1 Introduction

Data acquisition systems (DAQs) have a wide range of applications, from acceleration and velocity measurements in vehicle brake testing, to strain measurements in structural health monitoring.

Some of the sensors used in DAQ systems can have a specified measurement bandwidth in their data sheets. For example, a typical MEMS accelerometer used in vibration monitoring can have a frequency response from DC (0Hz) to 10kHz. However, outside of the measurement bandwidth, a sensor can have a resonance frequency, which is where the sensor is most sensitive, which can distort the measurement. The MEMS accelerometer previously mentioned can have a resonance frequency around 60kHz. Any stray signal close to 60kHz can be amplified, creating an error in the measurement. To fix this error. an anti-aliasing filter can be used to filter out the unwanted frequencies. Note that resonance frequency from sensors is only one example of noise which can be present in a testing environment.

This report focuses on a signal chain designed for bandwidths up to 100kHz using the PGA855, THS4551, and ADS127L21 as shown in Figure 1-1. The evaluation board for ADS127L21 has an anti-aliasing filter with THS4551 present on the board, however, the bandwidth and tolerance are not designed for higher frequencies. Adjustments were made to increase the filter bandwidth and were verified through simulation and hardware measurements.

The ADS127L21 is a 24-bit, high-speed, wide-bandwidth, delta-sigma (ΔΣ) analog-to-digital converter (ADC). The ADS127L21 offers excellent ac and dc performance, along with multiple internal digital filter options including a fully programmable finite impulse response (FIR) and infinite impulse response (IIR) filter, making the device useful for a wide variety of data acquisition applications. THS4551 is a differential amplifier used to drive ADS127L21. ADS127L21 is designed for exceptional dc accuracy, low noise, and robust capacitive load driving, making ADS127L21 designed for data acquisition systems where high precision is required along with excellent signal-to-noise ratio (SNR).

The PGA855 is the input amplifier. The PGA855 is a precision, high-bandwidth programmable gain instrumentation amplifier. The PGA855 is equipped with eight binary gain settings, from an attenuating gain of 0.125V/V to a maximum of 16V/V, using three digital gain-selection pins. The output common-mode voltage can be independently set using the VOCM pin. The device features super-beta input transistors from Texas Instruments, which provide ultra-low input offset voltage, offset drift, input bias current, input voltage noise, and current noise.