ADS8881 Datasheet, PDF (38/44 Pages) Texas Instruments – 18-Bit, 1-MSPS, Serial Interface, microPower, Miniature, True-Differential Input, SAR Analog-to-Digital Converter

ADS8881
SBAS547A – MAY 2013 – REVISED JULY 2013
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Ultralow-Power DAQ Circuit at 10 kSPS
The data acquisition circuit shown in Figure 71 is optimized for using the ADS8881 at a reduced throughput of 10
kSPS with ultralow-power consumption (< 1 mW) targeted at portable and battery-powered applications.
In order to save power, this circuit is operated on a single 3.3-V supply. The circuit uses extremely low-power,
dual amplifiers (such as the OPA2333) with a maximum quiescent current of 28 µA per channel to drive the
inputs of the ADC. The input amplifiers are configured in a modified unity-gain buffer configuration. The filter
capacitor at the inputs of the ADC attenuates the sampling charge-injection noise from the ADC but effects the
stability of the input amplifiers by degrading the phase margin. This attenuation requires a series isolation resistor
to maintain amplifier stability. The value of the series resistor is directly proportional to the open-loop output
impedance of the driving amplifier to maintain stability, which is high (in the order of kΩ) in the case of low-power
amplifiers such as the OPA333. Therefore, a high value of 1 kΩ is selected for the series resistor at the inputs of
the ADC. However, this series resistor creates an additional voltage drop in the signal path, thereby leading to
linearity and distortion issues. The dual-feedback configuration used in Figure 71 corrects for this additional
voltage drop and maintains system performance at ultralow-power consumption.
REFERENCE DRIVE CIRCUIT
1k 
1µF
REF3325
AVDD
IN OUT
GND
10k 
1µF
-
OPA313
++
AVDD
4.7 
22µF
10k 
10nF
VIN+
VCM
VIN-
INPUT DRIVER
-
OPA333
++
AVDD
1k 
++
OPA333
-
1k 
10nF
10k 
4.7nF
AVDD
REF AVDD
AINP
CONVST
ADS8881
AINM
GND
CONVST
18-Bit 10kSPS
SAR ADC
Figure 71. Ultralow-Power DAQ Circuit at 10 kSPS
For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, refer to 18-Bit, 10kSPS Data Acquisition (DAQ) Block Optimized for Ultra Low Power < 1mW
(SLAU514).
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