Data acquisition ConClusion
To design a very low noise and low power DAQ solution for seismology and energy exploration, a discrete PGIA can be designed with low noise and THD precision amplifiers to drive a high resolution precision ADC. This solution is flexible to balance the noise, THD, and ODR against its power consumption requirements.
Benefits from LTC2500-32’s low noise performance, as well as the ADA4084-2 and LTC2500-32, show the best noise performance without an MCU’s further filtering processing.
Both the ADA4522-2 and ADA4084-2 have good noise performance at PGIA gain = 1. The noise performance is about 0.8 µV rms.
ADA4084-2 has better noise performance at high gain. At gain = 16, ADA4084-2 and LTC2500-32’s noise is 0.19 µV rms, which is better than the 0.25 µV rms of the ADA4522-2.
For the AD7768-1, with MCU’s filtering, the ADA4084-2 and AD7768-1 solution shows noise performance similar to the ADA4084-2 and LTC2500-32 solution.
This article gives a solution to data
acquisition that requires both low noise and low power with limited bandwidth. There are other DAQ applications that require different performance. If low power consumption is not a must, then the following operational amplifiers can be used to build the PGIA:
Lowest noise: the LT1124 and LT1128 can be considered to have the best noise
Lowest drift: the ADA4523, a new zero- drift amplifier, has better noise specifications than the ADA4522-2 and LTC2500-32.
Lowest bias current: the ADA4625-1 is recommended if the sensor’s output resistance is high.
Higher BW: The ADA4807, LTC6226, and LTC6228 are good solutions when building high BW, low noise PGIAs in high BW DAQ applications.
In DAQ applications where noise and power
are not important but a small PCB area and high integrity are required, ADI’s new integrated PGIAs, ADA4254 and LTC6373, are also good choices. ADA4254 is a zero-drift, high voltage, 1/16 to ~176 gain robust PGIA, and LTC6373 is a 25 pA IBIAS, 36 V, 0.25 to ~16 gain, low THD PGIA.
Analog Devices
www.analog.com Instrumentation Monthly September 2021 Figure 8. A PGIA to drive the LTC2500-32. 49
RTI Noise at Gain = 1 (µV rms)
RTI Noise at Gain = 2 (µV rms)
RTI Noise at Gain = 4 (µV rms)
RTI Noise at Gain = 8 (µV rms)
RTI Noise at Gain = 16 (µV rms)
THD at Gain = 1 (dB) THD at Gain = 2 (dB) THD at Gain = 4 (dB) THD at Gain = 8 (dB) THD at Gain = 16 (dB) CMRR at Gain = 1 (dB) CMRR at Gain = 4 (dB) CMRR at Gain = 16 (dB) Pd Typical (mW)
RTI Integrated Noise Within 430 Hz BWand Gain = 1 (µV rms)
RTI Integrated Noise Within 430 Hz BW and Gain = 4 (µV rms)
RTI Integrated Noise Within 430 Hz BW and Gain = 16 (µV rms)
RTI Integrated Noise Within 430 Hz BW and Gain = 16 (µV rms)
ADA4084-2 and
AD7768-1 (Median Mode, FMOD = 4 MHz, ODR = 16 kSPS)+
3.718 1.996 1.217 0.909 0.808 -125 -125 -124 -120 -115 131 117 120 31.3
ADA4084 PGIA and AD7768-1
1.765 0.744 0.259 0.148 Table 4. Noise Simulation Result
ADA4084-2 and AD7768-1(Me- dian Mode, FMOD = 4 MHz, ODR = 16 kSPS)+ MCU FIR and DEC to ODR = 16 k/16 = 1 kSPS
0.868 0.464 0.286 0.208 0.186 -125 -125 -124 -120 -115 131 117 120 31.3
Table 5. Signal Chain Solution Test Results ADA4084-2
and LTC2500-32 ADC
MCLK = 1 MHz 0.82 0.42 0.3
0.24 0.19 -122 -119 -118 -117 -115 114 121 126 33.2
ADA4522 PGIA and AD7768-1
1.774 0.767 0.311 0.225
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