SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Description
Features
•
•
•
The SX8723 is a data acquisition system based on
Semtech’s low power ZoomingADC™ technology. It
directly connects most types of miniature sensors
with a general purpose microcontroller.
•
•
With 2 differential inputs, it can adapt to multiple
sensor systems. Its digital outputs are used to bias
or reset the sensing elements.
•
•
•
•
•
Applications
•
•
•
•
Industrial pressure sensing
Industrial temperature sensing
Barometer
Compass
Up to 16-bit differential data acquisition
Programmable gain: (1/12 to 1000)
Sensor offset compensation up to 15 times
full scale of input signal
2 differential or 4 single-ended signal inputs
Programmable Resolution versus Speed
versus Supply current
2 digital outputs to bias Sensors
Internal or external voltage reference
Internal time base
Low-power (250 uA for 16b @ 500 S/s)
I2C interface
Ordering Information
Device
Package
SX8723E083TRT
MLPD-W-12 4x4
1) Available in tape and reel only
2) Lead free, WEEE and RoHS compliant.
Functional Block Diagram
SX8723
VBATT
-
VREF
+
+
-
-
REF MUX
+
ZoomingADCTM
AC2
AC3
AC4
AC5
SIGNAL MUX
AC0
AC1
PGA
ADC
READY
CONTROL LOGIC
D0
D1
GPIO
CHARGE
PUMP
4MHz
OSC
I2C
SCL
MCU
SDA
VPUMP
VSS
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Table of Contents
Description..............................................................................................................................................................1
Applications............................................................................................................................................................1
Features ..................................................................................................................................................................1
Ordering Information .............................................................................................................................................1
Functional Block Diagram.....................................................................................................................................1
Absolute Maximum Ratings ..................................................................................................................................4
Electrical Characteristics ......................................................................................................................................5
ZoomingADC Specifications.................................................................................................................................6
Timing Characteristics ..........................................................................................................................................8
2
I C Timing Waveforms ...........................................................................................................................................8
Pin Configuration ...................................................................................................................................................9
Marking Information...............................................................................................................................................9
Pin Description .......................................................................................................................................................9
Circuit Description ...............................................................................................................................................10
General Description .............................................................................................................................................. 10
Block Diagram ....................................................................................................................................................... 10
VREF ..................................................................................................................................................................... 10
GPIO ..................................................................................................................................................................... 11
Charge Pump ........................................................................................................................................................ 12
RC Oscillator ......................................................................................................................................................... 13
2
I C.......................................................................................................................................................................... 14
2
I C Communication Format ................................................................................................................................... 14
2
I C Address ........................................................................................................................................................... 14
ZoomingADC ........................................................................................................................................................15
Features ................................................................................................................................................................ 15
Overview ............................................................................................................................................................... 15
ZADC Description.................................................................................................................................................. 15
Acquisition Chain................................................................................................................................................... 15
Registers ............................................................................................................................................................... 17
ZADC Detailed Functionality Description .............................................................................................................. 18
Continuous-Time vs. On-Request......................................................................................................................... 18
Input Multiplexers .................................................................................................................................................. 19
Programmable Gain Amplifiers ............................................................................................................................. 20
PGA & ADC Enabling............................................................................................................................................ 21
PGA1 ..................................................................................................................................................................... 21
PGA2 ..................................................................................................................................................................... 21
PGA3 ..................................................................................................................................................................... 21
ADC Characteristics .............................................................................................................................................. 22
Conversion Sequence ........................................................................................................................................... 22
Over-Sampling Frequency .................................................................................................................................... 22
Over-Sampling Ratio ............................................................................................................................................. 23
Elementary Conversions ....................................................................................................................................... 23
Resolution ............................................................................................................................................................. 24
Conversion Time and Throughput......................................................................................................................... 25
Output Code Format ............................................................................................................................................. 26
Power Saving Modes ............................................................................................................................................ 27
Registers Map ....................................................................................................................................................... 28
Registers Descriptions .......................................................................................................................................... 28
RC Register........................................................................................................................................................... 28
GPIO Registers ..................................................................................................................................................... 29
ZADC Registers .................................................................................................................................................... 30
Mode Register ....................................................................................................................................................... 31
Optional Operating Modes: External Voltage Reference Option .......................................................................... 32
Application Hints..................................................................................................................................................33
Recommended Operation Mode and Registers Settings...................................................................................... 33
Operation Mode..................................................................................................................................................... 33
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Registers Settings ................................................................................................................................................. 33
Schematic.............................................................................................................................................................. 34
Input Impedance.................................................................................................................................................... 35
Switched Capacitor Principle ................................................................................................................................ 36
PGA Settling or Input Channel Modifications ........................................................................................................ 37
PGA Gain & Offset, Linearity and Noise ............................................................................................................... 37
Frequency Response ............................................................................................................................................ 38
Power Reduction ................................................................................................................................................... 39
Typical Performance ............................................................................................................................................40
Linearity ................................................................................................................................................................. 40
Integral Non-Linearity............................................................................................................................................ 40
Differential Non-Linearity....................................................................................................................................... 43
Noise ..................................................................................................................................................................... 44
Gain Error and Offset Error ................................................................................................................................... 46
Power Consumption .............................................................................................................................................. 47
PCB Layout Considerations................................................................................................................................49
How to Evaluate....................................................................................................................................................49
Package Outline Drawing: MLPD-W-12 4x4.......................................................................................................50
Land Pattern Drawing: MLPD-W-12 4x4.............................................................................................................51
Tape and Reel Specification ...............................................................................................................................52
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Absolute Maximum Ratings
Exceeding the specifications below may result in permanent damage to the device or device malfunction.
Operation outside the parameters specified in the Electrical Characteristics section is not implied.
Parameter
Symbol
Comments / Conditions
Min
Max
Unit
VSS - 0.3
5.7
V
Power supply
VBATT
Storage temperature
TSTORE
-55
150
°C
Temperature under bias
TBIAS
-40
140
°C
VVR_P
VSS - 300
VBATT + 300
mV
VSS - 300
VBATT + 300
mV
Peak reflow temperature
TPKG
Notes: This device is ESD sensitive. Use of standard ESD handling precautions is required.
260
°C
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Max sensor common mode
VVR_N
Input voltage
4
SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Electrical Characteristics
All values are valid within the operating conditions unless otherwise specified.
Parameter
Symbol
Comments / Conditions
Min
Typ
Max
Unit
Operating conditions
Power supply
VBATT
2.4
5.5
V
Operating temperature
TOP
-40
125
°C
Current consumption
Active current, @ 30 °C, 5.5 V
Active current, @ 30 °C, 3.3 V
Sleep current
I OP
I OP
Isleep
16 b @ 250 Sample/s
ADC, fs = 125 kHz
250
300
16 b @ 1 kSample/s
PGA3 + ADC, fs= 500 kHz
700
800
16 b + gain 1000 @ 1 kSample/s
PGA3,2,1 + ADC, fs = 500kHz
1000
1200
16 b @ 250 Sample/s
PGA3 + ADC, fs = 125 kHz
150
16 b @ 1 kSample/s
PGA3 + ADC, fs= 500 kHz
300
16 b + gain 1000 @ 1 kSample/s
PGA3,2,1 + ADC, fs = 500kHz
850
@ 30 °C
75
up to 85 °C
100
@125 °C
150
µA
µA
200
nA
Time base
Max ADC over-sampling frequency
FSmax
@ 25 °C
450
500
550
kHz
Min ADC over-sampling frequency
FSmin
@ 25 °C
56.25
62.5
68.75
kHz
Digital I/O
Input logic high
VIH
Input logic low
VIL
0.7
VBATT
0.3
Output logic high
VOH
IOH < 4mA
Output logic low
VOL
IOL < 4mA
0.4
VBATT-0.4
VBATT > 3V
1.19
VBATT
V
V
VREF: Internal Bandgap Reference
Absolute output voltage
Variation over Temperature
VBATT > 3V, ref to 25° C
Total Output Noise
VBATT > 3V, rms, broadband
V1.5 © 2007 Semtech Corp.
-1
1.22
1.25
V
+1
%
1
mV
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
ZoomingADC Specifications
Unless otherwise specified: Temperature TA = +25° C, VDD = +5V, GND = 0V, VREF, ADC = +5V, VIN = 0V, over-sampling frequency
fS = 250 kHz, PGA3 on with Gain = 1, PGA1&PGA2 off, offsets GDOff2 = GDOff3 = 0. Power operation: normal (IB_AMP_ADC[1:0] =
IB_AMP_PGA[1:0] = '01'). For resolution n = 12 bits: OSR = 32 and NELCONV = 4. For resolution n = 16 bits: OSR = 512 and NELCONV = 2.
Bandgap chopped at NELCONV rate.
Parameter
Symbol
Comments / Conditions
Min
Typ
Max
Unit
Analog Input
Differential Input Voltage Ranges
VIN = (VINP - VINN)
Gain = 1, OSR = 32 (Note 1)
-2.42
2.42
V
Gain = 100, OSR = 32
-24.2
24.2
mV
Gain = 1000, OSR = 32
-2.42
2.42
mV
VDD
V
Reference Voltage Range
VREF, ADC = (VREFP – VREFN)
Programmable Gain Amplifier (PGA)
Total PGA Gain
GDTOT
(Note 1)
1/12
1000
V/V
PGA1 Gain
GD1
See Table 5
1
10
V/V
PGA2 Gain
GD2
See Table 6
1
10
V/V
PGA3 Gain
GD3
Step = 1/12 V/V, See Table 8
0
127/12
V/V
Gain Setting Precision (each stage)
-3
±0.5
3
±5
Gain Temperature Dependence
%
ppm/°C
PGA2 Offset
GDoff2
Step = 0.2 V/V, See Table 7
-1
1
V/V
PGA3 Offset
GDoff3
Step = 1/12 V/V, See Table 9
-63/12
63/12
V/V
Offset Setting Precision (PGA2 or 3)
(Note 2)
-3
±0.5
3
±5
Offset Temperature Dependence
Input Impedance PGA1
Input Impedance PGA2, PGA3
Output RMS noise
%
ppm/°C
Gain = 1 (Note 3)
1500
kΩ
Gain = 10 (Note 3)
150
kΩ
Maximal gain (Note 3)
150
kΩ
PGA1 (Note 4)
205
µV
PGA2 (Note 5)
340
µV
PGA3 (Note 6)
365
µV
ADC Static Performance
Resolution, n
(Note 7)
No Missing Codes
(Note 8)
Gain Error
(Note 9)
Offset Error
6
n = 16 bits (Note 10)
Integral Non-Linearity, INL
Differential Non-Linearity, DNL
±0.15
%
±1
%
±1
LSB
n = 12 Bits (Note 11)
±0.6
LSB
±1.5
LSB
n = 12 Bits (Note 12)
±0.5
LSB
n = 16 Bits (Note 12)
±0.5
LSB
VSS-0.3
PSRR
Bits
Bits
n = 16 Bits (Note 11)
Common Mode input range
Power Supply Rejection Ratio
16
16
VBATT+0.3
V
VDD = 5V ± 0.3V (Note 13)
78
dB
VDD = 3V ± 0.3V (Note 13)
72
dB
ADC Dynamic Performance
Conversion Time
TCONV
Throughput Rate (Continuous Mode)
1/TCONV
Nbr of Initialization Cycles
NINIT
n = 12 bits (Note 14)
133
cycles/fS
n = 16 bits (Note 14)
1027
cycles/fS
n = 12 bits, fS = 250kHz
1.88
kSps
n = 16 bits, fS = 250kHz
0.485
kSps
0
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Parameter
Nbr of End Conversion Cycles
Symbol
Comments / Conditions
NEND
PGA Stabilization Delay
Min
Typ
0
(Note 15)
Max
5
OSR
Unit
cycles
cycles
ADC Digital Output
Binary Two’s Complement
See Table 15 and Table 16
Output Data Coding
Power Supply
Voltage Supply Range
VDD
Analog Quiescent Current
2.4
5
5.5
V
Only ZoomingADC
VDD = 5V/3V
800/675
µA
ADC Only Consumption
VDD = 5V/3V
260/190
µA
PGA1 Consumption
VDD = 5V/3V
190/170
µA
PGA2 Consumption
VDD = 5V/3V
150/135
µA
PGA3 Consumption
VDD = 5V/3V
200/180
µA
Analog Power Dissipation
All PGAs & ADC Active
Normal Power Mode
VDD = 5V/3V (Note 16)
4.0/2.0
mW
3/4 Power Reduction Mode
VDD = 5V/3V (Note 17)
3.2/1.6
mW
1/2 Power Reduction Mode
VDD = 5V/3V (Note 18)
2.4/1.1
mW
1/4 Power Reduction Mode
VDD = 5V/3V (Note 19)
1.5/0.7
mW
Total Consumption
IQ
Temperature
Operating Range
-40
125
°C
Notes:
(1) Gain
defined
as
overall
PGA
gain
GDTOT
=
GD1⋅GD2⋅GD3.
Maximum
input
voltage
is
given
by:
VIN, MAX = ±(VREF,ADC/2)⋅(OSR/OSR+1).
(2) Offset due to tolerance on GDoff2 or GDoff3 setting. For small intrinsic offset, use only ADC and PGA1.
(3) Measured with block connected to inputs through AMUX block. Normalized input sampling frequency for input impedance is fS =
500kHz. This figure must be multiplied by 2 for fS = 250kHz, 4 for fS = 125kHz. Input impedance is proportional to 1/ fS.
(4) Figure independent on PGA1 gain and sampling frequency fS.
(5) Figure independent on PGA2 gain and sampling frequency fS.
(6) Figure independent on PGA3 gain and sampling frequency fS.
(7) Resolution is given by n = 2⋅log2(OSR) + log2(NELCONV). OSR can be set between 8 and 1024, in powers of 2. NELCONV can be set to 1,
2, 4 or 8.
(8) If a ramp signal is applied to the input, all digital codes appear in the resulting ADC output data.
(9) Gain error is defined as the amount of deviation between the ideal (theoretical) transfer function and the measured transfer function
(with the offset error removed).
(10) Offset error is defined as the output code error for a zero volt input (ideally, output code = 0). For ± 1 LSB offset, NELCONV must be ≥2.
(11) INL defined as the deviation of the DC transfer curve of each individual code from the best-fit straight line. This specification holds
over the full scale. (For 16 bits INL set PGA3 on).
(12) DNL is defined as the difference (in LSB) between the ideal (1 LSB) and measured code transitions for successive codes.
(13) Figures for Gains = 1 to 100. PSRR is defined as the amount of change in the ADC output value as the power supply voltage
changes.
(14) Conversion time is given by: TCONV = (NELCONV ⋅ (OSR + 1) + 1) / fS. OSR can be set between 8 and 1024, in powers of 2. NELCONV can
be set to 1, 2, 4 or 8.
(15) PGAs are reset after each writing operation to registers RegACCfg1-5. The ADC must be started after a PGA or inputs commonmode stabilization delay. This is done by writing bit Start several cycles after PGA settings modification or channel switching. Delay
between PGA start or input channel switching and ADC start should be equivalent to OSR (between 8 and 1024) number of cycles.
This delay does not apply to conversions made without the PGAs.
(16) Nominal (maximum) bias currents in PGAs and ADC, i.e. IB_AMP_PGA[1:0] = '11' and IB_AMP_ADC[1:0] = '11'.
(17) Bias currents in PGAs and ADC set to 3/4 of nominal values, i.e. IB_AMP_PGA[1:0] = '10', IB_AMP_ADC[1:0] = '10'.
(18) Bias currents in PGAs and ADC set to 1/2 of nominal values, i.e. IB_AMP_PGA[1:0] = '01', IB_AMP_ADC[1:0] = '01'.
(19) Bias currents in PGAs and ADC set to 1/4 of nominal values, i.e. IB_AMP_PGA[1:0] = '00', IB_AMP_ADC[1:0] = '00'.
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Timing Characteristics
Parameter
Symbol
Comments / Conditions
Min
Typ
Max
Unit
Interrupt (Ready) timing specification
READY pulse width (2)
tIRQ
1
1/FS
I2C timing specifications(1)
SCL clock frequency
fSCL
0
400
kHz
SCL low period
tLOW
1.3
µs
SCL high period
tHIGH
0.6
µs
Data setup time
tSU;DAT
100
ns
Data hold time
tHD;DAT
0
ns
Repeated start setup time
tSU;STA
0.6
µs
Start condition hold time
tHD;STA
0.6
µs
Stop condition hold time
tSU;STO
0.6
µs
Bus free time between stop and start
tBUF
1.3
µs
Input glitch suppression
tSP
50
ns
Notes:
(1) All timing specifications are referred to VILmin and VIHmax voltage levels defined for the SCL and SDA pins.
(2) The READY pulse indicates End of Conversion. This is a Low going pulse of duration equal to one cycle of the ADC sampling rate.
I2C Timing Waveforms
SDA
SCL
tSU;STA
tHD;STA
tSU;STO
tBUF
2
Figure 1 - I C Start and Stop timings
SDA
SCL
tLOW
tHIGH
tSU;DAT tHD;DAT
tSP
2
Figure 2 - I C Data timings
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Pin Configuration
Marking Information
8723
yyww
xxxxx
xxxxx
Eyww = Date code
xxxx = Semtech lot number
Pin Description
Pin
Name
Type
Function
1
AC4
Analog Input
Differential sensor input in conjunction with AC5
2
AC5
Analog Input
Differential sensor input in conjunction with AC4
3
VBATT
Power Input
2.4V to 5.5V power supply
4
VSS
Power Input
Chip Ground
5
READY
Digital Output
Conversion complete flag.
6
D1
Digital IO + analog
Digital output sensor drive (VBATT or VSS)
VREF Input in optional operating mode
7
D0
Digital IO + analog
Digital output sensor drive (VBATT or VSS)
VREF Output in optional operating mode
8
SDA
Digital IO
I2C Data
9
SCL
Digital IO
I2C Clock. Up to 400KHz.
10
VPUMP
Power IO
Charge pump output. Raises ADC supply above VBATT if VBATT supply is too low.
Recommended range for capacitor is 1nF to 10 nF. Connect the capacitor to GND.
11
AC2
Analog Input
Differential sensor input in conjunction with AC3
12
AC3
Analog Input
Differential sensor input in conjunction with AC2
13
VSS
Power Input
Bottom ground pad (1)
Notes:
(1) This pin is internally connected to VSS. It should also be connected to VSS on PCB to reduce noise and improve thermal behavior.
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Circuit Description
General Description
The SX8723 is a complete low-power acquisition path with programmable gain, acquisition speed and
resolution.
Block Diagram
SX8723
VBATT
-
VREF
+
+
-
-
ZoomingADC
REF MUX
+
TM
AC2
AC3
AC4
AC5
SIGNAL MUX
AC0
AC1
PGA
ADC
READY
CONTROL LOGIC
D0/REFOUT
D1/REFIN
GPIO
CHARGE
PUMP
4MHz
OSC
I 2C
SCL
SDA
VPUMP
VSS
Figure 3 - SX8723 Block Diagram
VREF
The internally generated VREF is a trimmed bandgap reference with a nominal value of 1.22V that provides a
stable voltage reference for the ZoomingADC.
This reference voltage is directly connected to one of the ZoomingADC reference multiplexer inputs.
The bandgap voltage stability is only guaranteed for VBATT voltages of 3V and above. As VBATT drops down to
2.4V, the bandgap voltage could reduce by up to 50mV.
The bandgap has relatively weak output drive so it is recommended that if the bandgap is required as a signal
input then PGA1 must be enabled with Gain = 1.
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
GPIO
The GPIO block is a multipurpose 4 bit input/output port. In addition to digital behavior, D0 and D1 pins can be
programmed as analog pins in order to be used as output (reference voltage monitoring) and input for an
external reference voltage (For further details see Figure 14, Figure 15, Figure 16 and Figure 17). Each port
terminal can be individually selected as digital input or output.
RegOut[4]
RegOut[0]
0
D0/REFOUT
1
RegIn[0]
RegMode[1]
VREF
+
0
-
1
RegMode[0]
ZoomingADC
RegOut[5]
RegOut[1]
1
D1/REFIN
0
RegIn[1]
Figure 4 - GPIO Block Diagram
th
The direction of each bit within the GPIO block (input only or input/output) can be individually set using the 4
th
and 5 bits of the RegOut register. If D[x]_DIR = 1, both the input and output buffer are active on the
corresponding GPIO block pin. If D[x]_DIR = 0, the corresponding GPIO block pin is an input only and the
output buffer is in high impedance. After power on reset the GPIO block pins are in input/output mode
(D[x]_DIR are reset to 1)
The input values of GPIO block are available in RegIn register (read only). Reading is always direct – there is
no debounce function in the GPIO block. In case of possible noise on input signals, an external hardware filter
has to be realized. The input buffer is also active when the GPIO block is defined as output and the effective
value on the pin can be read back.
st
nd
Data stored in the 1 and 2 bits of RegOut register are outputted at GPIO block if D[x]_DIR = 1. The default
values after power on reset is low (0).
The digital pins are able to deliver a driving current up to 8 mA.
When the bits VREF_D0_OUT and VREF_D1_IN in the RegMode register are set to 1 the D0 and D1 pins
digital behavior are automatically bypassed in order to either input or output the voltage reference signals.
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Charge Pump
This block generates a supply voltage able to power the analog switch drive levels on the chip.
The minimum acceptable switch supply is 3V which means that if VBATT drops below 3V then the block should
be activated to generate a voltage of 3V or above. If VBATT is greater than 3V then VBATT may be switched
straight through to the VPUMP output.
If control input bit MULT_FORCE_OFF = 1 in RegMode register then the charge pump is disabled and VBATT is
permanently connected to VPUMP.
If control input bit MULT_FORCE_ON = 1 in RegMode register then the charge pump is permanently enabled.
This overrides MULT_FORCE_OFF bit in RegMode register.
If MULT_FORCE_ON = 0 and MULT_FORCE_OFF = 0 bits in RegMode register then the charge pump will
start if VBATT drops below 3V, otherwise VBATT will be switched directly through to VPUMP.
These controls are supplied to give the user the option of fixing the charge pump state to avoid it turning off and
on when VBATT is close to 3V.
The cell will use the on-chip bandgap reference and comparator to detect when VBATT is too low. When
activated, the block will use the charge pump to boost the VBATT voltage to above 3V but with diode limiting to
ensure that the generated voltage never exceeds 0.7V above VBATT.
An external capacitor is required on VPUMP whenever the power supply is supposed to be less or drop below 3V.
This capacitor should be large enough to ensure that generated voltage is smooth enough to avoid affecting
conversion accuracy but not so large that it gives an unacceptable settling time. A recommended value is
around 2.2nF.
The block will also indicate when the pumped output voltage is sufficiently high to allow ADC conversions to be
started. This will be a simple comparison which will give a ready signal when the VPUMP output is 3V or above.
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
RC Oscillator
This block provides the master clock reference for the chip. It produces a clock at 4 MHz which is divided
internally in order to generate the clock sources needed by the other blocks.
The oscillator technique is a low power relaxation design and it is designed to vary as little as possible over
temperature and supply voltage.
This oscillator is trimmed at manufacture chip test.
The RC oscillator will start up after a chip reset to allow the trimming values to be read and calibration registers
2
and I C address set to their programmed values. Once this has been done, the oscillator will be shut down and
2
the chip will enter a sleep state while waiting for an I C communication.
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
I2C
2
2
The I C interface gives access to the chip registers. It complies with the I C protocol specifications, restricted to
the slave side of the communication.
General features:
• Slave only operation
• Fast mode operation (up to 400 kHz)
• Combined read and write mode support
• General call reset support
• 7-bit device address customization
2
• Stretch I C clock SCL only before sending ACK/NACK
2
The interface handles I C communication at the transaction level: the processor is only aware of read and writes
transactions. A read transaction is an external request to get the content of system memory location and a write
transaction is an external request to write the content of a system memory location.
2
I C Communication Format
Start
SDA
Slave Address
1
0
0
1
0
0
0
W
ACK
Memory Address
0
0
A7
9
1
A6
A5
A4
A3
ACK
A2
A1
Start
A0
Slave Address
1
0
0
1
0
R
0
0
ACK
1
Data
D7
D6
D5
D4
NACK Stop
D3
D2
D1
D0
SCL
1
Master
9
SX8723
Master
1
9
SX8723
1
9
Master
SX8723
Master
Figure 5 - Timing Diagram for Reading from SX8723
Start
SDA
Slave Address
1
0
0
1
0
0
0
W
ACK
Memory Address
0
0
A7
9
1
A6
A5
A4
A3
ACK
A2
A1
Start
A0
Slave Address
1
0
0
1
0
W
0
0
ACK
0
Data
D7
D6
D5
D4
ACK Stop
D3
D2
D1
D0
SCL
1
Master
9
SX8723
Master
1
9
SX8723
Master
1
9
SX8723
Master
SX8723
Master
Data
NACK Stop
Figure 6 - Timing Diagram for Writing to the SX8723
Start
SDA
Slave Address
1
0
0
1
0
0
0
W
ACK
RegACOutMsb
0
0
0
9
1
1
0
1
0
ACK
0
0
Start
Slave Address
1
1
0
0
1
0
R
0
0
ACK
1
D7
D6
D5
D4
D3
D2
D1
...
D0
...
SCL
1
Ready
Master
Start
SDA ...
Slave Address
1
0
0
1
0
0
0
9
SX8723
Master
SX8723
W
ACK
RegACOutLsb
ACK
0
0
0
9
1
1
0
1
0
0
0
0
1
9
1
9
Master
Start
SX8723
Slave Address
1
0
0
1
0
R
0
0
ACK
1
Master
Data
D7
D6
D5
D4
NACK Stop
D3
D2
D1
D0
SCL ...
Ready
1
Master
SX8723
9
Master
SX8723
1
9
Master
1
9
SX8723
Master
Figure 7 - Timing Diagram for Reading an ADC Sample from SX8723
2
I C Address
2
The default I C slave address is 1001000 in binary.
2
This is the standard part I C slave address. Other addresses between 1001001 and 1001111 are available by
special request.
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
ZoomingADC
Features
The ZoomingADC is a complete and versatile low-power analog front-end interface typically intended for sensing
applications.
In the following text the ZoomingADC will be referred as ZADC.
The key features of the ZADC are:
• Programmable 6 to 16-bit dynamic range over-sampled ADC
• Flexible gain programming between 0.5 and 1000
• Flexible and large range offset compensation
• 3-channel differential or 5-channel single-ended input
• 2-channel differential reference inputs
• Power saving modes
Overview
Analog
Inputs
VSS
VREF
AC2
AC3
AC4
AC5
fs
AC0
AC1
fs
PGA1
VIN
PGA2
PGA3
VD1
GD1
VD2
+
GD2
VIN,ADC
+
GD3
-
16
ADC
-
OFF2
OFF3
Input
Selection
VBATT
Reference VSS
Inputs VREF
VSS
+
-
VREF,ADC
+
-
Gain 1
Gain 2
Offset 2
Reference
Selection
Gain 3
Offset 3
ZOOM
Figure 8 - ZADC General Functional Block Diagram
The total acquisition chain consists of an input multiplexer, 3 programmable gain amplifier stages and an over
sampled A/D converter. The reference voltage can be selected on two different channels. Two offset compensation
amplifiers allow for a wide offset compensation range. The programmable gain and offset allow the application to
zoom in on a small portion of the reference voltage defined input range.
ZADC Description
Acquisition Chain
Figure 8 shows the general block diagram of the acquisition chain (AC). A control block (not shown in Figure 8)
manages all communications with the I2C peripheral. The clocking is derived from the internal 4 MHz Oscillator.
Analog inputs can be selected through a 6 input multiplexer, while reference input is selected between two differential
channels. It should however be noted that only 5 acquisition channels (including the VREF) are available when
configured as single ended since the input amplifier is always operating in differential mode with both positive and
negative input selected through the multiplexer.
The core of the zooming section is made of three differential programmable amplifiers (PGA). After selection of an
input and reference signals VIN and VREF,ADC combination, the input voltage is modulated and amplified through
stages 1 to 3. Fine gain programming up to 1'000 V/V is possible. In addition, the last two stages provide
programmable offset. Each amplifier can be bypassed if needed.
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
The output of the PGA stages is directly fed to the analog-to-digital converter (ADC), which converts the signal
VIN,ADC into digital.
Like most ADCs intended for instrumentation or sensing applications, the ZoomingADC is an over-sampled
1
converter (See Note ). The ADC is a so-called incremental converter; with bipolar operation (the ADC accepts
both positive and negative differential input voltages). In first approximation, the ADC output result relative to
full-scale (FS) delivers the quantity:
VIN , ADC
OUTADC
≅
FS / 2
VREF , ADC / 2
Equation 1
in two's complement (see Equation 4 and Equation 5 for details). The output code OUTADC is -FS/2 to +FS/2 for
VIN,ADC ≅ -VREF,ADC/2 to +VREF,ADC/2 respectively. As will be shown, VIN,ADC is related to input voltage VIN by the
relationship:
VIN , ADC = GDTOT ⋅ VIN − GDoff TOT ⋅ VREF , ADC
(V)
Equation 2
where GDTOT is the total PGA gain, and GDoffTOT is the total PGA offset.
1 Note: Over-sampled converters are operated with a sampling frequency fS much higher than the input signal's Nyquist rate (typically fS is
20-1'000 times the input signal bandwidth). The sampling frequency to throughput ratio is large (typically 10-500). These converters include
digital decimation filtering. They are mainly used for high resolution, and/or low-to-medium speed applications.
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Registers
The system has a bank of eight 8-bit registers: six registers are used to configure the acquisition chain
(RegAcCfg0 to 5), and two registers are used to store the output code of the analog-to-digital conversion
(RegAcOutMsb & Lsb).
Bit Position
Register Name
7
6
5
4
3
RegACOutLsb
OUT[7:0]
RegACOutMsb
OUT[15:8]
SET_NELC[1:0]
01
2
SET_OSR[2:0]
010
1
0
CONT
0
0
RegACCfg0
Default values:
START
0
RegACCfg1
Default values:
IB_AMP_ADC[1:0]
11
IB_AMP_PGA[1:0]
11
ENABLE[3:0]
0000
RegACCfg2
Default values:
FIN[1:0]
00
PGA2_GAIN[1:0]
00
PGA2_OFFSET[3:0]
0000
RegACCfg3
Default values:
PGA1_G
0
PGA3_GAIN[6:0]
0001100
RegACCfg4
Default values:
0
PGA3_OFFSET[6:0]
0000000
RegACCfg5
Default values:
BUSY
0
DEF
0
AMUX[4:0]
00000
VMUX
0
Table 1 - Peripheral Registers to Configure the Acquisition Chain (AC)
and to Store the Analog-to-Digital Conversion (ADC) Result
With:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
OUT: (r) digital output code of the analog-to-digital converter. (MSB = OUT[15])
START: (w) setting this bit triggers a single conversion (after the current one is finished). This bit always reads back 0.
SET_NELC: (rw) sets the number of elementary conversions to 2 SET_NELC[1:0]. To compensate for offsets, the input signal is
chopped between elementary conversions (1,2,4,8).
SET_OSR: (rw) sets the over-sampling rate (OSR) of an elementary conversion to 2(3+SET_OSR[2:0]). OSR = 8, 16, 32, ..., 512,
1024.
CONT: (rw) setting this bit starts a conversion. A new conversion will automatically begin as long as the bit remains at 1.
TEST: bit only used for test purposes. In normal mode, this bit is forced to 0 and cannot be overwritten.
IB_AMP_ADC: (rw) sets the bias current in the ADC to 0.25*(1+ IB_AMP_ADC[1:0]) of the normal operation current (25, 50, 75 or
100% of nominal current). To be used for low-power, low-speed operation.
IB_AMP_PGA: (rw) sets the bias current in the PGAs to 0.25*(1+IB_AMP_PGA[1:0]) of the normal operation current (25, 50, 75 or
100% of nominal current). To be used for low-power, low-speed operation.
ENABLE: (rw) enables the ADC modulator (bit 0) and the different stages of the PGAs (PGAi by bit i=1,2,3). PGA stages that are
disabled are bypassed.
FIN: (rw) These bits set the over sampling frequency of the acquisition chain. Expressed as a fraction of the oscillator frequency,
the sampling frequency is given as: 11 500 kHz, 10 250 kHz, 01 125 kHz, 00 62.5 kHz.
PGA1_GAIN: (rw) sets the gain of the first stage: 0 1, 1 10.
PGA2_GAIN: (rw) sets the gain of the second stage: 00 1, 01 2, 10 5, 11 10.
PGA3_GAIN: (rw) sets the gain of the third stage to PGA3_GAIN[6:0]⋅1/12.
PGA2_OFFSET: (rw) sets the offset of the second stage between –1 and +1, with increments of 0.2. The MSB gives the sign
(0 → positive, 1 → negative); amplitude is coded with the bits PGA2_OFFSET[5:0].
PGA3_OFFSET: (rw) sets the offset of the third stage between –5.25 and +5.25, with increments of 1/12. The MSB gives the sign
(0 → positive, 1 → negative); amplitude is coded with the bits PGA3_OFFSET[5:0].
BUSY: (r) set to 1 if a conversion is running.
DEF: (w) sets all values to their defaults (PGA disabled, max speed, nominal modulator bias current, 2 elementary conversions,
over-sampling rate of 32) and starts a new conversion without waiting the end of the preceding one.
AMUX(4:0): (rw) AMUX(4) sets the mode (0 differential inputs, 1 single ended inputs with A0 = common reference) AMUX(3)
sets the sign (0 straight, 1 cross) AMUX(2:0) sets the channel.
VMUX: (rw) sets the differential reference channel (0 VBATT, 1 VREF).
(r = read; w = write; rw = read & write)
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ZoomingADC™ for Pressure and Temperature Sensing
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ZADC Detailed Functionality Description
Continuous-Time vs. On-Request
The ADC can be operated in two distinct modes: "continuous-time" and "on-request" modes (selected using the
bit CONT).
In "continuous-time" mode, the input signal is repeatedly converted into digital. After a conversion is finished, a
new one is automatically initiated. The new value is then written in the result register, and the corresponding
internal trigger pulse is generated. This operation is sketched in Figure 9. The conversion time in this case is
defined as TCONV.
TCONV
Internal Trig
Output Code
RegACOut[15:0]
BUSY
IRQ/READY
Figure 9 - ADC "Continuous-Time" Operation
In the "on-request" mode, the internal behavior of the converter is the same as in the "continuous-time" mode,
but the conversion is initiated on user request (with the START bit). As shown in Figure 10, the conversion time
is also TCONV.
TCONV
Internal Trig
START Request
Output Code
RegACOut[15:0]
BUSY
IRQ/READY
Figure 10 - ADC "On-Request" Operation
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Input Multiplexers
The ZoomingADC has eight analog inputs AC0 to AC7 and four reference inputs AC_R0 to AC_R3. Let us first
define the differential input voltage VIN and reference voltage VREF,ADC respectively as:
VIN = VINP − VINN
(V)
Equation 3
and:
VREF , ADC = VREFP − VREFN
(V)
Equation 4
As shown in Table 2, the inputs can be configured in two ways: either as 4 differential channels (VIN1 = AC1 AC0,..., VIN3 = AC5 – AC4), or AC0 can be used as a common reference, providing 5 signal paths all referred to
AC0. The control word for the analog input selection is AMUX[4:0]. Notice that the bit AMUX[3] controls the sign
of the input voltage.
AMUX[4:0]
(RegACCfg5[5:1])
VINP
VINN
AMUX[4:0]
(RegACCfg5[5:1])
VINP
VINN
00x00
AC1 (VREF)
AC0 (VSS)
01x00
AC0 (VSS)
AC1 (VREF)
00x01
AC3
AC2
01x01
AC2
AC3
00x10
AC5
AC4
01x10
AC4
AC5
10000
AC0 (VSS)
11000
AC0 (VSS)
10001
AC1 (VREF)
11001
AC1 (VREF)
10010
AC2
11010
AC0 (VSS)
AC2
AC0 (VSS)
10011
AC3
11011
AC3
10100
AC4
11100
AC4
10101
AC5
11101
AC5
Table 2 - Analog Input Selection
Similarly, the reference voltage is chosen among two differential channels (VREF,ADC = AC_R1 - AC_R0 or
VREF,ADC = AC_R3 - AC_R2) as shown in Table 3. The selection bit is VMUX. The reference inputs VREFP and
VREFN (common-mode) can be up to the power supply range.
VMUX
(RegACCfg5[0])
VREFP
VREFN
0
AC_R1 (VBATT)
AC_R0 (VSS)
1
AC_R3 (VREF)
AC_R2 (VSS)
Table 3 - Analog Reference Input Selection
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Programmable Gain Amplifiers
As seen in Figure 8, the zooming function is implemented with three programmable gain amplifiers (PGA).
These are:
• PGA1: coarse gain tuning
• PGA2: medium gain and offset tuning
• PGA3: fine gain and offset tuning. Should be set ON for high linearity data acquisition
All gain and offset settings are realized with ratios of capacitors. The user has control over each PGA activation
and gain, as well as the offset of stages 2 and 3. These functions are examined hereafter.
ENABLE[3:0]
(RegACCfg1[3:0])
Block
PGA3_GAIN[6:0]
(RegACCfg3[6:0])
PGA3 Gain
GD3 (V/V)
xxx0
ADC disabled
0000000
0
xxx1
ADC enabled
0000001
1/12(=0.083)
xx0x
PGA1 disabled
...
...
xx1x
PGA1 enabled
0000110
6/12
x0xx
PGA2 disabled
...
...
x1xx
PGA2 enabled
0001100
12/12
0xxx
PGA3 disabled
0010000
16/12
1xxx
PGA3 enabled
...
0100000
Table 4 - ADC & PGA Enabling
32/12
...
PGA1_GAIN
(RegACCfg3[7])
PGA1 Gain
GD1 (V/V)
1000000
0
1
1111111
1
10
64/12
...
127/12(=10.58)
Table 8 - PGA3 Gain Settings
Table 5 - PGA1 Gain Settings
PGA3_OFFSET[6:0]
(RegACCfg4[6:0])
PGA3 Offset
GDoff3 (V/V)
PGA2_GAIN[1:0]
(RegACCfg2[5:4])
PGA2 Gain
GD2 (V/V)
0000000
0
00
1
0000001
+1/12(=+0.083)
01
2
0000010
+2/12
10
5
...
...
11
10
0010000
+16/12
Table 6 - PGA2 Gain Settings
...
...
0100000
+32/12
PGA2_OFFSET[3:0]
(RegACCfg2[3:0])
PGA2 Offset
GDoff2 (V/V)
0000
0
0001
+0.2
0010
+0.4
0011
+0.6
0100
+0.8
0101
+1
1001
-0.2
...
...
1010
-0.4
1100000
-32/12
1011
-0.6
...
...
1100
-0.8
1111111
-63/12(=-5.25)
1101
-1
...
...
0111111
+63/12(=+5.25)
1000000
0
1000001
-1/12(=-0.083)
1000010
-2/12
...
...
1010000
-16/12
Table 9 - PGA3 Offset Settings
Table 7 - PGA2 Offset Settings
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
PGA & ADC Enabling
Depending on the application objectives, the user may enable or bypass each PGA stage. This is done
according to the word ENABLE and the coding given in Table 4. To reduce power dissipation, the ADC can also
be inactivated while idle.
PGA1
The first stage can have a buffer function (unity gain) or provide a gain of 10 (see Table 5). The voltage VD1 at
the output of PGA1 is:
VD1 = GD1 ⋅ VIN
(V)
Equation 5
where GD1 is the gain of PGA1 (in V/V) controlled with the bit PGA1_GAIN.
PGA2
The second PGA has a finer gain and offset tuning capability, as shown in Table 6 and Table 7. The voltage VD2
at the output of PGA2 is given by:
VD 2 = GD2 ⋅ VD1 − GDoff 2 ⋅ VREF , ADC (V)
Equation 6
where GD2 and GDoff2 are respectively the gain and offset of PGA2 (in V/V). These are controlled with the
words PGA2_GAIN[1:0] and PGA2_OFFSET[3:0].
PGA3
The finest gain and offset tuning is performed with the third and last PGA stage, according to the coding of
Table 8 and Table 9. The output of PGA3 is also the input of the ADC. Thus, similarly to PGA2, we find that the
voltage entering the ADC is given by:
VIN , ADC = GD3 ⋅ VD 2 − GDoff 3 ⋅ VREF , ADC
(V)
Equation 7
where GD3 and GDoff3 are respectively the gain and offset of PGA3 (in V/V). The control words are
PGA3_GAIN[6:0] and PGA3_OFFSET[6:0]. To remain within the signal compliance of the PGA stages, the
condition:
VD1 ,VD 2 < VDD (V)
Equation 8
must be verified.
Finally, combining equations 5 to 7 for the three PGA stages, the input voltage VIN,ADC of the ADC is related to
VIN by:
VIN , ADC = GDTOT ⋅ VIN − GDoff TOT ⋅ VREF , ADC (V)
Equation 9
where the total PGA gain is defined as:
GDTOT = GD3 ⋅ GD2 ⋅ GD1
(V/V)
Equation 10
and the total PGA offset is:
GDoff TOT = GDoff 3 + GD3 ⋅ GDoff 2
(V/V)
Equation 11
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
ADC Characteristics
The main performance characteristics of the ADC (resolution, conversion time, etc.) are determined by three
programmable parameters. The setting of these parameters and the resulting performances are described later.
• Over-sampling frequency fs
• Over-Sampling Ratio OSR
• Number of Elementary Conversions NELCONV
Conversion Sequence
A conversion is started each time the bit START or the bit DEF is set. As depicted in Figure 11, a complete
analog-to-digital conversion sequence is made of a set of NELCONV elementary incremental conversions and a
final quantization step. Each elementary conversion is made of (OSR+1) over-sampling periods Ts=1/fs, i.e.:
TELCONV =
(OSR + 1)
fs
(s)
Equation 12
The result is the mean of the elementary conversion results. An important feature is that the elementary
conversions are alternatively performed with the offset of the internal amplifiers contributing in one direction and
the other to the output code. Thus, converter internal offset is eliminated if at least two elementary sequences
are performed (i.e. if NELCONV ≥ 2). A few additional clock cycles are also required to initiate and end the
conversion properly.
Init
Elementary
Conversion
Elementary
Conversion
Elementary
Conversion
Elementary
Conversion
Conversion index
Offset
1
+
2
-
NELCONV-1
+
NELCONV
-
TCONV
End
Conversion
Result
Figure 11 - Analog-to-Digital Conversion Sequence
Note:
The internal bandgap reference state may be forced High or Low, or may be set to toggle during conversion at either the same rate or half
the rate of the Elementary Conversion. This may be useful to help eliminate bandgap related internal offset voltage and 1/fs noise.
Over-Sampling Frequency
The word FIN[1:0] (see Table 10) is used to select the over-sampling frequency fs. The over-sampling frequency
is derived from the 4MHz oscillator clock.
FIN[1:0]
(RegACCfg2[7:6])
Over-Sampling Frequency fs (Hz)
00
62.5 kHz
01
125 kHz
10
250 kHz
11
500 kHz
Table 10 - Over-Sampling Frequency Settings
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Over-Sampling Ratio
The over-sampling ratio (OSR) defines the number of integration cycles per elementary conversion. Its value is
set with the word SET_OSR[2:0] in power of 2 steps (see Table 11) given by:
OSR = 2 3+ SET _ OSR [2:0 ]
Equation 13
SET_OSR[2:0]
(RegACCfg0[4:2])
Over-Sampling Ratio
OSR (-)
000
8
001
16
010
32
011
64
100
128
101
256
110
512
111
1024
Table 11 - Over-Sampling Ratio Settings
Elementary Conversions
As mentioned previously, the whole conversion sequence is made of a set of NELCONV elementary incremental
conversions. This number is set with the word SET_NELC[1:0] in power of 2 steps (see Table 12) given by:
N ELCONV = 2 SET _ NELC [1:0 ]
Equation 14
SET_NELC[1:0]
(RegACCfg0[6:5])
# of Elementary
Conversions
NELCONV (-)
00
1
01
2
10
4
11
8
Table 12 - Number of Elementary Conversion Settings
As already mentioned, NELCONV must be equal or greater than 2 to reduce internal amplifier offsets.
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Resolution
The theoretical resolution of the ADC, without considering thermal noise, is given by:
n = 2 ⋅ log 2 (OSR ) + log 2 ( N ELCONV )
(Bits)
Equation 15
17
Resolution - n [Bits]
15
13
11
SET_NELC= 11
10
01
00
9
7
5
000
001
010
011
100
101
110
111
SET_OSR
Figure 12 - Resolution vs. SET_OSR[2:0] and SET_NELC[1:0]
Using look-up Table 13 or the graph plotted in Figure 12, resolution can be set between 6 and 16 bits. Notice
that, because of 16-bit register use for the ADC output, practically the resolution is limited to 16 bits,
i.e. n ≤ 16. Even though the resolution is truncated to 16 bit by the output register size, it may make sense to set
OSR and NELCONV to higher values in order to reduce the influence of the thermal noise in the PGA and of
external noises (see section “PGA Gain & Offset, Linearity and Noise” in page 37).
SET_NELC[1:0]
SET_OSR
[2:0]
00
01
10
11
000
6
7
8
9
001
8
9
10
11
010
10
11
12
13
011
12
13
14
15
100
14
15
16
16
101
16
16
16
16
110
16
16
16
16
111
16
16
16
16
Note: shaded area: resolution truncated to 16 bits due to
output register size RegACOut[15:0]
Table 13 - Resolution vs. SET_OSR[2:0] and SET_NELC[1:0] Settings
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Conversion Time and Throughput
As explained in Figure 12, conversion time is given by:
TCONV =
N ELCONV ⋅ (OSR + 1) + 1
fs
(s)
Equation 16
and throughput is then simply 1/TCONV. For example, consider an over-sampling ratio of 256, 2 elementary
conversions, and a over-sampling frequency of 500kHz (SET_OSR = "101", SET_NELC = "01", FIN = "00"). In
this case, using Table 14, the conversion time is 515 over-sampling periods, or 1.03ms. This corresponds to a
throughput of 971Hz in continuous-time mode. The plot of Figure 7 illustrates the classic trade-off between
resolution and conversion time.
SET_OSR
[2:0]
00
000
001
010
011
100
101
110
111
10
18
34
66
130
258
514
1026
SET_NELC[1:0]
01
10
19
35
67
131
259
515
1027
2051
37
69
133
261
517
1029
2053
4101
11
73
137
265
521
1033
2057
4105
8201
Table 14 - Normalized Conversion Time (TCONV*fs) vs. SET_OSR[2:0] and SET_NELC[1:0]
(Normalized to Over-Sampling Period 1/fs)
Resolution - n [Bits]
16.0
14.0
12.0
10.0
8.0
10
6.0
00
11
SET_NELC
01
4.0
10.0
100.0
1000.0
10000.0
Normalized Conversion Time - TCONV*fS [-]
Figure 13 - Resolution vs. Normalized Conversion Time for Different SET_NELC[1:0]
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ZoomingADC™ for Pressure and Temperature Sensing
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Output Code Format
The ADC output code is a 16-bit word in two's complement format (see Table 15). For input voltages outside the
range, the output code is saturated to the closest full-scale value (i.e. 0x7FFF or 0x8000). For resolutions
smaller than 16 bits, the non-significant bits are forced to the values shown in Table 16. The output code,
expressed in LSBs, corresponds to:
OUTADC = 216 ⋅
VIN , ADC
VREF , ADC
⋅
OSR + 1
OSR
(LSB)
Equation 17
Recalling equation 9, this can be rewritten as:
OUTADC = 216 ⋅
VIN
VREF , ADC
V
⋅ GDTOT − GDoffTOT ⋅ REF , ADC
VIN
OSR + 1
⋅
OSR
(LSB)
Equation 18
where, from Equation 10and Equation 11, the total PGA gain and offset are respectively:
GDTOT = GD3 ⋅ GD2 ⋅ GD1
(V/V)
and:
GDoff TOT = GDoff 3 + GD3 ⋅ GDoff 2
(V/V)
ADC Input Voltage VIN,ADC
% of Full Scale (FS)
Output in LSBs
Output Code in Hex
+2.46146V
+0.5⋅FS
+215-1=+32'767
7FFF
+2.46138V
...
+215-2=+32'766
7FFE
...
...
...
...
+75µV
...
+1
0001
0V
0
0
0000
-75µV
...
-1
8FFF
...
...
...
...
-2.46146V
...
-215-1=-32'767
8001
-2.46154V
-0.5⋅FS
-215=-32'768
8000
Table 15 - Basic ADC Relationships (example for: VREF,ADC = 5V, OSR = 64, n = 16 bits)
SET_OSR[2:0]
SET_NELC = 00
SET_NELC = 01
SET_NELC = 10
SET_NELC = 11
000
1000000000
100000000
10000000
1000000
001
10000000
1000000
100000
10000
010
100000
10000
1000
100
011
1000
100
10
1
100
10
1
-
-
101
-
-
-
-
110
-
-
-
-
111
-
-
-
-
Table 16 - Last Forced LSBs in Conversion Output Registers for Resolution Settings Smaller than 16
bits (n < 16) (RegACOutMsb[7:0] & RegACOutLsb[7:0])
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ZoomingADC™ for Pressure and Temperature Sensing
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The equivalent LSB size at the input of the PGA chain is:
LSB =
1 VREF , ADC OSR
⋅
⋅
(V)
2 n GDTOT OSR + 1
Equation 19
Notice that the input voltage VIN,ADC of the ADC must satisfy the condition:
VIN , ADC ≤
1
OSR
⋅ (VREFP − VREFN ) ⋅
2
OSR + 1
(V)
Equation 20
to remain within the ADC input range.
Power Saving Modes
During low-speed operation, the bias current in the PGAs and ADC can be programmed to save power using
the control words IB_AMP_PGA[1:0] and IB_AMP_ADC[1:0] (see Table 17). If the system is idle, the PGAs and
ADC can even be disabled, thus, reducing power consumption to its minimum. This can considerably improve
battery life.
IB_AMP_ADC[1:0]
(RegACCfg1[7:6])
IB_AMP_PGA[1:0]
(RegACCfg1[5:4])
00
ADC Bias Current
PGA Bias Current
Max. fs [kHz]
125
1/4⋅IADC
x
01
11
1/2⋅ IADC
x
250
IADC
00
x
01
x
11
500
1/4⋅IPGA
125
1/2⋅ IPGA
250
IPGA
500
Table 17 - ADC & PGA Power Saving Modes and Maximum Sampling Frequency
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ZoomingADC™ for Pressure and Temperature Sensing
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Registers Map
Address
Register
Bits
Description
RC Register
0x30
RegRCen
1
RC oscillator control
GPIO Registers
0x40
RegOut
8
D0 to D3 pads data output and direction control
0x41
RegIn
4
D0 to D3 pads input data
0x50
RegACOutLsb
8
LSB of the ADC result
0x51
RegACOutMsb
8
MSB of the ADC result
0x52
RegACCfg0
7
ADC conversion control
0x53
RegACCfg1
8
ADC conversion control
0x54
RegACCfg2
8
ADC conversion control
0x55
RegACCfg3
8
ADC conversion control
0x56
RegACCfg4
7
ADC conversion control
0x57
RegACCfg5
8
ADC conversion control
6
Chip operating mode register
ADC Registers
Mode Register
0x70
RegMode
Registers Descriptions
The register descriptions are presented here in ascending order of Register Address. Some registers carry
several individual data fields of various sizes; from single-bit values (e.g. flags), upwards. Some data fields are
spread across multiple registers. Unused bits are ‘don't care’ and writing either 0 or 1 will not affect any function
of the device. After power on reset the registers will have the values indicated in the tables “Reset” column.
RC Register
Bit
Name
Mode
Reset
Description
7:1
-
r
000000
unused
0
RC_EN
rw
1
Enables RC oscillator. Set 0 for low power mode.
Table 18 - RegRCen (0x30)
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GPIO Registers
Bit
Name
Mode
Reset
Description
7:6
-
r
00
Unused
5
D1_DIR
rw
1
D1 pad direction:
1 = Output
0 = Input
4
D0_DIR
rw
1
D0 pad direction:
1 = Output
0 = Input
3:2
-
r
00
Unused
1
D1_OUT
rw
0
D1 pad output value. Only valid when D1_DIR = 1 and VREF_D1_IN = 0
0
D0_OUT
rw
0
D0 pad output value.
VREF_D0_OUT = 0
Only
valid
when
D0_DIR
=
1
and
Table - 19 RegOut (0x40)
Bit
Name
Mode
Reset
Description
7:2
-
r
000000
Unused
1
D1_IN
r
-
D1 pad value
0
D0_IN
r
-
D0 pad value
Table - 20 RegIn (0x41)
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ZADC Registers
Bit
7:0
Name
Mode
OUT[7:0]
r
Reset
00000000
Description
LSB of the ADC result
Table 21 - RegACOutLsb (0x50)
Bit
7:0
Name
Mode
OUT[15:8]
r
Reset
00000000
Description
MSB of the ADC result
Table 22 - RegACOutMsb (0x51)
Bit
Name
Mode
Reset
Description
7
START
rw
0
Starts an ADC conversion
6:5
SET_NELC[1:0]
rw
01
Sets the number of elementary conversions
4:2
SET_OSR[2:0]
rw
010
Sets the ADC over-sampling rate
1
CONT
rw
0
Sets continuos ADC conversion mode
0
-
r
0
unused
Table 23 - RegACCfg0 (0x52)
Bit
Name
Mode
Reset
Description
7:6
IB_AMP_ADC[1:0]
rw
11
Bias current selection for the ADC
5:4
IB_AMP_PGA[1:0]
rw
11
Bias current selection for the PGA
3:0
ENABLE[3:0]
rw
0000
ADC and PGA stage enables
Table 24 - RegACCfg1 (0x53)
Bit
Name
7:6
FIN[1:0]
5:4
PGA2_GAIN[1:0]
3:0
PGA2_OFFSET[3:0]
Mode
rw
Reset
Description
00
ADC Sampling Frequency selection
rw
00
PGA2 gain selection
rw
0000
PGA2 offset selection
Table 25 - RegACCfg2 (0x54)
Bit
Name
Mode
Reset
Description
7
PGA1_GAIN
rw
0
PGA1 gain selection
6:0
PGA3_GAIN[6:0]
rw
0001100
PGA3 gain selection
Table 26 - RegACCfg3 (0x55)
Bit
Name
7
-
6:0
PGA3_OFFSET[6:0]
Mode
rw
Reset
0000000
Description
PGA3 offset selection
Table 27 - RegACCfg4 (0x56)
Bit
Name
Mode
Reset
Description
7
BUSY
r
0
ADC activity flag
6
DEF
rw
0
Selects ADC & PGA default configuration
5:1
AMUX[4:0]
rw
00000
Input channel configuration selector
0
VMUX
rw
0
Reference source selector (VBATT = 0 or VREF = 1)
Table 28 - RegACCfg5 (0x57)
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ZoomingADC™ for Pressure and Temperature Sensing
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Mode Register
Bit
Name
Mode
Reset
Function
7
MULT_READY
r
1
1: Indicates that the charge pump has settled and the output voltage is
sufficient for conversion
6
MULT_ACTIVE
r
0
1: Indicates that the charge pump is running (either because VBATT<3V
or it has been forced on)
5:4
CHOP
rw
00
VREF chopping control
11: Chop at Nelconv/2 rate
10: Chop at Nelconv rate
01: Chop state = 1
00: Chop state = 0
(Note 1)
3
MULT_FORCE_ON
rw
0
Force charge pump On (takes priority) (Note 2)
2
MULT_FORCE_OFF
rw
1
Force charge pump Off (Note 2)
1
VREF_D0_OUT
rw
0
Enable VREF output on D0 pin
0
VREF_D1_IN
rw
0
Enable VREF input on D1 pin
Note1:
The chop control is to allow chopping of the internal bandgap reference. This may be useful to help eliminate bandgap
related internal offset voltage and 1/f noise. The bandgap chop state may be forced High or Low, or may be set to toggle
during conversion at either the same rate or half the rate of the Elementary Conversion. (See Conversion Sequence in the
ZoomingADC description)
Note2:
The internal charge pump may be forced On or Off to avoid conversion interruptions due to the pump switching off and on
when the VBATT supply is near 3V. If the pump is on automatic, then it will activate when VBATT drops below 3V to ensure
sufficient supply to the ADC. If the ADC is not being run at full rate or full accuracy then it may operate sufficiently well
when VBATT is less than 3V.
Table 29 - RegMode (0x70)
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ZoomingADC™ for Pressure and Temperature Sensing
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Optional Operating Modes: External Voltage Reference Option
D0 and D1 are multi-functional pins with the following functions in different operating modes (see RegMode
register for control settings):
0
D0/REFOUT
0
GPIO
D0/REFOUT
1
RegMode[1] = 0
VREF
GPIO
1
RegMode[1] = 1
+
0
-
1
VREF
ZoomingADC
+
0
-
1
RegMode[0] = 0
RegMode[0] = 0
1
D1/REFIN
0
1
GPIO
D1/REFIN
Figure 14 - D0 and D1 are Digital Inputs /
Outputs
0
0
D0/REFOUT
GPIO
Figure 16 - D0 is Reference Voltage Output and
D1 is Digital Input / Output
D0/REFOUT
0
ZoomingADC
GPIO
1
GPIO
1
RegMode[1] = 1
RegMode[1] = 0
VREF
+
0
-
1
VREF
+
0
-
1
ZoomingADC
ZoomingADC
RegMode[0] = 1
RegMode[0] = 1
1
D1/REFIN
1
D1/REFIN
0
0
GPIO
GPIO
Figure 17 - D0 is Reference Voltage Output and
D1 is Reference Voltage Input
Figure 15 - D0 is Digital Input / Output and D1
Reference Voltage Input
This allows external monitoring of the internal bandgap reference or the ability to use an external reference input
for the ADC, or the option to filter the internal VREF output before feeding back as VREF,ADC input.
The internally generated VREF is a trimmed bandgap reference with a nominal value of 1.22V.
When using an external VREF,ADC input, it may have any value between 0V and VBATT. Simply substitute
the external value for 1.22 V in the ADC conversion calculations.
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ZoomingADC™ for Pressure and Temperature Sensing
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Application Hints
Recommended Operation Mode and Registers Settings
Operation Mode
Parameter
Value
Units
Analog multiplexers
VMUX
VBATT
AMUX
VINP = AC3 & VINN = AC2
PGA1 Gain
OFF
V/V
PGA2 Gain
OFF
V/V
PGA3 Gain
1
V/V
Total Offset
0
V/V
PGA2 Offset
OFF
V/V
PGA3 Offset
0
V/V
ADC Bias
50
%
PGA Bias
50
PGA
%
ADC
fs
250
kHz
Resolution
16
Bits
OSR
512
OverSamples / ElementaryConversion
NELCONV
2
ElementaryConversion / Conversion
Continuous mode
ON
-
Table 30 - Recommended Operation Mode Values
Registers Settings
Register Name
Bit Position
7
6
5
RegACOutLsb
4
3
2
1
0
OUT[7:0]
RegACOutMsb
XXXXXXXX
OUT[15:8]
SET_NELC[1:0]
01
Hexadecimal
Value
XXXXXXXX
RegACCfg0
START
0
SET_OSR[2:0]
110
CONT
1
0
RegACCfg1
IB_AMP_ADC[1:0]
01
IB_AMP_PGA[1:0]
01
ENABLE[3:0]
1001
0x59
RegACCfg2
FIN[1:0]
10
PGA2_GAIN[1:0]
PGA2_OFFSET[3:0]
0000
0x80
0x3A
RegACCfg3
PGA1_G
0
PGA3_GAIN[6:0]
0001100
0x0C
RegACCfg4
0
PGA3_OFFSET[6:0]
0000000
0x00
RegACCfg5
BUSY
0
DEF
0
AMUX[4:0]
00001
VMUX
0
0x02
Table 31 - Registers Settings
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
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Schematic
SX8723
VBATT
TM
-
VREF
2.5V
-
+
-
-
+
5V
-
SIGNAL MUX
+
+
ZoomingADC
REF MUX
+
AC2
VIN
AC3
+
AC4
AC5
PGA
ADC
READY
CONTROL LOGIC
D0
D1
GPIO
CHARGE
PUMP
4MHz
OSC
I 2C
SCL
MCU
SDA
VPUMP
VSS
Figure 18 - Recommended Operation Mode Schematic
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ZoomingADC™ for Pressure and Temperature Sensing
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Input Impedance
The PGAs of the ZoomingADC are a switched capacitor based blocks (see Switched Capacitor Principle
chapter). This means that it does not use resistors to fix gains, but capacitors and switches. This has important
implications on the nature of the input impedance of the block.
Using switched capacitors is the reason why, while a conversion is done, the input impedance on the selected
channel of the PGAs is inversely proportional to the sampling frequency fs and to stage gain as given in
Equation 21.
768 ⋅109 ΩHz
Z in ≥
f s ⋅ gain
(Ω)
Equation 21
The input impedance observed is the input impedance of the first PGA stage that is enabled or the input
impedance of the ADC if all three stages are disabled.
PGA1 (with a gain of 10), PGA2 (with a gain of 10) and PGA3 (with a gain of 10) each have a minimum input
impedance of 150 kΩ at fs = 500 kHz (see ZoomingADC Specifications). Larger input impedance can be
obtained by reducing the gain and/or by reducing the sampling frequency. Therefore, with a gain of 1 and a
sampling frequency of 125 kHz, Zin > 6.1MΩ.
The input impedance on channels that are not selected is very high (>100MΩ).
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ZoomingADC™ for Pressure and Temperature Sensing
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Switched Capacitor Principle
Basically, a switched capacitor is a way to emulate a resistor by using a capacitor. The capacitors are much
easier to realize on CMOS technologies and they show a very good matching precision.
V1
V1
V2
f
f
V2
R
Figure 19 - The Switched Capacitor Principle
A resistor is characterized by the current that flows through it (positive current leaves node V1):
I=
V1 − V2
R
(A)
Equation 22
One can verify that the mean current leaving node V1 with a capacitor switched at frequency f is:
I = (V 1 − V 2) ⋅ f ⋅ C (A)
Equation 23
Therefore as a mean value, the switched capacitor
1
is equivalent to a resistor.
f ⋅C
It is important to consider that this is only a mean value. If the current is not integrated (low impedance source),
the impedance is infinite during the whole time but the transition.
What does it mean for the ZoomingADC?
If the fs clock is reduced, the mean impedance is increased. By dividing the fs clock by a factor 10, the
impedance is increased by a factor 10.
One can reduce the capacitor that is switched by using an amplifier set to its minimal gain. In particular if PGA1
is used with gain 1, its mean impedance is 10x bigger than when it is used with gain 10.
Current
integration
Sensor
impedence
V1
Sensor
Node
Capacitance
ZoomingADC (model)
f
f V2
C
Figure 20 - The Switched Capacitor Principle
One can increase the effective impedance by increasing the electrical bandwidth of the sensor node so that the
switching current is absorbed through the sensor before the switching period is over. Measuring the sensor
node will show short voltage spikes at the frequency fs, but these will not influence the measurement. Whereas
if the bandwidth of the node is lower, no spikes will arise, but a small offset can be generated by the integration
of the charges generated by the switched capacitors, this corresponds to the mean impedance effect.
Note:
One can increase the mean input impedance of the ZoomingADC by lowering the acquisition clock fs.
One can increase the mean input impedance of the ZoomingADC by decreasing the gain of the first enabled amplifier.
One can increase the effective input impedance of the ZoomingADC by having a source with a high electrical bandwidth (sensor electrical
bandwidth much higher than fs).
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ZoomingADC™ for Pressure and Temperature Sensing
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PGA Settling or Input Channel Modifications
PGAs are reset after each writing operation to registers RegAcCfg1-5. Similarly, input channels are switched
after modifications of AMUX[4:0] or VMUX. To ensure precise conversion, the ADC must be started after a PGA
or inputs common-mode stabilization delay. This is done by writing bit START several cycles after PGA settings
modification or channel switching. Delay between PGA start or input channel switching and ADC start should be
equivalent to OSR (between 8 and 1024) number of cycles. This delay does not apply to conversions made
without the PGAs.
If the ADC is not settled within the specified period, there is most probably an input impedance problem (see
previous section).
PGA Gain & Offset, Linearity and Noise
Hereafter are a few design guidelines that should be taken into account when using the ZoomingADC:
1. Keep in mind that increasing the overall PGA gain, or "zooming" coefficient, improves linearity but
degrades noise performance.
2. Use the minimum number of PGA stages necessary to produce the desired gain ("zooming") and offset.
Bypass unnecessary PGAs.
3. Put most gain on PGA3 and use PGA2 and PGA1 only if necessary.
4. PGA3 should be always ON for best linearity.
5. For low-noise applications where power consumption is not a primary concern, maintain the largest bias
currents in the PGAs and in the ADC; i.e. set IB_AMP_PGA[1:0] = IB_AMP_ADC[1:0] = '11'.
6. For lowest output offset error at the output of the ADC, bypass PGA2 and PGA3. Indeed, PGA2 and
PGA3 typically introduce an offset of about 5 to 10 LSB (16 bit) at their output. Note, however, that the
ADC output offset is easily calibrated out by software.
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ZoomingADC™ for Pressure and Temperature Sensing
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Frequency Response
The incremental ADC is an over-sampled converter with two main blocks: an analog modulator and a low-pass
digital filter. The main function of the digital filter is to remove the quantization noise introduced by the
modulator. This filter determines the frequency response of the transfer function between the output of the ADC
and the analog input VIN. Notice that the frequency axes are normalized to one elementary conversion period
OSR / fs. The plots of Figure 21 also show that the frequency response changes with the number of elementary
conversions NELCONV performed. In particular, notches appear for NELCONV ≥ 2. These notches occur at:
f NOTCH (i ) =
i ⋅ fs
OSR ⋅ N ELCONV
i = 1,2,..., ( N ELCONV − 1)
(Hz) for
Equation 24
and are repeated every fs / OSR.
Information on the location of these notches is particularly useful when specific frequencies must be filtered out
by the acquisition system. This chip has no dedicated 50/60 Hz rejection filtering but some rejection can be
achieved by using Equation 24 and setting the appropriate values of OSR, fs and NELCONV.
Examples:
Rejection [Hz]
fNOTCH [Hz]
fs [kHz]
OSR [-]
NELCONV [-]
61
125
1024
2
61
250
1024
4
61
500
1024
8
53
62.5
1024
8
46
62.5
1024
4
46
125
1024
8
60
50
1.2
1
Normalized Magnitude [-]
Normalized Magnitude [-]
Table 32 - 60/50 Hz Line Rejection Examples
N ELCONV = 1
0.8
0.6
0.4
0.2
0
1.2
1
N ELCONV = 2
0.8
0.6
0.4
0.2
0
0
1
2
3
4
0
1.2
1
N ELCONV = 4
0.8
0.6
0.4
0.2
0
0
1
2
3
1
2
3
4
Normalized Frequency - f *(OSR/fS) [-]
Normalized Magnitude [-]
Normalized Magnitude [-]
Normalized Frequency - f *(OSR/fS) [-]
4
1.2
NELCONV = 8
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
Normalized Frequency - f *(OSR/fS) [-]
Normalized Frequency - f *(OSR/fS) [-]
Figure 21 - Frequency Response: Normalized Magnitude vs. Frequency for Different NELCONV
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ZoomingADC™ for Pressure and Temperature Sensing
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Power Reduction
The ZoomingADC is particularly well suited for low-power applications. When very low power consumption is
of primary concern, such as in battery operated systems, several parameters can be used to reduce power
consumption as follows:
1.
2.
3.
4.
Operate the acquisition chain with a reduced supply voltage VDD.
Disable the PGAs which are not used during analog-to-digital conversion with ENABLE[3:0].
Disable all PGAs and the ADC when the system is idle and no conversion is performed.
Use lower bias currents in the PGAs and the ADC using the control words IB_AMP_PGA[1:0] and
IB_AMP_ADC[1:0].
5. Reduce sampling frequency.
Finally, remember that power reduction is typically traded off with reduced linearity, larger noise and slower
maximum sampling speed.
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Typical Performance
Note: The graphs and tables provided following this note are statistical summary based on limited number of
samples and are provided for informational purposes only. The performance characteristics listed herein are not
tested or guaranteed. In some graphs or tables, the data presented may be outside the specified operating
range and therefore outside the warranted range.
Linearity
Integral Non-Linearity
The different PGA stages have been designed to find the best compromise between the noise performance, the
integral non-linearity and the power consumption. To obtain this, the first stage has the best noise performance
and the third stage the best linearity performance. For large input signals (small PGA gains, i.e. up to about 50),
the noise added by the PGA is very small with respect to the input signal and the second and third stage of the
PGA should be used to get the best linearity. For small input signals (large gains, i.e. above 50), the noise level
in the PGA is important and the first stage of the PGA should be used.
The following figures show the Integral non linearity for different gain settings over the chip temperature range.
Gain 1
-40 °C
25 °C
85 °C
125 °C
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ZoomingADC™ for Pressure and Temperature Sensing
ADVANCED COMMUNICATIONS & SENSING
Gain 10
-40 °C
25 °C
85 °C
125 °C
Gain 100
-40 °C
25 °C
85 °C
125 °C
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SX8723
ZoomingADC™ for Pressure and Temperature Sensing
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Gain 1000
-40 °C
25 °C
85 °C
125 °C
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Differential Non-Linearity
The differential non-linearity is generated by the ADC. The PGA does not add differential non-linearity. Figure
22 shows the differential non-linearity.
Figure 22 - Differential Non-Linearity of the ADC Converter
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ZoomingADC™ for Pressure and Temperature Sensing
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Noise
Ideally, a constant input voltage VIN should result in a constant output code. However, because of circuit noise,
the output code may vary for a fixed input voltage. Thus, a statistical analysis on the output code of 1200
conversions for a constant input voltage was performed to derive the equivalent noise levels of PGA1, PGA2,
and PGA3. The extracted rms output noise of PGA1, 2, and 3 are given in Table 33: standard output deviation
and output rms noise voltage. Figure 23 shows the distribution for the ADC alone (PGA1, 2, and 3 bypassed).
Quantization noise is dominant in this case, and, thus, the ADC thermal noise is below 16 bits.
The simple noise model of Figure 24 is used to estimate the equivalent input referred rms noise VN,IN of the
acquisition chain in the model of Figure 25. This is given by the relationship:
2
VN ,IN
2
2
VN 1 VN 2
VN 3
+
+
GD1 GD1 ⋅ GD2 GD1 ⋅ GD2 ⋅ GD3
=
(OSR ⋅ N ELCONV )
2
2
(V rms)
Equation 25
where VN1, VN2, and VN3 are the output rms noise figures of Table 33, GD1, GD2, and GD3 are the PGA gains of
stages 1 to 3 respectively. As shown in this equation, noise can be reduced by increasing OSR and NELCONV
(increases the ADC averaging effect, but reduces noise).
Parameter
Standard deviation
output (LSB)
at
ADC
Output rms noise (µV)
PGA1
PGA2
PGA3
0.85
1.4
1.5
205 (VN1)
340 (VN2)
365 (VN3)
Table 33 - PGA Noise Measurements (n = 16 bits, OSR = 512, NELCONV = 2, VREF = 5 V)
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ZoomingADC™ for Pressure and Temperature Sensing
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Occurences
[% of total samples]
80
60
40
20
0
-5
-4
-3
-2
-1
0
1
2
3
4
5
Output Code Deviation From Mean Value [LSB]
Figure 23 - ADC Noise (PGA1, 2 & 3 Bypassed, OSR = 512, NELCONV = 2)
PGA1
PGA2
VN1
GD1
fs
PGA3
VN2
GD2
VN3
GD3
ADC
Figure 24 - Simple Noise Model for PGAs and ADC
PGA1
VN,IN
PGA2
VN1
GD1
fs
PGA3
VN2
GD2
VN3
GD3
ADC
Figure 25 - Total Input Referred Noise
As an example, consider the system where: GD2 = 10 (GD1 = 1; PGA3 bypassed), OSR = 512, NELCONV = 2,
VREF = 5 V. In this case, the noise contribution VN1 of PGA1 is dominant over that of PGA2. Using Equation 25,
we get: VN,IN = 6.4 µV (rms) at the input of the acquisition chain, or, equivalently, 0.85 LSB at the output of the
ADC. Considering 0.2 V (rms) maximum signal amplitude, the signal-to-noise ratio is 90dB.
Noise can also be reduced by implementing a software filter. By making an average on a number of subsequent
measurements, the apparent noise is reduced the square root of the number of measurement used to make the
average.
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ZoomingADC™ for Pressure and Temperature Sensing
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Gain Error and Offset Error
Gain error is defined as the amount of deviation between the ideal transfer function (theoretical Equation 18)
and the measured transfer function (with the offset error removed).
The actual gain of the different stages can vary depending on the fabrication tolerances of the different
elements. Although these tolerances are specified to a maximum of ±3%, they will be most of the time around
±0.5%. Moreover, the tolerances between the different stages are not correlated and the probability to get the
maximal error in the same direction in all stages is very low. Finally, these gain errors can be calibrated by the
software at the same time with the gain errors of the sensor for instance.
Figure 26 shows gain error drift vs. temperature for different PGA gains. The curves are expressed in % of FullScale Range (FSR) normalized to 25°C.
Offset error is defined as the output code error for a zero volt input (ideally, output code = 0). The offset of the
ADC and the PGA1 stage are completely suppressed if NELCONV > 1.
The measured offset drift vs. temperature curves for different PGA gains are depicted in Figure 27. The output
offset error, expressed in LSB for 16-bit setting, is normalized to 25°C. Notice that if the ADC is us ed alone, the
output offset error is below ±1 LSB and has no drift.
NORMALIZED TO 25°C
Gain Error [% of FSR]
0.2
0.1
0.0
-0.1
1
5
20
100
-0.2
-0.3
-0.4
-50
-25
0
25
50
75
100
Temperature [°C]
Figure 26 - Gain Error vs. Temperature for Different PGA Gains
Output Offset Error [LSB]
NORMALIZED TO 25°C
100
1
5
20
100
80
60
40
20
0
-20
-40
-50
-25
0
25
50
75
100
Temperature [°C]
Figure 27 - Offset Error vs. Temperature for Different PGA Gains
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ZoomingADC™ for Pressure and Temperature Sensing
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Power Consumption
Figure 28 plots the variation of quiescent current consumption with supply voltage VDD, as well as the
distribution between the 3 PGA stages and the ADC (see Table 34). As shown in Figure 29, if lower sampling
frequency is used, the quiescent current consumption can be lowered by reducing the bias currents of the PGAs
and the ADC with registers IB_AMP_PGA [1:0] and IB_AMP_ADC [1:0]. (In Figure 29, IB_AMP_PGA/ADC[1:0]
= '11', '10', '00' for fS = 500, 250, 62.5 kHz respectively.)
Quiescent current consumption vs. temperature is depicted in Figure 30, showing a relative increase of nearly
40% between -45 and +85°C.
Figure 28 - Quiescent Current Consumption vs. Supply Voltage
Figure 29 - Quiescent Current Consumption vs. Supply Voltage for Different Sampling Frequencies
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ZoomingADC™ for Pressure and Temperature Sensing
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Figure 30 - Absolute Change in Quiescent Current Consumption vs. Temperature
Figure 31 - Relative Change in Quiescent Current Consumption vs. Temperature
Supply
Back Bone
ADC
PGA1
PGA2
PGA3
TOTAL
VDD = 5 V
142
127
96
86
97
548
VDD = 3.3 V
98
105
87
73
96
459
VDD = 2.5 V
99
105
87
71
91
453
Unit
µA
Table 34 - Typical Quiescent Current Distributions in Acquisition Chain (n = 16 bits, fS = 250 kHz)
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ZoomingADC™ for Pressure and Temperature Sensing
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PCB Layout Considerations
PCB layout considerations to be taken when using the SX8723 are relatively simple to get the highest
performances out of the ZoomingADC. The most important to achieve good performances out the ZoomingADC
is to have a good voltage reference. The SX8723 has already an internal reference that is good enough to get
the best performances with a minimal amount of external components, but, in case an external reference is
needed this one must be as clean as possible in order to get the desired performance. Separating the digital
from the analog lines will be also a good choice to reduce the noise induced by the digital lines. It is also
advised to have separated ground planes for digital and analog signals with the shortest return path, as well as
making the power supply lines as wider as possible and to have good decoupling capacitors.
How to Evaluate
The SX8723 is a subset of the SX8724 thus for evaluation purposes the XE8000EV121 evaluation kit can be
ordered. This kit connects to any PC using a USB port. The “SX87xx Evaluation Tools” software gives the user
the ability to control the SX8724 registers as well as getting the raw data from the ZoomingADC and displaying
it on the “Graphical User interface”. For more information please look at SEMTECH web site
(http://www.semtech.com).
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ZoomingADC™ for Pressure and Temperature Sensing
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Package Outline Drawing: MLPD-W-12 4x4
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Land Pattern Drawing: MLPD-W-12 4x4
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Tape and Reel Specification
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© Semtech 2007
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Contact Information
Semtech Corporation
Advanced Communication and Sensing Products Division
200 Flynn Road, Camarillo, CA 93012
Phone (805) 498-2111 Fax : (805) 498-3804
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