How to suppress AI common mode of analog inputs in I/O modules?

Suppressing common-mode interference at the analog inputs of the I/O module is crucial for ensuring measurement accuracy. Instrumentation amplifiers with differential input structures are preferred due to their high common-mode rejection ratio (CMRR), which effectively suppresses common-mode voltage. A common-mode choke, such as the CMZ2012A-900T, can be connected before the signal enters the amplifier to attenuate common-mode noise on the line. The amplifier's own power supply rejection ratio (PSRR) is also important; a clean power supply must be provided, typically achieved through RC filtering or a linear regulator. On the PCB layout, the traces at the two input terminals of the amplifier must be symmetrical and grounded to maintain balance.

For current signals such as 4-20mA, connecting a high-voltage capacitor in parallel across the receiving resistor can provide high-frequency bypassing for common-mode noise. If the common-mode voltage in the field exceeds the amplifier's allowable range, an isolation amplifier or isolation transmitter must be used to electrically isolate the input side from the measurement side. When grounding the system, the grounding points on the sensor side and the measurement side should be good to minimize common-mode voltage introduced by the ground potential difference. Injecting common-mode interference can verify the circuit's suppression capability.