Data acquisition
A low noise and low power DAQ solution for seismology and energy exploration applications
By David Guo, product application engineer and Steven Xie, product application engineer at Analog Devices
P
recision data acquisition (DAQ) systems are popular in industrial applications. In some DAQ applications, low power and
ultralow noise are required. One example is seismic sensor-related applications, where a lot of information can be extracted from seismic data that is useful for a wide range of applications such as structural health monitoring, geophysical research, oil exploration, and even industrial and household safety.
DAQ SiGnAl ChAin ReQuiRementS Seismic geophones are electromechanical conversion devices that convert ground vibration signals into electrical signals. They are suitable for high resolution seismic exploration. They are implanted in the ground along arrays to measure the time of returns of seismic waves as they are reflected off discontinuity surfaces such as bedding planes, as shown in Figure 1. To capture the small output signal from the
geophones, a high sensitivity DAQ signal chain must be built for data analysis. The total rms noise should be = 1.0 µV rms with a limited flat low-pass bandwidth range of 300 Hz to ~400 Hz, while the signal chain should achieve a THD of around –120 dB. Since the seismic instrument is battery-supplied, the power dissipation should be balanced around 30 mW. This article introduces two signal chain
solutions to achieve the following targeted requirements:
Gain of PGIA: 1, 2, 4, 8, 16
ADC with integrated programmable wideband filter
RTI noise at gain = 1 (300 Hz to ~400 Hz with –3 dB bandwidth) with 1.0 µV rms THD: –120 dB at gain = 1 CMRR at gain = 1 with >100 dB
Power consumption (PGIA plus ADC): 33 mW Secondary channel for self-test
46 Figure 1. Seismic source and geophone array.
DAQ SiGnAl ChAin Solution There is no single precision ADC that has all the features and that can achieve such low noise and THD on the ADI website, nor is there a PGIA that can provide such low noise and low power. However, ADI provides great precision amplifiers and precision ADCs to build signal chains to achieve the target. To build a low noise, low distortion, and low
power consumption PGIA, the ultralow noise ADA4084-2 or zero-drift amplifier ADA4522-2 are good candidates. For very high precision ADCs, the 24-bit
sigma-delta ADC AD7768-1 or 32-bit SAR ADC LTC2500-32 can be the best options. They provide configurable ODR with an integrated flat low-pass FIR filter for different DAQ applications.
SeiSmiC SiGnAl ChAin Solution: ADA4084-2 PGiA AnD AD7768-1 The total signal chain is shown in Figure 2. The ADA4084-2, the ADG658, and 0.1 per cent resistors can build a low noise and low THD PGIA for up to eight different selectable gain options. The AD7768-1 is a single-channel low power, –120 dB THD platform. It has a low ripple programmable FIR, DC to 110.8 kHz digital filter, and it uses the LT6657 as its reference device. AD7768-1 can get 1.76 µV rms noise
running at an ODR of 1 kSPS with power consumption of 10 mW in low power mode. To achieve a final 1.0 µV rms noise, it can run at higher ODR, such as 16 kSPS in median mode. When AD7768-1 runs at higher modulator frequency, it has a lower noise floor, as shown in Figure 3, with higher power consumption. A flat low-pass FIR filter algorithm can be implemented in the MCU software to remove the higher bandwidth noise and decimate the final ODR to 1 kSPS. The final rms noise will be around one fourth of 3.55 µV, which is 0.9 µV. As one example, the MCU software FIR
filter can be made as shown in Figure 4 to balance performance and group delay.
Figure 2. ADA4084-2 PGIA and AD7768-1 plus MCU filtering signal chain solution. September 2021 Instrumentation Monthly
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