How to filter out common-mode interference in the sampling circuit of an electrical force measurement switch (PMS)?

Common-mode interference in the PMS sampling loop (voltage or current) originates from power grid-to-ground potential fluctuations, common-mode noise from the switching power supply, and spatial radiation coupling. It manifests as common-mode voltage fluctuations of the sampled signal to ground, which may exceed the amplifier's input range or be converted into differential-mode interference. The core of filtering common-mode interference is using a differential amplifier with a high common-mode rejection ratio (CMRR) and providing a low-impedance return path for common-mode noise. In terms of hardware, the sampled signal first enters a differential amplifier (such as INA188, CMRR>120dB), and the amplifier should be placed as close as possible to the sampling point.

A matching small capacitor (such as 10pF) is connected to ground at both the non-inverting and inverting inputs of the amplifier, forming a low-pass filter with the line impedance. Simultaneously, a clean analog ground is provided for the amplifier and connected to the chassis ground through a Y capacitor (such as 2.2nF/Y1) or a common-mode inductor CMZ2012A-900T, providing a discharge path for common-mode noise. If the interference is very severe, an isolation amplifier or an isolation ADC can be considered. When routing PCBs, differential pairs must be strictly of equal length, equidistant, and closely spaced. These measures can improve the common-mode interference rejection ratio of the sampling loop by more than 60dB, effectively filtering out common-mode components.