LINER LTC2481IDD 16-bit î î£ adc with easy drive input current cancellation and i2c interface Datasheet

LTC2481
16-Bit ∆Σ ADC with Easy Drive
Input Current Cancellation and I2C Interface
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FEATURES
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Easy Drive Technology Enables Rail-to-Rail Inputs
with Zero Differential Input Current
Directly Digitizes High Impedance Sensors with
Full Accuracy
Programmable Gain from 1 to 256
Integrated Temperature Sensor
GND to VCC Input/Reference Common Mode Range
2-Wire I2C Interface
Programmable 50Hz, 60Hz or Simultaneous
50Hz/60Hz Rejection Mode
2ppm (0.25LSB) INL, No Missing Codes
1ppm Offset and 15ppm Full-Scale Error
Selectable 2x Speed Mode
No Latency: Digital Filter Settles in a Single Cycle
Single Supply 2.7V to 5.5V Operation
Internal Oscillator
Six Addresses Available and One Global Address for
Synchronization
Available in a Tiny (3mm × 3mm) 10-Lead
DFN Package
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APPLICATIO S
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Direct Sensor Digitizer
Weight Scales
Direct Temperature Measurement
Strain Gauge Transducers
Instrumentation
Industrial Process Control
DVMs and Meters
The LTC2481 includes on-chip programmable gain, a
temperature sensor and an oscillator. The LTC2481 can be
configured through an I2C interface to provide a programmable gain from 1 to 256 in 8 steps, to digitize an external
signal or internal temperature sensor, reject line frequencies (50Hz, 60Hz or simultaneous 50Hz/60Hz) as well as
a 2x speed-up mode.
The LTC2481 allows a wide common mode input range
(0V to VCC) independent of the reference voltage. The
reference can be as low as 100mV or can be tied directly
to VCC. The LTC2481 includes an on-chip trimmed oscillator eliminating the need for external crystals or oscillators. Absolute accuracy and low drift are automatically
maintained through continuous, transparent, offset and
full-scale calibration.
, LTC and LT are registered trademarks of Linear Technology Corporation.
No Latency ∆Σ and Easy Drive are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Patent Pending.
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The LTC®2481 combines a 16-bit plus sign No Latency ∆ΣTM
analog-to-digital converter with patented Easy DriveTM technology and I2C digital interface. The patented sampling
scheme eliminates dynamic input current errors and the
shortcomings of on-chip buffering through automatic
cancellation of differential input current. This allows large
external source impedances and input signals, with rail-torail input range to be directly digitized while maintaining
exceptional DC accuracy.
TYPICAL APPLICATIO
+FS Error vs RSOURCE at IN+ and IN–
VCC
1µF
10k
IDIFF = 0
VIN+
1µF
SENSE
REF+
SCL
SDA
LTC2481
VIN–
10k
VCC
GND
REF–
CA0/F0
CA1
2481 TA01
2-WIRE
I2C INTERFACE
6 ADDRESSES
+FS ERROR (ppm)
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DESCRIPTIO
80
VCC = 5V
= 5V
60 VREF
VIN+ = 3.75V
–
40 VIN = 1.25V
FO = GND
20 TA = 25°C
CIN = 1µF
0
–20
–40
–60
–80
1
10
100
1k
RSOURCE (Ω)
10k
100k
2481 TA04
2481f
1
LTC2481
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ABSOLUTE
RATI GS
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PACKAGE/ORDER I FOR ATIO
(Notes 1, 2)
Supply Voltage (VCC) to GND...................... – 0.3V to 6V
Analog Input Voltage to GND ....... – 0.3V to (VCC + 0.3V)
Reference Input Voltage to GND .. – 0.3V to (VCC + 0.3V)
Digital Input Voltage to GND ........ – 0.3V to (VCC + 0.3V)
Digital Output Voltage to GND ..... – 0.3V to (VCC + 0.3V)
Operating Temperature Range
LTC2481C ................................................... 0°C to 70°C
LTC2481I ................................................ – 40°C to 85°C
Storage Temperature Range ................ – 65°C to 125°C
TOP VIEW
REF+ 1
10 CA0/F0
VCC 2
9 CA1
REF – 3
11
8 GND
IN+ 4
7 SDA
IN– 5
6 SCL
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 43°C/ W
EXPOSED PAD (PIN 11) IS GND
MUST BE SOLDERED TO PCB
DD PART MARKING*
LBPV
ORDER PART NUMBER
LTC2481CDD
LTC2481IDD
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
*The temperature grade is identified by a label on the shipping container.
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ELECTRICAL CHARACTERISTICS ( OR AL SPEED)
The ● denotes the specifications which
apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
Resolution (No Missing Codes)
0.1 ≤ VREF ≤ VCC, –FS ≤ VIN ≤ +FS (Note 5)
●
MIN
Integral Nonlinearity
5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V (Note 6)
2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V (Note 6)
Offset Error
2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Note 13)
Offset Error Drift
2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC
Positive Full-Scale Error
2.5V ≤ VREF ≤ VCC, IN+ = 0.75VREF, IN– = 0.25VREF
Positive Full-Scale Error Drift
2.5V ≤ VREF ≤ VCC
Negative Full-Scale Error
2.5V ≤ VREF ≤ VCC, IN– = 0.75VREF, IN+ = 0.25VREF
Negative Full-Scale Error Drift
2.5V ≤ VREF ≤ VCC, IN– = 0.75VREF, IN+ = 0.25VREF
0.1
ppm of
VREF/°C
Total Unadjusted Error
5V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V (Note 6)
5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V (Note 6)
2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V (Note 6)
15
15
15
ppm of VREF
ppm of VREF
ppm of VREF
Output Noise
5V ≤ VCC ≤ 5.5V, VREF = 5V, GND ≤ IN– = IN+ ≤ VCC (Note 12)
0.6
µVRMS
Internal PTAT Signal
TA = 27°C
420
mV
1.4
mV/°C
, IN+
TYP
MAX
●
2
1
10
●
0.5
2.5
16
●
0.1
●
See Table 2a
●
µV
ppm of VREF
ppm of
VREF/°C
25
1
ppm of VREF
ppm of VREF
nV/°C
25
Internal PTAT Temperature Coefficient
Programmable Gain
Bits
10
= 0.75VREF, IN– = 0.25VREF
UNITS
ppm of VREF
256
2481f
2
LTC2481
ELECTRICAL CHARACTERISTICS (2x SPEED)
The ● denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
Resolution (No Missing Codes)
0.1 ≤ VREF ≤ VCC, –FS ≤ VIN ≤ +FS (Note 5)
●
MIN
Integral Nonlinearity
5V ≤ VCC ≤ 5.5V, VREF = 5V, VIN(CM) = 2.5V (Note 6)
2.7V ≤ VCC ≤ 5.5V, VREF = 2.5V, VIN(CM) = 1.25V (Note 6)
●
Offset Error
2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC (Note 13)
●
Offset Error Drift
2.5V ≤ VREF ≤ VCC, GND ≤ IN+ = IN– ≤ VCC
Positive Full-Scale Error
2.5V ≤ VREF ≤ VCC, IN+ = 0.75VREF, IN– = 0.25VREF
Positive Full-Scale Error Drift
2.5V ≤ VREF ≤
Negative Full-Scale Error
2.5V ≤ VREF ≤ VCC, IN– = 0.75VREF, IN+ = 0.25VREF
Negative Full-Scale Error Drift
2.5V ≤ VREF ≤ VCC, IN– = 0.75VREF, IN+ = 0.25VREF
Output Noise
5V ≤ VCC ≤ 5.5V, VREF = 5V, GND ≤ IN– = IN+ ≤ VCC
Programmable Gain
See Table 2b
VCC, IN+
= 0.75VREF
, IN–
TYP
MAX
2
1
10
0.5
2
UNITS
16
Bits
ppm of VREF
mV
100
●
nV/°C
25
= 0.25VREF
ppm of VREF
0.1
●
ppm of
VREF/°C
25
ppm of VREF
0.1
ppm of
VREF/°C
µVRMS
0.84
●
1
128
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CO VERTER CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
IN– = IN+ ≤ VCC (Note 5)
IN– = IN+ ≤ VCC (Note 5)
Input Common Mode Rejection DC
2.5V ≤ VREF ≤ VCC, GND ≤
Input Common Mode Rejection
50Hz ±2%
2.5V ≤ VREF ≤ VCC, GND ≤
●
140
dB
●
140
dB
Input Common Mode Rejection
60Hz ±2%
2.5V ≤ VREF ≤ VCC, GND ≤ IN– = IN+ ≤ VCC (Note 5)
●
140
dB
Input Normal Mode Rejection
50Hz ±2%
2.5V ≤ VREF ≤ VCC, GND ≤ IN– = IN+ ≤ VCC (Notes 5, 7)
●
110
120
dB
Input Normal Mode Rejection
60Hz ±2%
2.5V ≤ VREF ≤ VCC, GND ≤ IN– = IN+ ≤ VCC (Notes 5, 8)
●
110
120
dB
Input Normal Mode Rejection
50Hz/60Hz ±2%
2.5V ≤ VREF ≤ VCC, GND ≤ IN– = IN+ ≤ VCC (Notes 5, 9)
●
87
Reference Common Mode
Rejection DC
2.5V ≤ VREF ≤ VCC, GND ≤ IN– = IN+ ≤ VCC (Note 5)
●
120
Power Supply Rejection DC
VREF = 2.5V, IN– = IN+ = GND
dB
140
dB
120
dB
Power Supply Rejection, 50Hz ±2%
VREF
= GND (Notes 7, 9)
120
dB
Power Supply Rejection, 60Hz ±2%
VREF = 2.5V, IN– = IN+ = GND (Notes 8, 9)
120
dB
= 2.5V, IN– = IN+
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A ALOG I PUT A D REFERE CE The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
IN+
Absolute/Common Mode IN+ Voltage
CONDITIONS
MIN
IN–
Absolute/Common Mode IN–
FS
Full Scale of the Differential Input (IN+
●
0.5VREF/GAIN
LSB
Least Significant Bit of the Output Code
●
FS/216
VIN
Input Differential Voltage Range (IN+ – IN–)
●
–FS
+FS
VREF
Reference Voltage Range (REF+ – REF–)
●
0.1
VCC
Voltage
– IN–)
TYP
MAX
UNITS
GND – 0.3V
VCC + 0.3V
V
GND – 0.3V
VCC + 0.3V
V
V
V
V
2481f
3
LTC2481
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A ALOG I PUT A D REFERE CE The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
CS
(IN+)
IN+
Sampling Capacitance
CS
(IN–)
IN–
Sampling Capacitance
MIN
VREF Sampling Capacitance
IDC_LEAK (IN+)
IN+ DC Leakage Current
Sleep Mode, IN+ = GND
●
IN–
Sleep Mode, IN–
= GND
Sleep Mode, VREF = VCC
IDC_LEAK (VREF)
DC Leakage Current
REF+, REF– DC Leakage Current
UNITS
pF
11
pF
11
pF
–10
1
10
nA
●
–10
●
–100
1
10
nA
1
100
nA
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IDC_LEAK
MAX
11
CS (VREF)
(IN–)
TYP
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I2C DIGITAL I PUTS A D DIGITAL OUTPUTS
The ● denotes the specifications which apply over
the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
VIH
High Level Input Voltage
CONDITIONS
●
VIL
Low Level Input Voltage
●
VIL(CA1)
Low Level Input Voltage for Address Pin
●
VIH(CA0/F0,CA1)
High Level Input Voltage for Address Pins
●
RINH
Resistance from CA0/F0, CA1 to VCC to Set
Chip Address Bit to 1
●
10
kΩ
RINL
Resistance from CA1 to GND to Set
Chip Address Bit to 0
●
10
kΩ
RINF
Resistance from CA0/F0, CA1 to VCC or
GND to Set Chip Address Bit to Float
●
2
II
Digital Input Current
●
–10
VHYS
Hysteresis of Schmitt Trigger Inputs
(Note 5)
VOL
Low Level Output Voltage SDA
I = 3mA
tOF
Output Fall Time from VIHMIN to VILMAX
Bus Load CB 10pF to 400pF (Note 14) ●
tSP
Input Spike Suppression
IIN
Input Leakage
CI
Capacitance for Each I/O Pin
●
CB
Capacitance Load for Each Bus Line
●
400
pF
CCAX
External Capacitive Load on Chip
Address Pins (CA0/F0,CA1) for Valid Float
●
10
pF
VIH(EXT,OSC)
High Level CA0/F0 External Oscillator
2.7V ≤ VCC < 5.5V
●
VIL(EXT,OSC)
Low Level CA0/F0 External Oscillator
2.7V ≤ VCC < 5.5V
●
0.5
V
0.1VCC ≤ VIN ≤ VCC
MIN
TYP
MAX
0.7VCC
UNITS
V
0.3VCC
V
0.05VCC
V
0.95VCC
V
MΩ
10
0.05VCC
µA
V
●
20+0.1CB
0.4
V
250
ns
●
50
ns
●
1
µA
10
pF
VCC – 0.5V
V
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POWER REQUIRE E TS
The ● denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
VCC
Supply Voltage
ICC
Supply Current
CONDITIONS
MIN
●
Conversion Mode (Note 11)
Sleep Mode (Note 11)
●
●
TYP
MAX
5.5
V
160
1
250
2
µA
µA
2.7
UNITS
2481f
4
LTC2481
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TI I G CHARACTERISTICS
The ● denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
MAX
UNITS
fEOSC
External Oscillator Frequency Range
CONDITIONS
●
MIN
10
TYP
4000
kHz
tHEO
External Oscillator High Period
●
0.125
100
µs
tLEO
External Oscillator Low Period
●
0.125
tCONV_1
Conversion Time for 1x Speed Mode
50Hz Mode
60Hz Mode
Simultaneous 50Hz/60Hz Mode
External Oscillator (Note 10)
●
●
●
●
157.2
131.0
144.1
160.3
163.5
133.6
136.3
146.9
149.9
41036/fEOSC
ms
ms
ms
ms
tCONV_2
Conversion Time for 2x Speed Mode
50Hz Mode
60Hz Mode
Simultaneous 50Hz/60Hz Mode
External Oscillator (Note 10)
●
●
●
●
78.7
65.6
72.2
80.3
66.9
73.6
20556/fEOSC
ms
ms
ms
ms
100
81.9
68.2
75.1
µs
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I2C TI I G CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 15)
SYMBOL
PARAMETER
CONDITIONS
fSCL
SCL Clock Frequency
●
0
tHD(SDA)
Hold Time (Repeated) START Condition
●
0.6
µs
tLOW
LOW Period of the SCL Clock Pin
●
1.3
µs
tHIGH
HIGH Period of the SCL Clock Pin
●
0.6
µs
tSU(STA)
Set-Up Time for a Repeated START Condition
●
0.6
µs
tHD(DAT)
Data Hold Time
●
0
tSU(DAT)
Data Set-Up Time
●
100
tr
Rise Time for Both SDA and SCL Signals
(Note 14)
●
20+0.1CB
300
ns
tf
Fall Time for Both SDA and SCL Signals
(Note 14)
●
20+0.1CB
300
ns
tSU(STO)
Set-Up Time for STOP Condition
●
0.6
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: All voltage values are with respect to GND.
Note 3: VCC = 2.7V to 5.5V unless otherwise specified.
VREF = REF+ – REF–, VREFCM = (REF+ + REF–)/2, FS = 0.5VREF/GAIN;
VIN = IN+ – IN–, VINCM = (IN+ + IN–)/2.
Note 4: Use internal conversion clock or external conversion clock source
with fEOSC = 307.2kHz unless otherwise specified.
Note 5: Guaranteed by design, not subject to test.
Note 6: Integral nonlinearity is defined as the deviation of a code from a
straight line passing through the actual endpoints of the transfer curve.
The deviation is measured from the center of the quantization band.
Note 7: 50Hz mode (internal oscillator) or fEOSC = 256kHz ±2% (external
oscillator).
MIN
TYP
MAX
UNITS
400
kHz
0.9
µs
ns
µs
Note 8: 60Hz mode (internal oscillator) or fEOSC = 307.2kHz ±2% (external
oscillator).
Note 9: Simultaneous 50Hz/60Hz mode (internal oscillator) or fEOSC =
280kHz ±2% (external oscillator).
Note 10: The external oscillator is connected to the CA0/F0 pin. The
external oscillator frequency, fEOSC, is expressed in kHz.
Note 11: The converter uses the internal oscillator.
Note 12: The output noise includes the contribution of the internal
calibration operations.
Note 13: Guaranteed by design and test correlation.
Note 14: CB = capacitance of one bus line in pF.
Note 15: All values refer to VIH(MIN) and VIL(MAX) levels.
2481f
5
LTC2481
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TYPICAL PERFOR A CE CHARACTERISTICS
Integral Nonlinearity
(VCC = 5V, VREF = 5V)
–45°C
1
25°C
0
85°C
–1
3
VCC = 5V
VREF = 2.5V
VIN(CM) = 1.25V
2
INL (ppm OF VREF)
2
INL (ppm OF VREF)
3
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
Integral Nonlinearity
(VCC = 2.7V, VREF = 2.5V)
1
–45°C, 25°C, 90°C
0
–1
–3
–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5
INPUT VOLTAGE (V)
2
–3
–1.25
2.5
1
–45°C, 25°C, 90°C
0
–1
–2
–2
–2
VCC = 2.7V
VREF = 2.5V
VIN(CM) = 1.25V
2
INL (ppm OF VREF)
3
Integral Nonlinearity
(VCC = 5V, VREF = 2.5V)
–0.75
–3
–1.25
1.25
–0.25
0.25
0.75
INPUT VOLTAGE (V)
–0.75
1.25
–0.25
0.25
0.75
INPUT VOLTAGE (V)
2481 G03
2481 G02
2481 G01
8
8
85°C
25°C
–45°C
–4
–8
4
–45°C
0
–4
2
–12
–1.25
2.5
Noise Histogram (6.8sps)
–0.75
1.2
2481 G07
1.25
2481 G06
VCC = 5V, VREF = 5V, VIN = 0V, VIN(CM) = 2.5V
4 GAIN = 256, TA = 25°C, RMS NOISE = 0.60µV
10,000 CONSECUTIVE
READINGS
RMS = 0.59µV
12
VCC = 2.7V
AVERAGE = –0.19µV
VREF = 2.5V
10 VIN = 0V
GAIN = 256
8 TA = 25°C
3
ADC READING (µV)
NUMBER OF READINGS (%)
1.8
–0.25
0.25
0.75
INPUT VOLTAGE (V)
Long-Term ADC Readings
6
4
2
1
0
–1
–2
–3
–4
–5
0
0
–0.75
5
2
2
–3 –2.4 –1.8 –1.2 –0.6 0 0.6
OUTPUT READING (µV)
–4
Noise Histogram (7.5sps)
4
–45°C
0
–12
–1.25
1.25
–0.25
0.25
0.75
INPUT VOLTAGE (V)
14
6
85°C
2481 G05
14
10,000 CONSECUTIVE
READINGS
12
RMS = 0.60µV
VCC = 5V
AVERAGE = –0.69µV
VREF = 5V
10 VIN = 0V
GAIN = 256
8 TA = 25°C
25°C
4
–8
2481 G04
NUMBER OF READINGS (%)
8
–8
–12
–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5
INPUT VOLTAGE (V)
VCC = 2.7V
VREF = 2.5V
VIN(CM) = 1.25V
25°C
4
0
12
VCC = 5V
VREF = 2.5V
VIN(CM) = 1.25V
85°C
TUE (ppm OF VREF)
TUE (ppm OF VREF)
12
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
TUE (ppm OF VREF)
12
Total Unadjusted Error
(VCC = 2.7V, VREF = 2.5V)
Total Unadjusted Error
(VCC = 5V, VREF = 2.5V)
Total Unadjusted Error
(VCC = 5V, VREF = 5V)
–3 –2.4 –1.8 –1.2 –0.6 0 0.6
OUTPUT READING (µV)
1.2
1.8
2481 G08
0
10
30
40
20
TIME (HOURS)
50
60
2481 G09
2481f
6
LTC2481
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TYPICAL PERFOR A CE CHARACTERISTICS
RMS Noise
vs Input Differential Voltage
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
TA = 25°C
VCC = 5V
VREF = 5V
VIN = 0V
VIN(CM) = GND
GAIN = 256
TA = 25°C
0.9
0.8
0.7
0.6
1.0
0.8
0.7
0.6
0.4
–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 2
INPUT DIFFERENTIAL VOLTAGE (V)
–1
0
2
1
3
5
4
0.5
OFFSET ERROR (ppm OF VREF)
VCC = 5V
VIN = 0V
VIN(CM) = GND
GAIN = 256
TA = 25°C
0.8
0.7
0.6
0.5
3.1
3.5
3.9 4.3
VCC (V)
4.7
5.1
0
1
2
3
VREF (V)
–0.1
–0.2
–1
0
–0.1
–0.2
75
90
2481 G16
0
1
3
2
VIN(CM) (V)
5
4
0.2
0.1
Offset Error vs VREF
0.3
REF+ = 2.5V
REF– = GND
VIN = 0V
VIN(CM) = GND
TA = 25°C
0
–0.1
VCC = 5V
REF– = GND
VIN = 0V
VIN(CM) = GND
TA = 25°C
0.2
0.1
0
–0.1
–0.2
–0.3
2.7
6
2481 G15
OFFSET ERROR (ppm OF VREF)
OFFSET ERROR (ppm OF VREF)
OFFSET ERROR (ppm OF VREF)
0.1
0 15 30 45 60
TEMPERATURE (°C)
0
Offset Error vs VCC
0.3
VCC = 5V
VREF = 5V
VIN = 0V
VIN(CM) = GND
–0.3
–45 –30 –15
0.1
2481 G14
Offset Error vs Temperature
0.2
0.2
5
4
2481 G13
0.3
VCC = 5V
VREF = 5V
VIN = 0V
TA = 25°C
–0.3
0.4
5.5
90
Offset Error vs VIN(CM)
0.3
0.9
0.6
75
2481 G12
RMS Noise vs VREF
0.7
0 15 30 45 60
TEMPERATURE (°C)
2481 G11
1.0
VREF = 2.5V
VIN = 0V
VIN(CM) = GND
GAIN = 256
TA = 25°C
0.4
2.7
0.4
–45 –30 –15
6
VIN(CM) (V)
RMS NOISE (µV)
RMS NOISE (µV)
0.8
0.6
0.4
2.5
RMS Noise vs VCC
0.9
0.7
0.5
2481 G10
1.0
0.8
0.5
0.5
VCC = 5V
VREF = 5V
VIN = 0V
VIN(CM) = GND
GAIN = 256
0.9
RMS NOISE (µV)
RMS NOISE (ppm OF VREF)
0.9
RMS Noise vs Temperature (TA)
RMS Noise vs VIN(CM)
1.0
RMS NOISE (µV)
1.0
–0.2
3.1
3.5
3.9 4.3
VCC (V)
4.7
5.1
5.5
2481 G17
–0.3
0
1
2
3
VREF (V)
4
5
2481 G18
2481f
7
LTC2481
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Temperature Sensor
vs Temperature
5
VCC = 5V
VREF = 1.4V
0.30
0.25
310
3
308
2
FREQUENCY (kHz)
VPTAT/VREF (V)
0.35
On-Chip Oscillator Frequency
vs Temperature
VCC = 5V
VREF = 1.4V
4
TEMPERATURE ERROR (°C)
0.40
Temperature Sensor Error
vs Temperature
1
0
–1
–2
306
304
–3
302
–4
0.20
–60
–30
0
30
60
TEMPERATURE (°C)
90
–5
–60
120
–30
30
60
0
TEMPERATURE (°C)
90
VCC = 4.1V
VREF = 2.5V
VIN = 0V
VIN(CM) = GND
300
–45 –30 –15
120
0 15 30 45 60
TEMPERATURE (°C)
75
90
2481 G19
2481 G20
On-Chip Oscillator Frequency
vs VCC
VREF = 2.5V
VIN = 0V
VIN(CM) = GND
308
–40
306
304
302
–20
–40
–60
–80
3.0
3.5
4.0
VCC (V)
4.5
5.0
–100
–120
–120
5.5
–140
1
10
10k 100k
1k
100
FREQUENCY AT VCC (Hz)
2481 G22
0 20 40 60 80 100 120 140 160 180 200 220
FREQUENCY AT VCC (Hz)
1M
2481 G24
2481 G23
Sleep Mode Current
vs Temperature
Conversion Current
vs Temperature
PSRR vs Frequency at VCC
0
2.0
200
VCC = 4.1V DC ±0.7V
= 2.5V
V
–20 INREF
+ = GND
–
IN = GND
–40 TA = 25°C
1.8
CONVERSION CURRENT (µA)
REJECTION (dB)
–80
–100
–140
2.5
VCC = 4.1V DC ±1.4V
VREF = 2.5V
IN+ = GND
IN– = GND
TA = 25°C
–60
–60
–80
–100
–120
180
SLEEP MODE CURRENT (µA)
300
PSRR vs Frequency at VCC
0
VCC = 4.1V DC
VREF = 2.5V
IN+ = GND
IN– = GND
TA = 25°C
–20
REJECTION (dB)
FREQUENCY (kHz)
PSRR vs Frequency at VCC
0
REJECTION (dB)
310
2481 G21
VCC = 5V
160
140
VCC = 2.7V
120
1.6
1.4
1.2
VCC = 5V
1.0
0.8
0.6
VCC = 2.7V
0.4
0.2
–140
30600
30650
30700
30750
FREQUENCY AT VCC (Hz)
30800
2481 G25
100
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
2481 G26
0
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
2481 G27
2481f
8
LTC2481
U W
TYPICAL PERFOR A CE CHARACTERISTICS
450
400
2
VCC = 5V
350
300
VCC = 3V
250
1
25°C, 90°C
0
–1
200
VCC = 5V
VREF = 2.5V
VIN(CM) = 1.25V
2
1
90°C
0
–45°C, 25°C
–1
–45°C
–2
150
100
0
–2
–3
–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5
INPUT VOLTAGE (V)
10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
1
90°C
0
–1
–45°C, 25°C
–2
RMS = 0.86µV
10,000 CONSECUTIVE
AVERAGE = 0.184mV
14 READINGS
VCC = 5V
12 VREF = 5V
VIN = 0V
GAIN = 256
10
TA = 25°C
8
6
0.8
0.6
0.4
VCC = 5V
VIN = 0V
VIN(CM) = GND
TA = 25°C
0.2
0
0
179
1.25
–0.25
0.25
0.75
INPUT VOLTAGE (V)
181.4
183.8
186.2
OUTPUT READING (µV)
2481 G31
0
1
3
2
VREF (V)
4
5
2481 G33
Offset Error vs Temperature
(2x Speed Mode)
200
196
188.6
2481 G32
Offset Error vs VIN(CM)
(2x Speed Mode)
198
1.25
1.0
4
240
VCC = 5V
VREF = 5V
VIN = 0V
TA = 25°C
230
OFFSET ERROR (µV)
–0.75
–0.25
0.25
0.75
INPUT VOLTAGE (V)
RMS Noise vs VREF
(2x Speed Mode)
2
OFFSET ERROR (µV)
–3
–1.25
–0.75
2481 G30
16
VCC = 2.7V
VREF = 2.5V
VIN(CM) = 1.25V
NUMBER OF READINGS (%)
2
–3
–1.25
2.5
Noise Histogram
(2x Speed Mode)
Integral Nonlinearity (2x Speed
Mode; VCC = 2.7V, VREF = 2.5V)
3
2
2481 G29
2481 G28
INL (ppm OF VREF)
3
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
RMS NOISE (µV)
SUPPLY CURRENT (µA)
3
VREF = VCC
IN+ = GND
IN– = GND
CA0/F0 = EXT OSC
TA = 25°C
INL (ppm OF VREF)
500
Integral Nonlinearity (2x Speed
Mode; VCC = 5V, VREF = 2.5V)
Integral Nonlinearity (2x Speed
Mode; VCC = 5V, VREF = 5V)
INL (ppm OF VREF)
Conversion Current
vs Output Data Rate
194
192
190
188
186
220
VCC = 5V
VREF = 5V
VIN = 0V
VIN(CM) = GND
210
200
190
180
184
170
182
180
–1
0
1
3
VIN(CM) (V)
2
4
5
6
2481 G34
160
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
2481 G35
2481f
9
LTC2481
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TYPICAL PERFOR A CE CHARACTERISTICS
Offset Error vs VREF
(2x Speed Mode)
Offset Error vs VCC
(2x Speed Mode)
250
VCC = 5V
VIN = 0V
VIN(CM) = GND
TA = 25°C
230
150
100
220
–40
210
200
190
4
4.5
VCC (V)
3.5
2.7 3
5
5.5
160
–140
1
0
2
4
3
VREF (V)
5
2481 G36
10k 100k
1k
100
FREQUENCY AT VCC (Hz)
1M
2481 G38
0
VCC = 4.1V DC ±0.7V
REF+ = 2.5V
REF– = GND
IN+ = GND
–40 IN– = GND
TA = 25°C
–20
REJECTION (dB)
RREJECTION (dB)
–40
10
PSRR vs Frequency at VCC
(2x Speed Mode)
VCC = 4.1V DC ±1.4V
REF+ = 2.5V
REF– = GND
IN+ = GND
IN– = GND
TA = 25°C
–20
1
2481 G37
PSRR vs Frequency at VCC
(2x Speed Mode)
0
–80
–120
170
0
–60
–100
180
50
VCC = 4.1V DC
REF+ = 2.5V
REF– = GND
IN+ = GND
IN– = GND
TA = 25°C
–20
REJECTION (dB)
OFFSET ERROR (µV)
OFFSET ERROR (µV)
200
0
240
VREF = 2.5V
VIN = 0V
VIN(CM) = GND
TA = 25°C
PSRR vs Frequency at VCC
(2x Speed Mode)
–60
–80
–60
–80
–100
–100
–120
–120
–140
0 20 40 60 80 100 120 140 160 180 200 220
FREQUENCY AT VCC (Hz)
2481 G39
–140
30600
30650
30700
30750
FREQUENCY AT VCC (Hz)
30800
2481 G40
U
U
U
PI FU CTIO S
REF+ (Pin 1), REF– (Pin 3): Differential Reference Input.
The voltage on these pins can have any value between GND
and VCC as long as the reference positive input, REF+, is
more positive than the reference negative input, REF –, by
at least 0.1V.
GND – 0.3V and VCC + 0.3V. Within these limits the
converter bipolar input range (VIN = IN+ – IN–) extends
from –0.5 • VREF /GAIN to 0.5 • VREF/GAIN. Outside this
input range the converter produces unique overrange
and underrange output codes.
VCC (Pin 2): Positive Supply Voltage. Bypass to GND
(Pin 8) with a 1µF tantalum capacitor in parallel with 0.1µF
ceramic capacitor as close to the part as possible.
SCL (Pin 6): Serial Clock Pin of the I2C Interface. The
LTC2481 can only act as a slave and the SCL pin only
accepts external serial clock. Data is shifted into the SDA
pin on the rising edges of the SCL clock and output
through the SDA pin on the falling edges of the SCL clock.
IN+ (Pin 4), IN– (Pin 5): Differential Analog Input. The
voltage on these pins can have any value between
2481f
10
LTC2481
U
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PI FU CTIO S
SDA (Pin 7): Bidirectional Serial Data Line of the I2C
Interface. In the transmitter mode (Read), the conversion
result is output through the SDA pin, while in the receiver
mode (Write), the device configuration bits are input
through the SDA pin. At data input mode, the pin is high
impedance; while at data output mode, it is an open-drain
N-channel driver and therefore an external pull-up resistor
or current source to VCC is needed.
GND (Pin 8): Ground. Connect this pin to a ground plane
through a low impedance connection.
CA1 (Pin 9): Chip Address Control Pin. The CA1 pin is
configured as a three state (LOW, HIGH, or Floating)
address control bit for the device I2C address.
CA0/F0 (Pin 10): Chip Address Control Pin/External Clock
Input Pin. When no transition is detected on the CA0/F0
pin, it is a two state (HIGH or Floating) address control bit
for the device I2C address. When the pin is driven by an
external clock signal with a frequency fEOSC of at least
10kHz, the converter uses this signal as its system clock
and the fundamental digital filter rejection null is located at
a frequency fEOSC/5120 and sets the Chip Address CA0
internally to a HIGH.
W
FU CTIO AL BLOCK DIAGRA
U
2
1
4
5
REF+
VCC
IN+
IN+
IN –
SCL
REF+
I2 C
SERIAL
INTERFACE
3RD ORDER
∆Σ ADC
(1-256)
IN –
REF –
GAIN
MUX
TEMP
SENSOR
SDA
CA1
CA0/F0
6
7
9
10
AUTOCALIBRATION
AND CONTROL
REF–
3
GND
8
INTERNAL
OSCILLATOR
2481 FD
2481f
11
U
LTC2481
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APPLICATIO S I FOR ATIO
CONVERTER OPERATION
Converter Operation Cycle
The LTC2481 is a low power, ∆Σ analog-to-digital converter with an I2C interface. After power on reset, its
operation is made up of three states. The converter
operating cycle begins with the conversion, followed by
the low power sleep state and ends with the data output/
input (see Figure 1).
POWER ON RESET
DEFAULT CONFIGURATION:
EXTERNAL INPUT GAIN = 1
50/60Hz REJECTION
1X SPEED, AUTOCAL
CONVERSION
SLEEP
NO
I2C INTERFACE
ACKNOWLEDGE
YES
DATA OUTPUT/INPUT
NO
The device will not acknowledge an external request
during the conversion state. After a conversion is finished,
the device is ready to accept a read/write request. Once the
LTC2481 is addressed for a read operation, the device
begins outputting the conversion result under control of
the serial clock (SCL). There is no latency in the conversion result. The data output is 24 bits long and contains
a16-bit plus sign conversion result plus a readback of the
configuration bits corresponds to the conversion just
performed. This result is shifted out on the SDA pin under
the control of the SCL. Data is updated on the falling edges
of SCL allowing the user to reliably latch data on the rising
edge of SCL. In write operation, the device accepts one
configuration byte and the data is shifted in on the rising
edges of the SCL. A new conversion is initiated by a STOP
condition following a valid write operation or at the conclusion of a data read operation (read out all 24 bits).
STOP
OR READ
24-BITS
YES
2481 F01
Figure 1. LTC2481 State Transition Diagram
Initially, the LTC2481 performs a conversion. Once the
conversion is complete, the device enters the sleep state.
While in this sleep state, power consumption is reduced
by two orders of magnitude. The part remains in the sleep
state as long as it is not addressed for a read/write
operation. The conversion result is held indefinitely in a
static shift register while the converter is in the sleep state.
The LTC2481 communicates through an I2C interface.
The I2C interface is a 2-wire open-drain interface supporting multiple devices and masters on a single bus. The
connected devices can only pull the bus wires LOW and
can never drive the bus HIGH. The bus wires are externally connected to a positive supply voltage via a currentsource or pull-up resistor. When the bus is free, both
lines are HIGH. Data on the I2C-bus can be transferred at
rates of up to 100kbit/s in the Standard-mode and up to
400kbit/s in the Fast-mode.
Each device on the I2C bus is recognized by a unique
address stored in that device and can operate as either a
transmitter or receiver, depending on the function of the
device. In addition to transmitters and receivers, devices
can also be considered as masters or slaves when performing data transfers. A master is the device which
initiates a data transfer on the bus and generates the clock
signals to permit that transfer. At the same time any device
addressed is considered a slave.
2481f
12
LTC2481
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APPLICATIO S I FOR ATIO
The LTC2481 can only be addressed as a slave. Once
addressed, it can receive configuration bits or transmit the
last conversion result. Therefore the serial clock line SCL
is an input only and the data line SDA is bidirectional. The
device supports the Standard-mode and the Fast-mode
for data transfer speeds up to 400kbit/s. Figure 2 shows
the definition of timing for Fast/Standard-mode devices
on the I2C-bus.
The START and STOP Conditions
A START condition is generated by transitioning SDA from
HIGH to LOW while SCL is HIGH. The bus is considered to
be busy after the START condition. When the data transfer
is finished, a STOP condition is generated by transitioning
SDA from LOW to HIGH while SCL is HIGH. The bus is free
again a certain time after the STOP condition. START and
STOP conditions are always generated by the master.
When the bus is in use, it stays busy if a repeated START
(Sr) is generated instead of a STOP condition. The
repeated START (Sr) conditions are functionally identical
to the START (S).
Data Transferring
After the START condition, the I2C bus is busy and data
transfer is set between a master and a slave. Data is
transferred over I2C in groups of nine bits (one byte)
followed by an acknowledge bit, therefore each group
takes nine SCL cycles. The transmitter releases the SDA
line during the acknowledge clock pulse and the receiver
issues an Acknowledge (ACK) by pulling SDA LOW or
leaves SDA HIGH to indicate a Not Acknowledge (NAK)
condition. Change of data state can only happen while SCL
is LOW.
Accessing the Special Features of the LTC2481
The LTC2481 combines a high resolution, low noise ∆Σ
analog-to-digital converter with an on-chip selectable temperature sensor, programmable gain, programmable digital
filter and output rate control. These special features are
selected through a single 8-bit serial input word during the
data input/output cycle (see Figure 3).
The LTC2481 powers up in a default mode commonly
used for most measurements. The device will remain in
this mode until a valid write cycle is performed. In this
default mode, the measured input is external, the GAIN is
1, the digital filter simultaneously rejects 50Hz and 60Hz
line frequency noise, and the speed mode is 1x (offset
automatically, continuously calibrated).
The I2C serial interface grants access to any or all special
functions contained within the LTC2481. In order to
change the mode of operation, a valid write address
followed by 8 bits of data are shifted into the device (see
Table 1). The first 3 bits (GS2, GS1, GS0) control the
GAIN of the converter from 1 to 256. The 4th bit is
reserved and should be low. The 5th bit (IM) is used to
select the internal temperature sensor as the conversion
input, while the 6th and 7th bits (FA, FB) combine to
determine the line frequency rejection mode. The 8th bit
(SPD) is used to double the output rate by disabling the
offset auto calibration.
SDA
tf
tLOW
tSU;DAT
tr
tr
tHD;STA
tSP
tr
tBUF
SCL
S
tHD;STA
tHD;DAT
tHIGH
tSU;STA
Sr
tSU;STO
P
S
2481 F02
Figure 2. Definition of Timing for F/S-Mode Devices on the I2C-Bus
2481f
13
LTC2481
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APPLICATIO S I FOR ATIO
1
2
…
7
8
9
1
2
3
GS2
GS1
GS0
4
5
6
7
8
IM
FA
FB
SPD
9
SCL
7-BIT ADDRESS
SDA
W
ACK BY
LTC2481
ACK BY
LTC2481
START BY
MASTER
SLEEP
DATA INPUT
2481 F03
Figure 3. Timing Diagram for Writing to the LTC2481
Table 1. Selecting Special Modes
Rejection
Mode
Gain
GS2 GS1 GS0 IM FA
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Any Gain
X
X
X
X
X
X
X
X
X
X
X
X
FB
SPD
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Any
0
0
Rejection
1
0
Mode
1
0
1
0
1
0
1
0
1
0
1
0
1
0
0
0
0
0
1
0
Any
1
0 Speed
0
1
1
0
1
0
0
X
1
0
1
X
1
1
0
X
1
1
1
X
Comments
External Input, Gain = 1, Autocalibration
External Input, Gain = 4, Autocalibration
External Input, Gain = 8, Autocalibration
External Input, Gain = 16, Autocalibration
External Input, Gain = 32, Autocalibration
External Input, Gain = 64, Autocalibration
External Input, Gain = 128, Autocalibration
External Input, Gain = 256, Autocalibration
External Input, Gain = 1, 2x Speed
External Input, Gain = 2, 2x Speed
External Input, Gain = 4, 2x Speed
External Input, Gain = 8, 2x Speed
External Input, Gain = 16, 2x Speed
External Input, Gain = 32, 2x Speed
External Input, Gain = 64, 2x Speed
External Input, Gain = 128, 2x Speed
External Input, Simultaneous 50Hz/60Hz Rejection
External Input, 50Hz Rejection
External Input, 60Hz Rejection
Reserved, Do Not Use
Temperature Input, 50Hz/60Hz Rejection, Gain = 1, Autocalibration
Temperature Input, 50Hz Rejection, Gain = 1, Autocalibration
Temperature Input, 60Hz Rejection, Gain = 1, Autocalibration
Reserved, Do Not Use
2481 TBL1
2481f
14
LTC2481
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APPLICATIO S I FOR ATIO
Table 2a. The LTC2481 Performance vs GAIN in Normal Speed Mode (VCC = 5V, VREF = 5V)
GAIN
1
4
8
16
32
64
128
256
Input Span
±2.5
±0.625
±0.312
±0.156
±78m
±39m
±19.5m
±9.76m
V
LSB
38.1
9.54
4.77
2.38
1.19
0.596
0.298
0.149
µV
65536
65536
65536
65536
65536
65536
32768
16384
Counts
Noise Free Resolution*
UNIT
Gain Error
5
5
5
5
5
5
5
8
Offset Error
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
ppm of FS
µV
UNIT
Table 2b. The LTC2481 Performance vs GAIN in 2x Speed Mode (VCC = 5V, VREF = 5V)
GAIN
Input Span
1
2
4
8
16
32
64
128
±2.5
±1.25
±0.625
±0.312
±0.156
±78m
±39m
±19.5m
V
38.1
19.1
9.54
4.77
2.38
1.19
0.596
0.298
µV
65536
65536
65536
65536
65536
65536
45875
22937
Counts
Gain Error
5
5
5
5
5
5
5
5
Offset Error
200
200
200
200
200
200
200
200
LSB
Noise Free Resolution*
ppm of FS
µV
*The resolution in counts is calculated as the FS divided by LSB or the RMS noise value, whichever is larger.
GAIN (GS2, GS1, GS0)
Rejection Mode (FA, FB)
The input referred gain of the LTC2481 is adjustable from
1 to 256. With a gain of 1, the differential input range is
±VREF/2 and the common mode input range is rail-to-rail.
As the GAIN is increased, the differential input range is
reduced to ±VREF/2 • GAIN but the common mode input
range remains rail-to-rail. As the differential gain is increased, low level voltages are digitized with greater
resolution. At a gain of 256, the LTC2481 digitizes an input
signal range of ±9.76mV with over 16,000 counts.
The LTC2481 includes a high accuracy on-chip oscillator
with no required external components. Coupled with a 4th
order digital lowpass filter, the LTC2481 rejects line frequency noise. In the default mode, the LTC2481 simultaneously rejects 50Hz and 60Hz by at least 87dB. The
LTC2481 can also be configured to selectively reject 50Hz
or 60Hz to better than 110dB.
Temperature Sensor (IM)
The LTC2481 includes an on-chip temperature sensor. The
temperature sensor is selected by setting IM = 1 in the serial
input data stream. Conversions are performed directly on
the temperature sensor by the converter. While operating
in this mode, the device behaves as a temperature to bits
converter. The digital reading is proportional to the absolute temperature of the device. This feature allows the
converter to linearize temperature sensors or continuously
remove temperature effects from external sensors. Several
applications leveraging this feature are presented in more
detail in the applications section. While operating in this
mode, the gain is set to 1 and the speed is set to normal independent of the control bits (GS2, GS1, GS0 and SPD).
Speed Mode (SPD)
The LTC2481 continuously performs offset calibrations.
Every conversion cycle, two conversions are automatically performed (default) and the results combined. This
result is free from offset and drift. In applications where
the offset is not critical, the autocalibration feature can be
disabled with the benefit of twice the output rate.
Linearity, full-scale accuracy and full-scale drift are identical for both 2x and 1x speed modes. In both the 1x and
2x speed there is no latency. This enables input steps or
multiplexer channel changes to settle in a single conversion
cycle easing system overhead and increasing the effective
conversion rate.
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LTC2481 Data Format
Table 3. LTC2481 Status Bits
After a START condition, the master sends a 7-bit address
followed by a R/W bit. The bit R/W is 1 for a Read request
and 0 for a Write request. If the 7-bit address agrees with
an LTC2481’s address, that device is selected. When the
device is in the conversion state, it does not accept the
request and issues a Not-Acknowledge (NAK) by leaving
SDA HIGH. If the conversion is complete, it issues an
acknowledge (ACK) by pulling SDA LOW.
The LTC2481 has two registers. The output register
contains the result of the last conversion and a user
programmable configuration register that sets the converter operation mode.
The output register contains the last conversion result.
After each conversion is completed, the device automatically enters the sleep state where the supply current is
reduced to 1µA. When the LTC2481 is addressed for a
Read operation, it acknowledges (by pulling SDA LOW)
and acts as a transmitter. The master and receiver can read
up to three bytes from the LTC2481. After a complete Read
operation (3 bytes), the output register is emptied, a new
conversion is initiated, and a following Read request in the
same input/output phase will be NAKed. The LTC2481
output data stream is 24 bits long, shifted out on the falling
edges of SCL. The first bit is the conversion result sign bit
(SIG), see Tables 3 and 4. This bit is HIGH if VIN ≥ 0. It is
LOW if VIN <0. The second bit is the most significant bit
INPUT RANGE
BIT 23
SIG
BIT 22
MSB
VIN ≥ 0.5 • VREF
1
1
0V ≤ VIN < 0.5 • VREF
1
0
–0.5 • VREF ≤ VIN < 0V
0
1
VIN < – 0.5 • VREF
0
0
(MSB) of the result. The first two bits (SIG and MSB) can
be used to indicate over range conditions. If both bits are
HIGH, the differential input voltage is above +FS and the
following 16 bits are set to LOW to indicate an overrange
condition. If both bits are LOW, the input voltage is below
–FS and the following 16 bits are set to HIGH to indicate an
underrange condition. The function of these two bits is
summarized in Table 3. The next 16 bits contain the
conversion results in binary two’s complement format.
The remaining six bits are a readback of the configuration
register.
As long as the voltage on the IN+ and IN– pins is maintained within the – 0.3V to (VCC + 0.3V) absolute maximum
operating range, a conversion result is generated for any
differential input voltage VIN from –FS = –0.5 • VREF/GAIN
to +FS = 0.5 • VREF/GAIN. For differential input voltages
greater than +FS, the conversion result is clamped to the
value corresponding to the +FS + 1LSB. For differential
input voltages below –FS, the conversion result is clamped
to the value corresponding to –FS – 1LSB.
Table 4. LTC2481 Output Data Format
DIFFERENTIAL INPUT VOLTAGE
VIN *
VIN* ≥ FS**
FS** – 1LSB
0.5 • FS**
0.5 • FS** – 1LSB
0
–1LSB
– 0.5 • FS**
– 0.5 • FS** – 1LSB
– FS**
VIN* < –FS**
BIT 23
SIG
1
1
1
1
1
0
0
0
0
0
BIT 22
MSB
1
0
0
0
0
1
1
1
1
0
BIT 21
BIT 20
BIT 19
…
BIT 6
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
…
…
…
…
…
…
…
…
…
…
0
1
0
1
0
1
0
1
0
1
*The differential input voltage VIN = IN+ – IN–. **The full-scale voltage FS = 0.5 • VREF/GAIN.
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1 … 7
8
7-BIT
ADDRESS
R
9
1
SGN
2 …
MSB
1
D15
ACK BY
LTC2481
START BY
MASTER
9
2
3
4
5
LSB
PG2
PG1
PG0
ACK BY
MASTER
SLEEP
6
X
7
IM
8
9
SPD
NAK BY
MASTER
DATA OUTPUT
2481 F04
Figure 4. Timing Diagram for Reading from the LTC2481
Initiating a New Conversion
OPERATION SEQUENCE
When the LTC2481 finishes a conversion, it automatically
enters the sleep state. Once in the sleep state, the device
is ready for Read/Write operations. After the device acknowledges a Read or Write request, the device exits the
sleep state and enters the data input/output state. The data
input/output state concludes and the LTC2481 starts a
new conversion once a STOP condition is issued by the
master or all 24-bits of data are read out of the device.
The LTC2481 acts as a transmitter or receiver. The device
may be programmed to perform several functions. These
include measuring an external differential input signal or
an integrated temperature sensor, setting a programmable gain (from 1 to 256), selecting line frequency
rejection (50Hz, 60Hz, or simultaneous 50Hz and 60Hz),
and a 2x speed up mode.
During the data read cycle, a stop command may be issued
by the master controller in order to start a new conversion
and abort the data transfer. This stop command must be
issued during the 9th clock cycle of a byte read when the
bus is free (the ACK/NAK cycle).
LTC2481 Address
The LTC2481 has two address pins, enabling one in 6
possible addresses, as shown in Table 5.
Table 5. LTC2481 Address Assignment
CA1
CA0/F0 *
Address
LOW
HIGH
001 01 00
LOW
Floating
001 01 01
Floating
HIGH
001 01 11
Floating
Floating
010 01 00
HIGH
HIGH
010 01 10
HIGH
Floating
010 01 11
Continuous Read
In applications where the configuration does not need to
change for each conversion cycle, the conversion result
can be continuously read. The configuration remains
unchanged from the last value written into the device. If
the device has not been written to since power up, the
configuration is set to the default value (Input External,
GAIN=1, simultaneous 50Hz/60Hz rejection, and 1x speed
mode). The operation sequence is shown in Figure 6.
When the conversion is finished, the device may be
addressed for a read operation. At the end of a read
operation, a new conversion begins. At the conclusion
of the conversion cycle, the next result may be read
using the method described above. If the conversion
cycle is not concluded and a valid address selects the
device, the LTC2481 generates a NAK signal indicating
the conversion cycle is in progress.
* CA0/F0 is treated as HIGH when driven by a valid external clock.
In addition to the configurable addresses listed in Table 5,
the LTC2481 also contains a global address (1110111)
which may be used for synchronizing multiple LTC2481s.
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S
CONVERSION
ACK
R/W
7-BIT ADDRESS
DATA
SLEEP
Sr
DATA TRANSFERRING
P
DATA INPUT/OUTPUT
CONVERSION
2481 F05
Figure 5. The LTC2481 Conversion Sequence
S
7-BIT ADDRESS
R ACK
READ
P
S
7-BIT ADDRESS
R ACK
READ
P
CONVERSION
CONVERSION
SLEEP
DATA OUTPUT
SLEEP
DATA OUTPUT
CONVERSION
2481 F06
Figure 6. Consecutive Reading at the Same Configuration
S
7-BIT ADDRESS
CONVERSION
W ACK
SLEEP
WRITE
Sr
DATA INPUT
7-BIT ADDRESS
ADDRESS
Figure 7. Write, Read, Start Conversion
Continuous Read/Write
Once the conversion cycle is concluded, the LTC2481 can
be written to then read from, using the repeated Start (Sr)
command.
Figure 7 shows a cycle which begins with a data Write, a
repeated start, followed by a read, and concluded with a
stop command. The following conversion begins after all
24-bits are read out of the device or after the STOP
command and uses the newly programmed configuration data.
Discarding a Conversion Result and Initiating a New
Conversion with Optional Configuration Updating
At the conclusion of a conversion cycle, a Write cycle can
be initiated. Once the Write cycle is acknowledged, a stop
(P) command initiates a new conversion. If a new
configuration is required, this data can be written into the
device and a stop command initiates a new conversion,
see Figure 8.
R ACK
READ
DATA OUTPUT
P
CONVERSION
2481 F08
Synchronizing Multiple LTC2481s with the Global
Address Call
In applications where several LTC2481s are used on the
same I2C bus, all LTC2481s can be synchronized with the
global address call. To achieve this, first all the LTC2481s
must have completed the conversion cycle. The master
issues a Start, followed by the LTC2481 global address
1110111 and a Write request. All LTC2481s will be selected
and acknowledge the request. The master then sends the
write byte (Optional) and ends the Write operation with a
STOP. This will update the configuration registers (if a
write byte was sent) and initiate a new conversion simultaneously on all the LTC2481s, as shown in Figure 9. In
order to synchronize the start of conversion without
affecting the configuration registers, the Write operation
can be aborted with a STOP. This initiates a new conversion
on all the LTC2481s without changing the configuration
registers.
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S
7-BIT ADDRESS
CONVERSION
W ACK
SLEEP
WRITE (OPTIONAL)
DATA INPUT
P
CONVERSION
2481 F09
Figure 8. Start a New Conversion without Reading Old Conversion Result
SCL
SDA
LTC2481
S
LTC2481
GLOBAL ADDRESS
W ACK
…
WRITE (OPTIONAL)
ALL LTC2481s IN SLEEP
LTC2481
P
CONVERSION OF ALL LTC2481s
DATA INPUT
2481 F10
Figure 9. Synchronize the LTC2481s with the Global Address Call
Easy Drive Input Current Cancellation
The LTC2481 combines a high precision delta-sigma ADC
with an automatic differential input current cancellation
front end. A proprietary front-end passive sampling
network transparently removes the differential input current. This enables external RC networks and high impedance sensors to directly interface to the LTC2481 without
external amplifiers. The remaining common mode input
current is eliminated by either balancing the differential
input impedances or setting the common mode input
equal to the common mode reference (see Automatic
Input Current Cancellation section). This unique architecture does not require on-chip buffers enabling input
signals to swing all the way to ground and up to VCC.
Furthermore, the cancellation does not interfere with the
transparent offset and full-scale auto-calibration and the
absolute accuracy (full scale + offset + linearity) is maintained even with external RC networks.
Conversion Clock
A major advantage the delta-sigma converter offers over
conventional type converters is an on-chip digital filter
(commonly implemented as a SINC or Comb filter). For high
resolution, low frequency applications, this filter is typically
designed to reject line frequencies of 50Hz or 60Hz plus their
harmonics. The filter rejection performance is directly re-
lated to the accuracy of the converter system clock. The
LTC2481 incorporates a highly accurate on-chip oscillator.
This eliminates the need for external frequency setting components such as crystals or oscillators.
Frequency Rejection Selection (CA0/F0)
The LTC2481 internal oscillator provides better than 110dB
normal mode rejection at the line frequency and all its
harmonics (up to the 255th) for 50Hz ±2% or 60Hz ±2%,
or better than 87dB normal mode rejection from 48Hz to
62.4Hz. The rejection mode is selected by writing to the
on-chip configuration register (the default mode at power
up is simultaneous 50Hz/60Hz rejection).
When a fundamental rejection frequency different from
50Hz or 60Hz is required or when the converter must be
synchronized with an outside source, the LTC2481 can
operate with an external conversion clock. The converter
automatically detects the presence of an external clock
signal at the CA0/F0 pin and turns off the internal oscillator. The chip address for CA0 is internally set HIGH. The
frequency fEOSC of the external signal must be at least
10kHz to be detected. The external clock signal duty cycle
is not significant as long as the minimum and maximum
specifications for the high and low periods tHEO and tLEO
are observed.
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While operating with an external conversion clock of a
frequency fEOSC, the LTC2481 provides better than 110dB
normal mode rejection in a frequency range of fEOSC/5120
±4% and its harmonics. The normal mode rejection as a
function of the input frequency deviation from fEOSC/5120
is shown in Figure 10.
Table 6 summarizes the duration of the conversion state of
each state and the achievable output data rate as a function
of fEOSC.
Ease of Use
The LTC2481 data output has no latency, filter settling
delay or redundant data associated with the conversion
cycle. There is a one-to-one correspondence between the
conversion and the output data. Therefore, multiplexing
multiple analog voltages is easy.
The LTC2481 performs offset and full-scale calibrations
every conversion cycle. This calibration is transparent to the
user and has no effect on the cyclic operation described
–85
NORMAL MODE REJECTION (dB)
Whenever an external clock is not present at the CA0/F0
pin, the converter automatically activates its internal oscillator and enters the Internal Conversion Clock mode.
CA0/F0 may be tied HIGH or left floating in order to set the
chip address. The LTC2481 operation will not be disturbed if the change of conversion clock source occurs
during the sleep state or during the data output state while
the converter uses an external serial clock. If the change
occurs during the conversion state, the result of the
conversion in progress may be outside specifications but
the following conversions will not be affected.
–80
–90
–95
–100
–105
–110
–115
–120
–125
–130
–135
–140
–12
–8
–4
0
4
8
12
DIFFERENTIAL INPUT SIGNAL FREQUENCY
DEVIATION FROM NOTCH FREQUENCY fEOSC/5120(%)
2481 F11
Figure 10. LTC2481 Normal Mode Rejection When
Using an External Oscillator
above. The advantage of continuous calibration is extreme
stability of offset and full-scale readings with respect to
time, supply voltage change and temperature drift.
Power-Up Sequence
The LTC2481 automatically enters an internal reset state
when the power supply voltage VCC drops below approximately 2V. This feature guarantees the integrity of the conversion result.
When the VCC voltage rises above this critical threshold,
the converter creates an internal power-on-reset (POR)
signal with a duration of approximately 4ms. The POR
signal clears all internal registers. Following the POR
signal, the LTC2481 starts a normal conversion cycle and
follows the succession of states described in Figure 1. The
Table 6. LTC2481 State Duration
STATE
OPERATING MODE
CONVERSION
Internal Oscillator
External Oscillator
DURATION
60Hz Rejection
133ms, Output Data Rate ≤ 7.5 Readings/s for 1x Speed Mode
67ms, Output Data Rate ≤ 15 Readings/s for 2x Speed Mode
50Hz Rejection
160ms, Output Data Rate ≤ 6.2 Readings/s for 1x Speed Mode
80ms, Output Data Rate ≤ 12.5 Readings/s for 2x Speed Mode
50Hz/60Hz Rejection
147ms, Output Data Rate ≤ 6.8 Readings/s for 1x Speed Mode
73.6ms, Output Data Rate ≤ 13.6 Readings/s for 2x Speed Mode
CA0/F0 = External Oscillator
with Frequency fEOSC Hz
(fEOSC/5120 Rejection)
41036/fEOSCs, Output Data Rate ≤ fEOSC/41036 Readings/s for
1x Speed Mode
20556/fEOSCs, Output Data Rate ≤ fEOSC/20556 Readings/s for
2x Speed Mode
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On-Chip Temperature Sensor
The LTC2481 contains an on-chip PTAT (proportional to
absolute temperature) signal that can be used as a temperature sensor. The internal PTAT has a typical value of 420mV
at 27°C and is proportional to the absolute temperature
value with a temperature coefficient of 420/(27 + 273) =
1.40mV/°C (SLOPE), as shown in Figure 11. The internal
PTAT signal is used in a single-ended mode referenced to
device ground internally. The GAIN is automatically set to
one (independent of the values of GS0, GS1, GS2) in order
to preserve the PTAT property at the ADC output code and
avoid an out of range error. The 1x speed mode with automatic offset calibration is automatically selected for the internal PTAT signal measurement as well.
When using the internal temperature sensor, if the output
code is normalized to RSDA = VPTAT/VREF, the temperature
is calculated using the following formula:
TK =
RSDA • VREF
in Kelvin
SLOPE
and
TC =
RSDA • VREF
– 273 in °C
SLOPE
where SLOPE is nominally 1.4mV/°C.
Since the PTAT signal can have an initial value variation
which results in errors in SLOPE, to achieve absolute
temperature measurements, a one-time calibration is
needed to adjust the SLOPE value. The converter output of
the PTAT signal, R0SDA, is measured at a known temperature T0 (in °C) and the SLOPE is calculated as:
SLOPE =
R0SDA • VREF
T 0 + 273
If the same VREF source is used during calibration and
temperature measurement, the actual value of the VREF is
not needed to measure the temperature as shown in the
calculation below:
RSDA • VREF
– 273
SLOPE
R
= SDA • T 0 + 273 – 273
R0SDA
TC =
(
600
)
VCC = 5V
IM = 1
SLOPE = 1.40mV/°C
500
VPTAT (mV)
first conversion result following POR is accurate within the
specifications of the device if the power supply voltage is
restored within the operating range (2.7V to 5.5V) before
the end of the POR time interval.
400
300
200
–60
–30
0
30
60
TEMPERATURE (°C)
90
120
2481 F12
Figure 11. Internal PTAT Signal vs Temperature
Reference Voltage Range
The LTC2481 external reference voltage range is 0.1V to
VCC. The converter output noise is determined by the
thermal noise of the front-end circuits, and as such, its
value in nanovolts is nearly constant with reference voltage. Since the transition noise (600nV) is much less than
the quantization noise (VREF/217), a decrease in the reference voltage will increase the converter resolution. A
reduced reference voltage will also improve the converter
performance when operated with an external conversion
clock (external FO signal) at substantially higher output
data rates (see the Output Data Rate section). VREF must
be ≥1.1V to use the internal temperature sensor.
The reference input is differential. The differential reference input range (VREF = REF+ – REF–) is 100mV to VCC
and the common mode reference input range is 0V to VCC.
This calibrated SLOPE can be used to calculate the
temperature.
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Input Voltage Range
Driving the Input and Reference
The analog input is truly differential with an absolute/
common mode range for the IN+ and IN– input pins
extending from GND – 0.3V to VCC + 0.3V. Outside
these limits, the ESD protection devices begin to turn on
and the errors due to input leakage current increase rapidly.
Within these limits, the LTC2481 converts the bipolar
differential input signal, VIN = IN+ – IN–, from – FS to +FS
where FS = 0.5 • VREF/GAIN. Beyond this range, the
converter indicates the overrange or the underrange condition using distinct output codes. Since the differential
input current cancellation does not rely on an on-chip
buffer, current cancellation as well as DC performance is
maintained rail-to-rail.
The input and reference pins of the LTC2481 converter are
directly connected to a network of sampling capacitors.
Depending upon the relation between the differential input
voltage and the differential reference voltage, these capacitors are switching between these four pins transferring
small amounts of charge in the process. A simplified equivalent circuit is shown in Figure 12.
For a simple approximation, the source impedance RS
driving an analog input pin (IN+, IN–, REF+ or REF–) can be
considered to form, together with RSW and CEQ (see
Figure 12), a first order passive network with a time constant τ = (RS + RSW) • CEQ. The converter is able to sample
the input signal with better than 1ppm accuracy if the
sampling period is at least 14 times greater than the input
circuit time constant τ. The sampling process on the four
input analog pins is quasi-independent so each time constant should be considered by itself and, under worst-case
circumstances, the errors may add.
Input signals applied to IN+ and IN– pins may extend by
300mV below ground and above VCC. In order to limit any
fault current, resistors of up to 5k may be added in series
with the IN+ and IN– pins without affecting the performance of the devices. The effect of the series resistance on
the converter accuracy can be evaluated from the curves
presented in the Input Current/Reference Current sections. In addition, series resistors will introduce a temperature dependent offset error due to the input leakage
current. A 1nA input leakage current will develop a 1ppm
offset error on a 5k resistor if VREF = 5V. This error has a
very strong temperature dependency.
IREF+
VCC
RSW (TYP)
10k
ILEAK
VREF+
( )
I IN+
ILEAK
I REF +
ILEAK
VIN+
RSW (TYP)
10k
( )
= I IN –
AVG
AVG
=
VIN(CM) − VREF(CM)
0.5 • REQ
(
)
2
VIN 2
1.5 • VREF − VINCM + VREFCM
0.5 • VREF • DT 1.5VREF + VREF(CM) – VIN(CM)
VIN
–
−
−
≅
=
0.5 • REQ
0.5 • REQ
VREF • REQ
REQ
VREF • REQ
where:
CEQ
12pF
(TYP)
ILEAK
VCC
⎛ REF + + REF – ⎞
VREFCM = ⎜
⎟ , VREF = REF + − REF –
2
⎠
⎝
VIN = IN+ − IN−
⎛ IN+ + IN− ⎞
VINCM = ⎜
⎟
2
⎠
⎝
RSW (TYP)
10k
ILEAK
VIN–
REQ = 2.71MΩ INTERNAL OSCILLATOR 60Hz MODE
REQ = 2.98MΩ INTERNAL OSCILLATOR 50Hz AND 60Hz MODE
ILEAK
IREF–
AVG
( )
VCC
IIN+
IIN–
When using the internal oscillator, the LTC2481’s frontend switched-capacitor network is clocked at 123kHz
corresponding to an 8.1µs sampling period. Thus, for
settling errors of less than 1ppm, the driving source
impedance should be chosen such that τ ≤ 8.1µs/14 =
580ns. When an external oscillator of frequency fEOSC is
used, the sampling period is 2.5/fEOSC and, for a settling
error of less than 1ppm, τ ≤ 0.178/fEOSC.
VCC
ILEAK
VREF–
(
2480 F13
ILEAK
)
REQ = 0.833 • 1012 / f EOSC EXTERNAL OSCILLATOR
RSW (TYP)
10k
DT IS THE DENSITY OF A DIGITAL TRANSITION AT THE MODULATOR OUTPUT
WHERE REF– IS INTERNALLY TIED TO GND
SWITCHING FREQUENCY
fSW = 123kHz INTERNAL OSCILLATOR
fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR
Figure 12. LTC2481 Equivalent Analog Input Circuit
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Automatic Differential Input Current Cancellation
The switching algorithm forces the average input current
on the positive input (IIN+) to be equal to the average input
current on the negative input (IIN–). Over the complete
conversion cycle, the average differential input current
(IIN+ – IIN–) is zero. While the differential input current is
zero, the common mode input current (IIN++ IIN–)/2 is
proportional to the difference between the common mode
input voltage (VINCM) and the common mode reference
voltage (VREFCM).
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balance bridge type application, both the differential and common mode input current are zero. The
accuracy of the converter is unaffected by settling errors.
Mismatches in source impedances between IN+ and IN–
also do not affect the accuracy.
In applications where the input common mode voltage is
constant but different from the reference common mode
voltage, the differential input current remains zero while
IN +
CPAR
≅ 20pF
CEXT
LTC2481
RSOURCE
VINCM – 0.5VIN
IN –
CPAR
≅ 20pF
CEXT
2481 F14
Figure 13. An RC Network at IN+ and IN–
80
+FS ERROR (ppm)
For many applications, the sensor output impedance combined with external bypass capacitors produces RC time
constants much greater than the 580ns required for 1ppm
accuracy. For example, a 10kΩ bridge driving a 0.1µF
bypass capacitor has a time constant several orders of
magnitude greater than the required maximum. Historically, settling issues were solved using buffers. These
buffers led to increased noise, reduced DC performance
(Offset/Drift), limited input/output swing (cannot digitize
signals near ground or VCC), added system cost and increased power. The LTC2481 uses a proprietary switching
algorithm that forces the average differential input current
to zero independent of external settling errors. This allows
accurate direct digitization of high impedance sensors
without the need of buffers (see Figures 13 to 15). Additional errors resulting from mismatched leakage currents
must also be taken into account.
VINCM + 0.5VIN
VCC = 5V
= 5V
60 VREF
VIN+ = 3.75V
– = 1.25V
40 VIN
TA = 25°C
20
CEXT = 0pF
CEXT = 100pF
0
CEXT = 1nF, 0.1µF, 1µF
–20
–40
–60
–80
1
10
100
1k
RSOURCE (Ω)
10k
100k
2481 F15
Figure 14. +FS Error vs RSOURCE
at IN+
and IN–
80
–FS ERROR (ppm)
In applications where the sensor output impedance is low
(up to 10kΩ with no external bypass capacitor or up to
500Ω with 0.001µF bypass), complete settling of the input
occurs. In this case, no errors are introduced and direct
digitization of the sensor is possible.
RSOURCE
VCC = 5V
= 5V
60 VREF
VIN+ = 1.25V
–
40 VIN = 3.75V
TA = 25°C
20
CEXT = 1nF, 0.1µF, 1µF
0
CEXT = 100pF
–20
CEXT = 0pF
–40
–60
–80
1
10
100
1k
RSOURCE (Ω)
10k
100k
2481 F16
Figure 15. –FS Error vs RSOURCE at IN+ and IN–
the common mode input current is proportional to the
difference between VINCM and VREFCM. For a reference
common mode of 2.5V and an input common mode of
1.5V, the common mode input current is approximately
2481f
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0.74µA (in simultaneous 50Hz/60Hz rejection mode). This
common mode input current has no effect on the accuracy
if the external source impedances tied to IN+ and IN– are
matched. Mismatches in these source impedances lead to
a fixed offset error but do not affect the linearity or fullscale reading. A 1% mismatch in 1kΩ source resistances
leads to a 15ppm shift (74µV) in offset voltage.
In applications where the common mode input voltage
varies as a function of input signal level (single-ended
input, RTDs, half bridges, current sensors, etc.), the
common mode input current varies proportionally with
input voltage. For the case of balanced input impedances,
the common mode input current effects are rejected by the
large CMRR of the LTC2481 leading to little degradation in
accuracy. Mismatches in source impedances lead to gain
errors proportional to the difference between the common
mode input voltage and the common mode reference
voltage. 1% mismatches in 1kΩ source resistances lead
to worst-case gain errors on the order of 15ppm or 1LSB
(for 1V differences in reference and input common mode
voltage). Table 7 summarizes the effects of mismatched
source impedance and differences in reference/input common mode voltages.
Table 7. Suggested Input Configuration for LTC2481
BALANCED INPUT
RESISTANCES
UNBALANCED INPUT
RESISTANCES
Constant
VIN(CM) – VREF(CM)
CEXT > 1nF at Both
IN+ and IN–. Can Take
Large Source Resistance
with Negligible Error
CEXT > 1nF at Both IN+
and IN–. Can Take Large
Source Resistance.
Unbalanced Resistance
Results in an Offset
Which Can be Calibrated
Varying
VIN(CM) – VREF(CM)
CEXT > 1nF at Both IN+
and IN–. Can Take Large
Source Resistance with
Negligible Error
Minimize IN+ and IN–
Capacitors and Avoid
Large Source Impedance
(< 5k Recommended)
The magnitude of the dynamic input current depends upon
the size of the very stable internal sampling capacitors and
upon the accuracy of the converter sampling clock. The
accuracy of the internal clock over the entire temperature
and power supply range is typically better than 0.5%. Such
a specification can also be easily achieved by an external
clock. When relatively stable resistors (50ppm/°C) are
used for the external source impedance seen by IN+ and
IN–, the expected drift of the dynamic current and offset
will be insignificant (about 1% of their respective values
over the entire temperature and voltage range). Even for
the most stringent applications, a one-time calibration
operation may be sufficient.
In addition to the input sampling charge, the input ESD
protection diodes have a temperature dependent leakage
current. This current, nominally 1nA (±10nA max), results
in a small offset shift. A 1k source resistance will create a
1µV typical and 10µV maximum offset voltage.
Reference Current
In a similar fashion, the LTC2481 samples the differential
reference pins REF+ and REF– transferring small amount
of charge to and from the external driving circuits thus
producing a dynamic reference current. This current does
not change the converter offset, but it may degrade the
gain and INL performance. The effect of this current can be
analyzed in two distinct situations.
For relatively small values of the external reference capacitors (CREF < 1nF), the voltage on the sampling capacitor
settles almost completely and relatively large values for the
source impedance result in only small errors. Such values
for CREF will deteriorate the converter offset and gain
performance without significant benefits of reference filtering and the user is advised to avoid them.
Larger values of reference capacitors (CREF > 1nF) may be
required as reference filters in certain configurations. Such
capacitors will average the reference sampling charge and
the external source resistance will see a quasi constant
reference differential impedance.
In the following discussion, it is assumed the input and
reference common mode are the same. Using internal
oscillator for 60Hz mode, the typical differential reference
resistance is 1MΩ which generates a full-scale (VREF/2)
gain error of 0.51ppm for each ohm of source resistance
driving the REF+ and REF– pins. For 50Hz/60Hz mode, the
related difference resistance is 1.1MΩ and the resulting fullscale error is 0.46ppm for each ohm of source resistance
driving the REF+ and REF– pins. For 50Hz mode, the related
difference resistance is 1.2MΩ and the resulting full-scale
error is 0.42ppm for each ohm of source resistance driving
the REF+ and REF– pins. When CA0/F0 is driven by an
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In addition to this gain error, the converter INL performance is degraded by the reference source impedance.
The INL is caused by the input dependent terms
–VIN2/(VREF • REQ) – (0.5 • VREF • DT)/REQ in the reference
pin current as expressed in Figure 12. When using internal
oscillator and 60Hz mode, every 100Ω of reference source
resistance translates into about 0.67ppm additional INL
90
60
200
CREF = 0.01µF
0
0
20
1k
100
RSOURCE (Ω)
10k
–200
CREF = 1µF, 10µF
–300
–500
100k
CREF = 0.1µF
VCC = 5V
VREF = 5V
VIN+ = 1.25V
VIN– = 3.75V
TA = 25°C
0
200
600
400
RSOURCE (Ω)
800
2481 F17
Figure 16. +FS Error vs RSOURCE at REF+ or REF– (Small CREF)
2481 F20
10
0
–30
CREF = 0.01µF
CREF = 0.001µF
CREF = 100pF
CREF = 0pF
INL (ppm OF VREF)
–FS ERROR (ppm)
–20
1000
Figure 19. –FS Error vs RSOURCE at REF+ or REF– (Large CREF)
10
–10
1000
CREF = 0.01µF
–400
10
800
2481 F19
0
0
600
400
RSOURCE (Ω)
Figure 18. +FS Error vs RSOURCE at REF+ or REF– (Large CREF)
10
–10
200
–100
CREF = 0.01µF
CREF = 0.001µF
CREF = 100pF
CREF = 0pF
30
CREF = 0.1µF
300
0
50
40
CREF = 1µF, 10µF
100
–FS ERROR (ppm)
70
VCC = 5V
VREF = 5V
VIN+ = 3.75V
VIN– = 1.25V
TA = 25°C
400
VCC = 5V
VREF = 5V
VIN+ = 3.75V
VIN– = 1.25V
TA = 25°C
80
+FS ERROR (ppm)
500
+FS ERROR (ppm)
external oscillator with a frequency fEOSC (external conversion clock operation), the typical differential reference resistance is 0.30 • 1012/fEOSC Ω and each ohm of source
resistance driving the REF+ or REF– pins will result in 1.67
• 10–6 • fEOSCppm gain error. The typical +FS and –FS errors
for various combinations of source resistance seen by the
REF+ or REF– pins and external capacitance connected to
that pin are shown in Figures 16-19.
–40
–50
–60 VCC = 5V
VREF = 5V
–70 V + = 1.25V
IN
–
–80 VIN = 3.75V
TA = 25°C
–90
10
0
VCC = 5V
8 VREF = 5V
VIN(CM) = 2.5V
6 T = 25°C
A
4 CREF = 10µF
R = 1k
2
R = 500Ω
0
R = 100Ω
–2
–4
–6
–8
1k
100
RSOURCE (Ω)
10k
100k
2481 F18
Figure 17. –FS Error vs RSOURCE at REF+ or REF– (Small CREF)
–10
– 0.5
– 0.3
0.1
– 0.1
VIN/VREF (V)
0.3
0.5
2481 F21
Figure 20. INL vs DIFFERENTIAL Input Voltage and
Reference Source Resistance for CREF > 1µF
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The magnitude of the dynamic reference current depends
upon the size of the very stable internal sampling capacitors and upon the accuracy of the converter sampling
clock. The accuracy of the internal clock over the entire
temperature and power supply range is typically better
than 0.5%. Such a specification can also be easily achieved
by an external clock. When relatively stable resistors
(50ppm/°C) are used for the external source impedance
seen by VREF+ and VREF–, the expected drift of the dynamic
current gain error will be insignificant (about 1% of its
value over the entire temperature and voltage range). Even
for the most stringent applications a one-time calibration
operation may be sufficient.
In addition to the reference sampling charge, the reference pins
ESD protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±100nA max),
results in a small gain error. A 100Ω source resistance will
create a 0.05µV typical and 5µV maximum full-scale error.
OFFSET ERROR (ppm OF VREF)
VIN(CM) = VREF(CM)
VCC = VREF = 5V
VIN = 0V
CA0/F0 = EXT CLOCK
40
30
TA = 85°C
20
10
0
TA = 25°C
–10
0
10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
2481 F22
Figure 21. Offset Error vs Output Data Rate and Temperature
3500
3000
+FS ERROR (ppm OF VREF)
In applications where the reference and input common
mode voltages are different, extra errors are introduced.
For every 1V of the reference and input common mode
voltage difference (VREFCM – VINCM) and a 5V reference,
each Ohm of reference source resistance introduces an
extra (VREFCM – VINCM)/(VREF • REQ) full-scale gain error,
which is 0.074ppm when using internal oscillator and
60Hz mode. When using internal oscillator and 50Hz/60Hz
mode, the extra full-scale gain error is 0.067ppm. When
using internal oscillator and 50Hz mode, the extra gain
error is 0.061ppm. If an external clock is used, the corresponding extra gain error is 0.24 • 10–6 • fEOSCppm.
50
VIN(CM) = VREF(CM)
VCC = VREF = 5V
CA0/F0 = EXT CLOCK
2500
TA = 85°C
2000
1500
TA = 25°C
1000
500
0
0 10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
2481 F23
Figure 22. +FS Error vs Output Data Rate and Temperature
0
–500
–FS ERROR (ppm OF VREF)
error. When using internal oscillator and 50Hz/60Hz mode,
every 100Ω of reference source resistance translates into
about 0.61ppm additional INL error. When using internal
oscillator and 50Hz mode, every 100Ω of reference source
resistance translates into about 0.56ppm additional INL
error. When CA0/F0 is driven by an external oscillator with
a frequency fEOSC, every 100Ω of source resistance driving REF+ or REF– translates into about 2.18 • 10–6 •
fEOSCppm additional INL error. Figure 20 shows the typical INL error due to the source resistance driving the REF+
or REF– pins when large CREF values are used. The user is
advised to minimize the source impedance driving the
REF+ and REF– pins.
–1000
TA = 25°C
–1500
TA = 85°C
–2000
–2500
–3000
–3500
VIN(CM) = VREF(CM)
VCC = VREF = 5V
CA0/F0 = EXT CLOCK
0 10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
2481 F24
Figure 23. –FS Error vs Output Data Rate and Temperature
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Output Data Rate
When using its internal oscillator, the LTC2481 produces
up to 7.5 samples per second (sps) with a notch frequency
of 60Hz, 6.25sps with a notch frequency of 50Hz and
6.82sps with the 50Hz/60Hz rejection mode. The actual
output data rate will depend upon the length of the sleep
and data output phases which are controlled by the user
and which can be made insignificantly short. When operated with an external conversion clock (CA0/F0 connected
to an external oscillator), the LTC2481 output data rate can
be increased as desired. The duration of the conversion
phase is 41036/fEOSC. If fEOSC = 307.2kHz, the converter
behaves as if the internal oscillator is used and the notch
is set at 60Hz.
An increase in fEOSC over the nominal 307.2kHz will
translate into a proportional increase in the maximum
output data rate. The increase in output rate is nevertheless accompanied by three potential effects, which must
be carefully considered.
First, a change in fEOSC will result in a proportional change
in the internal notch position and in a reduction of the
converter differential mode rejection at the power line
frequency. In many applications, the subsequent performance degradation can be substantially reduced by relying
upon the LTC2481’s exceptional common mode rejection
and by carefully eliminating common mode to differential
mode conversion sources in the input circuit. The user
should avoid single-ended input filters and should maintain a very high degree of matching and symmetry in the
circuits driving the IN+ and IN– pins.
Second, the increase in clock frequency will increase
proportionally the amount of sampling charge transferred
through the input and the reference pins. If large external
input and/or reference capacitors (CIN, CREF) are used, the
previous section provides formulae for evaluating the
effect of the source resistance upon the converter performance for any value of fEOSC. If small external input and/or
reference capacitors (CIN, CREF) are used, the effect of the
external source resistance upon the LTC2481 typical performance can be inferred from Figures 14, 15, 16 and 17
in which the horizontal axis is scaled by 307200/fEOSC.
Third, an increase in the frequency of the external oscillator
above 1MHz (a more than 3X increase in the output data
rate) will start to decrease the effectiveness of the internal
autocalibration circuits. This will result in a progressive
degradation in the converter accuracy and linearity. Typical measured performance curves for output data rates up
to 100 readings per second are shown in Figures 21 to 28.
In order to obtain the highest possible level of accuracy
from this converter at output data rates above 20 readings
per second, the user is advised to maximize the power
supply voltage used and to limit the maximum ambient
operating temperature. In certain circumstances, a reduction of the differential reference voltage may be beneficial.
Input Bandwidth
The combined effect of the internal SINC4 digital filter and
of the analog and digital autocalibration circuits determines the LTC2481 input bandwidth. When the internal
oscillator is used with the notch set at 60Hz, the 3dB input
bandwidth is 3.63Hz. When the internal oscillator is used
with the notch set at 50Hz, the 3dB input bandwidth is
3.02Hz. If an external conversion clock generator of frequency fEOSC is connected to the CA0/F0 pin, the 3dB input
bandwidth is 11.8 • 10–6 • fEOSC.
Due to the complex filtering and calibration algorithms
utilized, the converter input bandwidth is not modeled
very accurately by a first order filter with the pole located
at the 3dB frequency. When the internal oscillator is used,
the shape of the LTC2481 input bandwidth is shown in
Figure 29. When an external oscillator of frequency fEOSC
is used, the shape of the LTC2481 input bandwidth can be
derived from Figure 29, 60Hz mode curve in which the
horizontal axis is scaled by fEOSC/307200.
The conversion noise (600nVRMS typical for VREF = 5V) can
be modeled by a white noise source connected to a noise
free converter. The noise spectral density is 47nV√Hz for
an infinite bandwidth source and 64nV√Hz for a single
0.5MHz pole source. From these numbers, it is clear that
particular attention must be given to the design of external
amplification circuits. Such circuits face the simultaneous
requirements of very low bandwidth (just a few Hz) in order
to reduce the output referred noise and relatively high
2481f
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24
24
VCC = VREF = 5V
TA = 25°C
22
22
RESOLUTION (BITS)
RESOLUTION (BITS)
TA = 85°C
20
18
16
14
12
VIN(CM) = VREF(CM)
VCC = VREF = 5V
VIN = 0V
CA0/F0 = EXT CLOCK
RES = LOG 2 (VREF/NOISERMS)
10
0 10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
20
VCC = 5V, VREF = 2.5V
18
16
14 VIN(CM) = VREF(CM)
VIN = 0V
CA0/F0 = EXT CLOCK
12 T = 25°C
A
RES = LOG 2 (VREF/NOISERMS)
10
0 10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
2481 F28
2481 F25
22
22
20
20
18
TA = 25°C
TA = 85°C
16
14
VIN(CM) = VREF(CM)
12 VCC = VREF = 5V
CA0/F0 = EXT CLOCK
RES = LOG 2 (VREF/INLMAX)
10
0 10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
2481 F26
Figure 25. Resolution (INLMAX ≤ 1LSB)
vs Output Data Rate and Temperature
20
OFFSET ERROR (ppm OF VREF)
Figure 27. Resolution (NoiseRMS ≤ 1LSB)
vs Output Data Rate and Reference Voltage
RESOLUTION (BITS)
RESOLUTION (BITS)
Figure 24. Resolution (NoiseRMS ≤ 1LSB)
vs Output Data Rate and Temperature
18
VCC = VREF = 5V
16
VCC = 5V, VREF = 2.5V
14
VIN(CM) = VREF(CM)
VIN = 0V
12 CA0/F0 = EXT CLOCK
TA = 25°C
RES = LOG 2 (VREF/INLMAX)
10
0 10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
2481 F29
Figure 28. Resolution (INLMAX ≤ 1LSB)
vs Output Data Rate and Reference Voltage
bandwidth (at least 500kHz) necessary to drive the input
switched-capacitor network. A possible solution is a high
gain, low bandwidth amplifier stage followed by a high
bandwidth unity-gain buffer.
VIN(CM) = VREF(CM)
VIN = 0V
15 CA0/F0 = EXT CLOCK
TA = 25°C
10
VCC = VREF = 5V
5
0
–5
VCC = 5V, VREF = 2.5V
–10
0 10 20 30 40 50 60 70 80 90 100
OUTPUT DATA RATE (READINGS/SEC)
2481 F27
Figure 26. Offset Error vs Output
Data Rate and Reference Voltage
When external amplifiers are driving the LTC2481, the
ADC input referred system noise calculation can be
simplified by Figure 30. The noise of an amplifier driving
the LTC2481 input pin can be modeled as a band limited
white noise source. Its bandwidth can be approximated
by the bandwidth of a single pole lowpass filter with a
corner frequency fi. The amplifier noise spectral density
is ni. From Figure 30, using fi as the x-axis selector, we
can find on the y-axis the noise equivalent bandwidth
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If the CA0/F0 pin is driven by an external oscillator of
frequency fEOSC, Figure 30 can still be used for noise
calculation if the x-axis is scaled by fEOSC/307200. For
large values of the ratio fEOSC/307200, the Figure 30 plot
accuracy begins to decrease, but at the same time the
LTC2481 noise floor rises and the noise contribution of the
driving amplifiers lose significance.
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2481 significantly
simplifies antialiasing filter requirements. Additionally,
the input current cancellation feature of the LTC2481
allows external lowpass filtering without degrading the DC
performance of the device.
The SINC4 digital filter provides greater than 120dB normal mode rejection at all frequencies except DC and
integer multiples of the modulator sampling frequency
(fS). The LTC2481’s autocalibration circuits further simplify the antialiasing requirements by additional normal
mode signal filtering both in the analog and digital domain.
Independent of the operating mode, fS = 256 • fN = 2048
• fOUTMAX where fN is the notch frequency and fOUTMAX is
the maximum output data rate. In the internal oscillator
mode with a 50Hz notch setting, fS = 12800Hz, with
50Hz/60Hz rejection, fS = 13960Hz and with a 60Hz notch
setting fS = 15360Hz. In the external oscillator mode, fS =
fEOSC/20. The performance of the normal mode rejection
is shown in Figures 31 and 32.
In 1x speed mode, the regions of low rejection occurring
at integer multiples of fS have a very narrow bandwidth.
Magnified details of the normal mode rejection curves are
INPUT SIGNAL ATTENUATION (dB)
0
–1
50Hz AND
60Hz MODE
–2
50Hz MODE
–3
60Hz MODE
–4
–5
–6
1
3
4
0
5
2
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2481 F30
Figure 29. Input Signal Using the Internal Oscillator
100
INPUT REFERRED NOISE
EQUIVALENT BANDWIDTH (Hz)
freqi of the input driving amplifier. This bandwidth includes the band limiting effects of the ADC internal
calibration and filtering. The noise of the driving amplifier referred to the converter input and including all these
effects can be calculated as N = ni • √freqi. The total
system noise (referred to the LTC2481 input) can now be
obtained by summing as square root of sum of squares
the three ADC input referred noise sources: the LTC2481
internal noise, the noise of the IN+ driving amplifier and
the noise of the IN– driving amplifier.
10
60Hz MODE
50Hz MODE
1
0.1
0.1
1
10 100 1k 10k 100k 1M
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz) 2481 F31
Figure 30. Input Refered Noise Equivalent Bandwidth
of an Input Connected White Noise Source
shown in Figure 33 (rejection near DC) and Figure 34
(rejection at fS = 256fN) where fN represents the notch
frequency. These curves have been derived for the external oscillator mode but they can be used in all operating
modes by appropriately selecting the fN value.
The user can expect to achieve this level of performance
using the internal oscillator as it is demonstrated by
Figures 35, 36 and 37. Typical measured values of the
normal mode rejection of the LTC2481 operating with an
internal oscillator and a 60Hz notch setting are shown in
Figure 35 superimposed over the theoretical calculated
curve. Similarly, the measured normal mode rejection of
the LTC2481 for the 50Hz rejection mode and 50Hz/60Hz
rejection mode are shown in Figures 36 and 37.
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As a result of these remarkable normal mode specifications, minimal (if any) antialias filtering is required in front
of the LTC2481. If passive RC components are placed in
front of the LTC2481, the input dynamic current should be
considered (see Input Current section). In this case, the
differential input current cancellation feature of the LTC2481
allows external RC networks without significant degradation in DC performance.
0
0
–10
–10
INPUT NORMAL MODE REJECTION (dB)
INPUT NORMAL MODE REJECTION (dB)
Traditional high order delta-sigma modulators, while providing very good linearity and resolution, suffer from
potential instabilities at large input signal levels. The proprietary architecture used for the LTC2481 third order
modulator resolves this problem and guarantees a predictable stable behavior at input signal levels of up to 150% of
full scale. In many industrial applications, it is not uncom-
mon to have to measure microvolt level signals superimposed on volt level perturbations and the LTC2481 is
eminently suited for such tasks. When the perturbation is
differential, the specification of interest is the normal mode
rejection for large input signal levels. With a reference
voltage VREF = 5V, the LTC2481 has a full-scale differential
input range of 5V peak-to-peak. Figures 38 and 39 show
measurement results for the LTC2481 normal mode rejection ratio with a 7.5V peak-to-peak (150% of full scale)
input signal superimposed over the more traditional normal mode rejection ratio results obtained with a 5V peakto-peak (full scale) input signal. In Figure 38, the LTC2481
uses the internal oscillator with the notch set at 60Hz and
in Figure 39 it uses the internal oscillator with the notch set
at 50Hz. It is clear that the LTC2481 rejection performance
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS11fS12fS
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2481 F33
2481 F32
INPUT NORMAL MODE REJECTION (dB)
0
fN = fEOSC/5120
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
0
fN
2fN 3fN 4fN 5fN 6fN 7fN
INPUT SIGNAL FREQUENCY (Hz)
8fN
2481 F34
Figure 33. Input Normal Mode Rejection at DC
Figure 32. Input Normal Mode Rejection at DC
0
INPUT NORMAL MODE REJECTION (dB)
Figure 31. Input Normal Mode Rejection,
Internal Oscillator and 50Hz Notch Mode
0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
250fN 252fN 254fN 256fN 258fN 260fN 262fN
INPUT SIGNAL FREQUENCY (Hz)
2481 F35
Figure 34. Input Normal Mode Rejection at fs = 256fN
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VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
VIN(P-P) = 5V
TA = 25°C
MEASURED DATA
CALCULATED DATA
–20
–40
– 60
–80
–100
–120
0
15
30
45
60
75
0
NORMAL MODE REJECTION (dB)
NORMAL MODE REJECTION (dB)
0
–40
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
VIN(P-P) = 5V
TA = 25°C
– 60
–80
–100
–120
90 105 120 135 150 165 180 195 210 225 240
INPUT FREQUENCY (Hz)
MEASURED DATA
CALCULATED DATA
–20
0
12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200
INPUT FREQUENCY (Hz)
2481 F36
2481 F37
Figure 35. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (60Hz Notch)
–20
–40
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
VIN(P-P) = 5V
TA = 25°C
– 60
–80
–100
–120
0
20
40
60
80
100
120
140
INPUT FREQUENCY (Hz)
160
180
200
220
2481 F38
Figure 37. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (50Hz/60Hz Mode)
0
NORMAL MODE REJECTION (dB)
MEASURED DATA
CALCULATED DATA
VIN(P-P) = 5V
VIN(P-P) = 7.5V
(150% OF FULL SCALE)
–20
– 60
–80
–100
–120
0
15
30
45
60
75
90 105 120 135 150 165 180 195 210 225 240
INPUT FREQUENCY (Hz)
Figure 38. Measured Input Normal Mode Rejection vs
Input Frequency with Input Perturbation of 150% Full
Scale (60Hz Notch)
VIN(P-P) = 5V
VIN(P-P) = 7.5V
(150% OF FULL SCALE)
–20
VCC = 5V
VREF = 5V
VINCM = 2.5V
TA = 25°C
–40
0
NORMAL MODE REJECTION (dB)
NORMAL MODE REJECTION (dB)
0
Figure 36. Input Normal Mode Rejection vs Input Frequency
with Input Perturbation of 100% Full Scale (50Hz Notch)
2481 F39
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
TA = 25°C
–40
– 60
–80
–100
–120
0
12.5 25 37.5 50 62.5 75 87.5 100 112.5 125 137.5 150 162.5 175 187.5 200
INPUT FREQUENCY (Hz)
2481 F40
Figure 39. Measured Input Normal Mode Rejection vs
Input Frequency with Input Perturbation of 150% Full
Scale (50Hz Notch)
2481f
31
LTC2481
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0
INPUT NORMAL REJECTION (dB)
INPUT NORMAL REJECTION (dB)
0
–20
–40
–60
–80
–100
–120
0
fN
2fN 3fN 4fN 5fN 6fN 7fN
INPUT SIGNAL FREQUENCY (fN)
–20
–40
–60
–80
–100
–120
248 250 252 254 256 258 260 262 264
INPUT SIGNAL FREQUENCY (fN)
8fN
2481 F42
2481 F41
Figure 41. Input Normal Mode Rejection 2x Speed Mode
Figure 40. Input Normal Mode Rejection 2x Speed Mode
–70
MEASURED DATA
VCC = 5V
CALCULATED DATA VREF = 5V
VINCM = 2.5V
VIN(P-P) = 5V
TA = 25°C
–20
–40
NORMAL MODE REJECTION (dB)
NORMAL MODE REJECTION (dB)
0
–60
–80
–100
–120
–80
NO AVERAGE
–90
WITH
RUNNING
AVERAGE
–100
–110
–120
–130
–140
0
25
50 75 100 125 150 175 200 225
INPUT FREQUENCY (Hz)
60
62
54 56
58
48 50
52
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2481 F43
2481 F44
Figure 43. Input Normal Mode Rejection 2x Speed Mode
Figure 42. Input Normal Mode Rejection vs Input
Frequency, 2x Speed Mode and 50Hz/60Hz Mode
5V
C8
1µF
C7
0.1µF
ISOTHERMAL
LT1236
2
+
G1
NC1M4V0
IN OUT
TRIM
GND
4
6
5
R2
2k
R7
8k
R8
1k
4
+
IN
IN–
5
1
2
REF+ VCC SCL
SDA
LTC2481 CA1
CA0/F0
1.7k
6
7
9
10
1.7k
REF– GND
3
8
2481 F45
TYPE K
THERMOCOUPLE
JACK
(OMEGA MPJ-K-F)
26.3C
Figure 44. Calibration Setup
2481f
32
LTC2481
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APPLICATIO S I FOR ATIO
is maintained with no compromises in this extreme situation. When operating with large input signal levels, the user
must observe that such signals do not violate the device
absolute maximum ratings.
Using the 2x speed mode of the LTC2481, the device
bypasses the digital offset calibration operation to double
the output data rate. The superior normal mode rejection
is maintained as shown in Figures 31 and 32. However, the
magnified details near DC and fS = 256fN are different, see
Figures 40 and 41. In 2x speed mode, the bandwidth is
11.4Hz for the 50Hz rejection mode, 13.6Hz for the 60Hz
rejection mode and 12.4Hz for the 50Hz/60Hz rejection
mode. Typical measured values of the normal mode
rejection of the LTC2481 operating with the internal oscillator and 2x speed mode is shown in Figure 42.
When the LTC2481 is configured in 2x speed mode, by
performing a running average, a SINC1 notch is combined
with the SINC4 digital filter, yielding the normal mode
rejection identical as that for the 1x speed mode. The
averaging operation still keeps the output rate with the
following algorithm:
Result 1 = average (sample 0, sample 1)
Result 2 = average (sample 1, sample 2)
……
Result n = average (sample n – 1, sample n)
The main advantage of the running average is that it
achieves simultaneous 50Hz/60Hz rejection at twice the
effective output rate, as shown in Figure 43. The raw
output data provides a better than 70dB rejection over
48Hz to 62.4Hz, which covers both 50Hz ±2% and 60Hz
±2%. With running average on, the rejection is better than
87dB for both 50Hz ±2% and 60Hz ±2%.
Complete Thermocouple Measurement System with
Cold Junction Compensation
The LTC2481 is ideal for direct digitization of thermocouples
and other low voltage output sensors. The input has a typical
offset error of 500nV (2.5µV max) offset drift of 10nV/°C
and a noise level of 600nVRMS. The input span may be
optimized for various sensors by setting the gain of the
PGA. Using an external 5V reference with a PGA gain of 64
gives a ±78mV input range—perfect for thermocouples.
Figure 45 (page 39 of this data sheet) is a complete type
K thermocouple meter. The only signal conditioning is a
simple surge protection network. In any thermocouple
meter, the cold junction temperature sensor must be at the
same temperature as the junction between the thermocouple materials and the copper printed circuit board
traces. The tiny LTC2481 can be tucked neatly underneath
an Omega MPJ-K-F thermocouple socket ensuring close
thermal coupling.
The LTC2481’s 1.4mV/°C PTAT circuit measures the cold
junction temperature. Once the thermocouple voltage and
cold junction temperature are known, there are many ways
of calculating the thermocouple temperature including a
straight-line approximation, lookup tables or a polynomial
curve fit. Calibration is performed by applying an accurate
500mV to the ADC input derived from an LT®1236 reference and measuring the local temperature with an accurate thermometer as shown in Figure 44. In calibration
mode, the up and down buttons are used to adjust the local
temperature reading until it matches an accurate thermometer. Both the voltage and temperature calibration
are easily automated.
The complete microcontroller code for this application is
available on the LTC2481 product webpage at:
http://www.linear.com
It can be used as a template for may different instruments
and it illustrates how to generate calibration coefficients
for the onboard temperature sensor. Extensive comments
detail the operation of the program. The read_LTC2481()
function controls the operation of the LTC2481 and is
listed below for reference.
2481f
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LTC2481
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/*
LTC248X.h
Processor setup and
Lots of useful defines for configuring the LTC2481 and LTC2485.
*/
#include <16F73.h>
#use delay(clock=6000000)
// Device
// 6MHz clock
//#fuses NOWDT,HS, PUT, NOPROTECT, NOBROWNOUT
// Configuration fuses
#rom 0x2007={0x3F3A} // Equivalent and more reliable fuse config.
#use I2C(master, sda=PIN_C5, scl=PIN_C3, SLOW)// Set up i2c port
#include "PCM73A.h"
// Various defines
#include "lcd.c"
// LCD driver functions
// Useful defines for the LTC2481 and LTC2485 - OR them together to make the
// 8 bit config word.
#define READ
0x01
// bitwise OR with address for read or write
#define WRITE
0x00
#define LTC248XADDR 0b01001000
// The one and only LTC248X in this circuit,
// with both address lines floating.
// Select gain - 1 to 256 (also depends on speed setting)
#define GAIN1 0b00000000
// G = 1
(SPD = 0), G = 1
(SPD = 1)
#define GAIN2 0b00100000
// G = 4
(SPD = 0), G = 2
(SPD = 1)
#define GAIN3 0b01000000
// G = 8
(SPD = 0), G = 4
(SPD = 1)
#define GAIN4 0b01100000
// G = 16 (SPD = 0), G = 8
(SPD = 1)
#define GAIN5 0b10000000
// G = 32 (SPD = 0), G = 16 (SPD = 1)
#define GAIN6 0b10100000
// G = 64 (SPD = 0), G = 32 (SPD = 1)
#define GAIN7 0b11000000
// G = 128 (SPD = 0), G = 64 (SPD = 1)
#define GAIN8 0b11100000
// G = 256 (SPD = 0), G = 128 (SPD = 1)
// Select ADC source - differential input or PTAT circuit
#define VIN
0b00000000
#define PTAT
0b00001000
// Select rejection frequency - 50, 55, or 60Hz
#define R50
0b00000010
#define R55
0b00000000
#define R60
0b00000100
// Speed settings is bit 7 in the 2nd byte
#define SLOW
0b00000000 // slow output rate with autozero
#define FAST
0b00000001 // fast output rate with no autozero
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LTC2481
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/*
LTC2481.c
Basic voltmeter test program for LTC2481
Reads LTC2481 input at gain = 1, 1X speed mode, converts to volts,
and prints voltage to a 2 line by 16 character LCD display.
Mark Thoren
Linear Technonlgy Corporation
June 23, 2005
Written for CCS PCM compiler, Version 3.182
*/
#include "LTC248X.h"
/*** read_LTC2481() ************************************************************
This is the function that actually does all the work of talking to the LTC2481.
Arguments:
addr - device address
config - configuration bits for next conversion
Returns:
zero if conversion is in progress,
32 bit signed integer with lower 8 bits clear, 24 bit LTC2481
output word in the upper 24 bits. Data is left-justified for
compatibility with the 24 bit LTC2485.
the i2c_xxxx() functions do the following:
void i2c_start(void): generate an i2c start or repeat start condition
void i2c_stop(void): generate an i2c stop condition
char i2c_read(boolean): return 8 bit i2c data while generating an ack or nack
boolean i2c_write(): send 8 bit i2c data and return ack or nack from slave device
These functions are very compiler specific, and can use either a hardware i2c
port or software emulation of an i2c port. This example uses software emulation.
A good starting point when porting to other processors is to write your own
i2c functions. Note that each processor has its own way of configuring
the i2c port, and different compilers may or may not have built-in functions
for the i2c port.
When in doubt, you can always write a "bit bang" function for troubleshooting
purposes.
The "fourbytes" structure allows byte access to the 32 bit return value:
struct fourbytes // Define structure of four consecutive bytes
{
// To allow byte access to a 32 bit int or float.
int8 te0;
//
int8 te1;
// The make32() function in this compiler will
int8 te2;
// also work, but a union of 4 bytes and a 32 bit int
int8 te3;
// is probably more portable.
};
Also note that the lower 4 bits are the configuration word from the previous
conversion.
2481f
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*******************************************************************************/
signed int32 read_LTC2481(char addr, char config)
{
struct fourbytes // Define structure of four consecutive bytes
{
// To allow byte access to a 32 bit int or float.
int8 te0;
//
int8 te1;
// The make32() function in this compiler will
int8 te2;
// also work, but a union of 4 bytes and a 32 bit int
int8 te3;
// is probably more portable.
};
union
// adc_code.bits32
all 32 bits
{
// adc_code.by.te0
byte 0
signed int32 bits32;
// adc_code.by.te1
byte 1
struct fourbytes by;
// adc_code.by.te2
byte 2
} adc_code;
// adc_code.by.te3
byte 3
// Start communication with LTC2481:
i2c_start();
if(i2c_write(addr | WRITE))// If no acknowledge, return zero
{
i2c_stop();
return 0;
}
i2c_write(config);
i2c_start();
i2c_write(addr | READ);
adc_code.by.te3 = i2c_read();
adc_code.by.te2 = i2c_read();
adc_code.by.te1 = i2c_read();
adc_code.by.te0 = 0;
i2c_stop();
return adc_code.bits32;
} // End of read_LTC2481()
/*** initialize() **************************************************************
Basic hardware initialization of controller and LCD, send Hello message to LCD
*******************************************************************************/
void initialize(void)
{
// General initialization stuff.
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_counters(RTCC_INTERNAL,RTCC_DIV_1);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
// This is the important part - configuring the SPI port
setup_spi(SPI_MASTER|SPI_L_TO_H|SPI_CLK_DIV_16|SPI_SS_DISABLED); // fast SPI clock
CKP = 0; // Set up clock edges - clock idles low, data changes on
CKE = 1; // falling edges, valid on rising edges.
2481f
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lcd_init();
delay_ms(6);
printf(lcd_putc, "Hello!");
delay_ms(500);
} // End of initialize()
// Initialize LCD
// Obligatory hello message
// for half a second
*** main() ********************************************************************
Main program initializes microcontroller registers, then reads the LTC2481
repeatedly
*******************************************************************************/
void main()
{
signed int32 x;
// Integer result from LTC2481
float voltage;
// Variable for floating point math
int16 timeout;
initialize();
// Hardware initialization
while(1)
{
delay_ms(1);
//
//
//
//
//
// Pace the main loop to something more than 1 ms
This is a basic error detection scheme. The LTC248X will never take more than
163.5ms, 149.9ms, or 136.5ms to complete a conversion in the 50Hz, 55Hz, and 60Hz
rejection modes, respectively.
If read_LTC248X() does not return non-zero within this time period, something
is wrong, such as an incorrect i2c address or bus conflict.
if((x = read_LTC2481(LTC248XADDR, GAIN1 | VIN | R55)) != 0)
{
// No timeout, everything is okay
timeout = 0;
// reset timer
x &= 0xFFFFFFC0;
// clear config bits so they don't affect math
x ^= 0x80000000;
// Invert MSB, result is 2's complement
voltage = (float) x;
// convert to float
voltage = voltage * 5.0 / 2147483648.0;// Multiply by Vref, divide by 2^31
lcd_putc('\f');
// Clear screen
lcd_gotoxy(1,1);
// Goto home position
printf(lcd_putc, "V %01.4f", voltage); // Display voltage
}
else
{
++timeout;
}
if(timeout > 200)
{
timeout = 200;
// Prevent rollover
lcd_gotoxy(1,1);
printf(lcd_putc, "ERROR - TIMEOUT");
delay_ms(500);
}
} // End of main loop
} // End of main()
2481f
37
LTC2481
U
PACKAGE DESCRIPTIO
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698)
0.675 ±0.05
3.50 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50
BSC
2.38 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
R = 0.115
TYP
6
3.00 ±0.10
(4 SIDES)
0.38 ± 0.10
10
1.65 ± 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
(DD10) DFN 1103
5
0.200 REF
1
0.75 ±0.05
0.00 – 0.05
0.25 ± 0.05
0.50 BSC
2.38 ±0.10
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
2481f
38
LTC2481
U
TYPICAL APPLICATIO
5V
PIC16F73
C8
1µF
C7
0.1µF
ISOTHERMAL
3
R2
2k
4
REF
IN+
2
VCC
LTC2481
TYPE K
THERMOCOUPLE
JACK
(OMEGA MPJ-K-F)
1.7k
1.7k
5
SCL
SDA
6
7
IN–
10
CA1 GND REF– CAO/FO
9
8
3
5V
D7
D6
2 × 16 CHARACTER
D5
LCD DISPLAY
D4
(OPTREX DMC162488
EN
OR SIMILAR)
RW
CONTRAST
GND D0 D1 D2 D3 RS
VCC
5V
1
3
R6
5k
2
5V
18
17
16
15
14
13
12
11
28
27
26
25
24
23
22
21
7
6
5
4
3
2
RC7
RC6
RC5
RC4
RC3
RC2
RC1
RC0
RB7
RB6
RB5
RB4
RB3
RB2
RB1
RB0
RA5
RA4
RA3
RA2
RA1
RA0
VDD
OSC1
OSC2
MCLR
VSS
VSS
20
5V
C6
0.1µF
9
Y1
6MHz
10
R1
1 10k
D1
BAT54
5V
9
19
2481 F46
CALIBRATE
2
1
R3
10k
DOWN
R4
10k
R5
10k
UP
Figure 45. Complete Type K Thermocouple Meter
2481f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
39
LTC2481
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1236A-5
Precision Bandgap Reference, 5V
0.05% Max Initial Accuracy, 5ppm/°C Drift
LT1460
Micropower Series Reference
0.075% Max Initial Accuracy, 10ppm/°C Max Drift
LT1790
Micropower SOT-23 Low Dropout Reference Family
0.05% Max Initial Accuracy, 10ppm/°C Max Drift
LTC2400
24-Bit, No Latency ∆Σ ADC in SO-8
0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA
LTC2410
24-Bit, No Latency ∆Σ ADC with Differential Inputs
0.8µVRMS Noise, 2ppm INL
LTC2411/LTC2411-1 24-Bit, No Latency ∆Σ ADCs with Differential Inputs in MSOP
1.45µVRMS Noise, 4ppm INL,
Simultaneous 50Hz/60Hz Rejection (LTC2411-1)
LTC2413
24-Bit, No Latency ∆Σ ADC with Differential Inputs
Simultaneous 50Hz/60Hz Rejection, 800nVRMS Noise
LTC2415/
LTC2415-1
24-Bit, No Latency ∆Σ ADCs with 15Hz Output Rate
Pin Compatible with the LTC2410
LTC2414/LTC2418
8-/16-Channel 24-Bit, No Latency ∆Σ ADCs
0.2ppm Noise, 2ppm INL, 3ppm Total Unadjusted Errors 200µA
LTC2440
High Speed, Low Noise 24-Bit ∆Σ ADC
3.5kHz Output Rate, 200nV Noise, 24.6 ENOBs
LTC2480
16-Bit ∆Σ ADC with Easy Drive Inputs, 600nV Noise,
Programmable Gain, and Temperature Sensor
Pin Compatible with LTC2482/LTC2484
LTC2482
16-Bit ∆Σ ADC with Easy Drive Inputs
Pin Compatible with LTC2480/LTC2484
LTC2483
16-Bit ∆Σ ADC with Easy Drive Inputs, I2C Interface
Pin Compatible with LTC2481/LTC2483
LTC2484
24-Bit ∆Σ ADC with Easy Drive Inputs
Pin Compatible with LTC2480/LTC2482
LTC2485
24-Bit ∆Σ ADC with Easy Drive Inputs, I2C Interface and
Temperature Sensor
Pin Compatible with LTC2481/LTC2483
2481f
40
Linear Technology Corporation
LT/LWI/TP 0805 500 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
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