voltage and reference voltage is reduced compared to the 3-wire RTD
configuration. Therefore, some further modifications may be required in case the 3-wire RTD design is used to
measure 2- and 4-wire RTDs as well. If the decreased absolute input voltages does not meet the minimum
absolute voltage requirements of the PGA anymore, either increase the value of RREF by switching in a larger
resistor or, alternatively, increase the excitation current and decrease the gain at the same time.
10.2.2.3 Application Curves
Figure 42 and Figure 43 show the measurement results. The measurements are taken at TA = 25°C. A system
offset calibration is performed using a reference resistor of 100 Ω. No gain calibration is implemented. The data
in Figure 42 are taken using precision resistors instead of a 3-wire Pt100. The respective temperature
measurement error in Figure 43 is calculated from the data in Figure 42 using the NIST tables.
0.2
0.05
0.1
0
-0.05
-0.1
0
50
100
150
200
250
RTD Value (:)
300
350
400
0
-0.1
-0.2
-200
D003
Figure 42. Resistance Measurement Error vs RRTD
ADVANCE INFORMATION
0.1
Measurement Error (°C)
Measurement Error (:)
The design meets the required temperature measurement accuracy given in Table 26. However, the
measurement error shown in Figure 43 does not include the error of the RTD itself.
0
200
400
600
Temperature (°C)
800
1000
D004
Figure 43. Temperature Measurement Error vs T(RTD)
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10.2.3 Resistive Bridge Measurement
The device offers several features to ease the implementation of ratiometric bridge measurements (such as a
PGA with gains up to 128, buffered, and differential reference inputs).
5.0 V
3.3 V
5.0 V
CDIF2
0.1
10 A to
1.5 mA
F
REFP
AVDD
AIN0
0.1
2.048-V
Reference
5.0 V
REFN
Reference MUX
F
DVDD
TI Device
CCM2
TX
RF2
AIN1
RX
AINP
CDIF1
PGA
MUX
RF1
AIN2
24-Bit
û ADC
Digital Filter and
UART
Interface
AINN
GPIO0
GPIO1
GPIO2/ DRDY
RESET
CCM1
AIN3
ADVANCE INFORMATION
Precision
Temperature
Sensor
Low-Drift
Oscillator
AVSS
DGND
Copyright © 2017, Texas Instruments Incorporated
Figure 44. Resistive Bridge Measurement
10.2.3.1 Design Requirements
Table 28. Design Requirements
DESIGN PARAMETER
VALUE
Analog supply voltage
5.0 V
Digital supply voltage
3.3 V
Load cell type
4-wire load cell
Load cell maximum capacity
1 kg
Load cell sensitivity
3 mV/V
Excitation voltage
5V
Repeatability
50 mg
10.2.3.2 Detailed Design Procedure
As shown in Figure 44, the bridge excitation voltage is simultaneously used as the reference voltage for the ADC
to implement a ratiometric bridge measurement. With this configuration, any drift in excitation voltage also shows
up on the reference voltage, consequently canceling out drift error. Either the dedicated reference inputs can be
used, or the analog supply can be used as the reference if the supply is used to excite the bridge.
The PGA offers gains up to 128, which helps amplify the small differential bridge output signal to make optimal
use of the ADC full-scale range. Using a symmetrical bridge with the excitation voltage equal to the supply
voltage of the device ensures that the output signal of the bridge meets the absolute input voltage requirement of
the PGA.
Using a 3-mV/V load cell with a 5-V excitation yields a maximum differential voltage at the ADC inputs of VINMAX
= 15 mV at maximum load. Equation 29 then calculates the maximum gain that can be used.
Gain ≤ VREF / VINMAX = 5 V / 15 mV = 333.3
(29)
Accordingly Gain = 128 is used in this example.
54
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A first-order differential and common-mode RC filter (RF1, RF2, CDIF1, CCM1, and CCM2) is placed on the ADC
inputs. The reference has an additional capacitor CDIF2 to limit reference noise. Care must be taken to maintain a
limited amount of filtering or the measurement is no longer ratiometric.
To find the repeatability of the readings, perform the following calculation. The load cell produces an output
voltage of 15 mV at the maximum load of 1 kg. At a Gain = 128 and DR = 20 SPS the ADS122U04 offers a
noise-free resolution of 0.41 µVPP. Equation 30 then calculates the repeatability.
Repeatability = (1 kg / 15 mV) · 0.41 µV = 27 mg
(30)
Table 29 shows the register settings for this design.
Table 29. Register Settings
SETTING
DESCRIPTION
00h
4Eh
AINP = AIN1, AINN = AIN2, gain = 128, PGA enabled
01h
0Ch
DR = 20 SPS, normal mode, continuous conversion mode, analog
supply as reference
02h
98h
Data counter disabled, data integrity disabled, burnout current
sources disabled, IDACs off
03h
00h
No IDACs used, manual data read mode
04h
48h
GPIO2/DRDY pin configured as a DRDY output
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REGISTER
55
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11 Power Supply Recommendations
The device requires two power supplies: analog (AVDD, AVSS) and digital (DVDD, DGND). The analog power
supply can be bipolar (for example, AVDD = 2.5 V, AVSS = –2.5 V) or unipolar (for example, AVDD = 3.3 V,
AVSS = 0 V) and is independent of the digital power supply. The digital supply sets the digital I/O levels.
11.1 Power-Supply Sequencing
The power supplies can be sequenced in any order, but in no case must any analog or digital inputs exceed the
respective analog or digital power-supply voltage and current limits. Wait approximately 50 µs after all power
supplies are stabilized before communicating with the device to allow the power-up reset process to complete.
11.2 Power-Supply Ramp Rate
As shown in Figure 45, the power-supply ramp rate must be monotonic and slower than 1 V per 50 µs for proper
device power-up over the entire temperature range.
DVDD
ADVANCE INFORMATION
1V
-50 s-
Figure 45. Power-Supply Ramp Rate
11.3 Power-Supply Decoupling
Good power-supply decoupling is important to achieve optimum performance. As shown in Figure 46 and
Figure 47, AVDD, AVSS (when using a bipolar supply), and DVDD must be decoupled with at least a 0.1-µF
capacitor. Place the bypass capacitors as close to the power-supply pins of the device as possible using lowimpedance connections. TI recommends using multi-layer ceramic chip capacitors (MLCCs) that offer low
equivalent series resistance (ESR) and inductance (ESL) characteristics for power-supply decoupling purposes.
For very sensitive systems, or for systems in harsh noise environments, avoiding the use of vias for connecting
the capacitors to the device pins may offer superior noise immunity. The use of multiple vias in parallel lowers
the overall inductance and is beneficial for connections to ground planes. Connect analog and digital grounds
together as close to the device as possible.
1
GPIO1
RX
16
1
GPIO1
RX
16
2
GPIO0
TX
15
2
GPIO0
TX
15
3
RESET
GPIO2/DRDY
14
3
RESET
GPIO2/ DRDY
14
4
DGND
DVDD
13
4
DGND
DVDD
13
5
AVSS
AVDD
12
5
AVSS
AVDD
12
6
AIN3
AIN0
11
6
AIN3
AIN0
11
7
AIN2
AIN1
10
7
AIN2
AIN1
10
8
REFN
REFP
9
8
REFN
REFP
9
3.3 V
Device
Device
3.3 V
0.1 PF
-2.5 V
0.1 µF
0.1 PF
Figure 46. Unipolar Analog Power Supply
56
3.3 V
2.5 V
0.1 µF
0.1 µF
Figure 47. Bipolar Analog Power Supply
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12 Layout
12.1 Layout Guidelines
Use best design practices when laying out a printed circuit board (PCB) for both analog and digital components.
This recommendation generally means that the layout separates analog components [such as ADCs, amplifiers,
references, digital-to-analog converters (DACs), and analog MUXs] from digital components [such as
microcontrollers, complex programmable logic devices (CPLDs), field-programmable gate arrays (FPGAs), radio
frequency (RF) transceivers, universal serial bus (USB) transceivers, and switching regulators]. Figure 48 shows
an example of good component placement. Although Figure 48 provides a good example of component
placement, the best placement for each application is unique to the geometries, components, and PCB
fabrication capabilities employed. That is, there is no single layout that is perfect for every design and careful
consideration must always be used when designing with any analog component.
Interface
Transceiver
Connector
or Antenna
Ground Fill or
Ground Plane
Figure 48. System Component Placement
The use of split analog and digital ground planes is not necessary for improved noise performance (although for
thermal isolation this option is a worthwhile consideration). However, the use of a solid ground plane or ground
fill in PCB areas with no components is essential for optimum performance. If the system being used employs a
split digital and analog ground plane, TI generally recommends that the ground planes be connected together as
close to the device as possible. A two-layer board is possible using common grounds for both analog and digital
grounds. Additional layers can be added to simplify PCB trace routing. Ground fill can also reduce EMI and RFI
issues.
TI also strongly recommends that digital components, especially RF portions, be kept as far as practically
possible from analog circuitry in a given system. Additionally, minimize the distance that digital control traces run
through analog areas and avoid placing these traces near sensitive analog components. Digital return currents
usually flow through a ground path that is as close to the digital path as possible. If a solid ground connection to
a plane is not available, these currents may find paths back to the source that interfere with analog performance.
The implications that layout has on the temperature-sensing functions are much more significant than for ADC
functions.
Supply pins must be bypassed to ground with a low-ESR ceramic capacitor. The optimum placement of the
bypass capacitors is as close as possible to the supply pins. If AVSS is connected to a negative supply, then
connect an additional bypass capacitor from AVSS to AGND as well. The ground-side connections of the bypass
capacitors must be low-impedance connections for optimum performance. The supply current flows through the
bypass capacitor terminal first and then to the supply pin to make the bypassing most effective.
Analog inputs with differential connections must have a capacitor placed differentially across the inputs. Best
input combinations for differential measurements are AIN0, AIN1 and AIN2, AIN3. The differential capacitors
must be of high quality. The best ceramic chip capacitors are C0G (NPO) types that have stable properties and
low noise characteristics. Thermally isolate a copper region around the thermocouple input connections to create
a thermally-stable cold junction. Obtaining acceptable performance with alternate layout schemes is possible as
long as the above guidelines are followed.
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ADVANCE INFORMATION
Ground Fill or
Ground Plane
Supply
Generation
Microcontroller
Device
Optional: Split
Ground Cut
Signal
Conditioning
(RC Filters
and
Amplifiers)
Ground Fill or
Ground Plane
Optional: Split
Ground Cut
Ground Fill or
Ground Plane
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Vias connect to either the bottom layer or
an internal plane. The bottom layer or
internal plane are dedicated GND planes
(GND = DGND = AVSS).
REFN
REFP
12.2 Layout Example
AIN1
AIN2
AIN0
ADVANCE INFORMATION
AVDD
9: REFP
8: REFN
10: AIN1
7: AIN2
11: AIN0
6: AIN3
12: AVDD
5: AVSS
13: DVDD
4: DGND
14: GPIO2/DRDY
3: RESET
15: TX
2: GPIO0
16: RX
1: GPIO1
AIN3
RESET
GPIO0
GPIO1
RX
TX
GPIO2/DRDY
DVDD
Figure 49. Layout Example
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13 Device and Documentation Support
13.1 Documentation Support
13.1.1 Related Documentation
For related documentation see the following:
• REF50xx Low-Noise, Very Low Drift, Precision Voltage Reference
• RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248 Application Report
13.2 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper
right corner, click on Alert me to register and receive a weekly digest of any product information that has
changed. For change details, review the revision history included in any revised document.
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
13.4 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
13.5 Electrostatic Discharge Caution
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
13.6 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
14 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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ADVANCE INFORMATION
13.3 Community Resources
PACKAGE OPTION ADDENDUM
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30-May-2017
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
Lead/Ball Finish
MSL Peak Temp
(2)
(6)
(3)
Op Temp (°C)
Device Marking
(4/5)
ADS122U04IPW
PREVIEW
TSSOP
PW
16
90
TBD
Call TI
Call TI
-40 to 125
ADS122U
ADS122U04IPWR
PREVIEW
TSSOP
PW
16
2000
TBD
Call TI
Call TI
-40 to 125
ADS122U
ADS122U04IRTER
PREVIEW
WQFN
RTE
16
3000
TBD
Call TI
Call TI
-40 to 125
ADS122U04IRTET
PREVIEW
WQFN
RTE
16
250
TBD
Call TI
Call TI
-40 to 125
PADS122U04IPWR
ACTIVE
TSSOP
PW
16
2000
TBD
Call TI
Call TI
-40 to 125
122U4
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based
flame retardants must also meet the <=1000ppm threshold requirement.
(3)
MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4)
There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5)
Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
(6)
Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish
value exceeds the maximum column width.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
Addendum-Page 1
Samples
PACKAGE OPTION ADDENDUM
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30-May-2017
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
Addendum-Page 2
IMPORTANT NOTICE
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remain responsible for using their independent analysis, evaluation and judgment in designing their applications and that Designers have
full and exclusive responsibility to assure the safety of Designers' applications and compliance of their applications (and of all TI products
used in or for Designers’ applications) with all applicable regulations, laws and other applicable requirements. Designer represents that, with
respect to their applications, Designer has all the necessary expertise to create and implement safeguards that (1) anticipate dangerous
consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures that might cause harm and
take appropriate actions. Designer agrees that prior to using or distributing any applications that include TI products, Designer will
thoroughly test such applications and the functionality of such TI products as used in such applications.
TI’s provision of technical, application or other design advice, quality characterization, reliability data or other services or information,
including, but not limited to, reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to
assist designers who are developing applications that incorporate TI products; by downloading, accessing or using TI Resources in any
way, Designer (individually or, if Designer is acting on behalf of a company, Designer’s company) agrees to use any particular TI Resource
solely for this purpose and subject to the terms of this Notice.
TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TI
products, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,
enhancements, improvements and other changes to its TI Resources. TI has not conducted any testing other than that specifically
described in the published documentation for a particular TI Resource.
Designer is authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that
include the TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE
TO ANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTY
RIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
regarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty or
endorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES OR
REPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TO
ACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL
PROPERTY RIGHTS. TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY DESIGNER AGAINST ANY CLAIM,
INCLUDING BUT NOT LIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF
PRODUCTS EVEN IF DESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL,
DIRECT, SPECIAL, COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN
CONNECTION WITH OR ARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
Unless TI has explicitly designated an individual product as meeting the requirements of a particular industry standard (e.g., ISO/TS 16949
and ISO 26262), TI is not responsible for any failure to meet such industry standard requirements.
Where TI specifically promotes products as facilitating functional safety or as compliant with industry functional safety standards, such
products are intended to help enable customers to design and create their own applications that meet applicable functional safety standards
and requirements. Using products in an application does not by itself establish any safety features in the application. Designers must
ensure compliance with safety-related requirements and standards applicable to their applications. Designer may not use any TI products in
life-critical medical equipment unless authorized officers of the parties have executed a special contract specifically governing such use.
Life-critical medical equipment is medical equipment where failure of such equipment would cause serious bodily injury or death (e.g., life
support, pacemakers, defibrillators, heart pumps, neurostimulators, and implantables). Such equipment includes, without limitation, all
medical devices identified by the U.S. Food and Drug Administration as Class III devices and equivalent classifications outside the U.S.
TI may expressly designate certain products as completing a particular qualification (e.g., Q100, Military Grade, or Enhanced Product).
Designers agree that it has the necessary expertise to select the product with the appropriate qualification designation for their applications
and that proper product selection is at Designers’ own risk. Designers are solely responsible for compliance with all legal and regulatory
requirements in connection with such selection.
Designer will fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of Designer’s noncompliance with the terms and provisions of this Notice.
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