How to eliminate common-mode interference of digital signals in I/O modules?

The key to eliminating common-mode interference (CMI) in I/O module digital signals lies in blocking its flow path. Use common-mode chokes, such as the CML3225A-510T, at all digital interfaces, which present high impedance to common-mode current. Connect safety Y capacitors in parallel between signal lines and local ground to provide high-frequency bypass for common-mode noise. For differential digital signals such as RS-485, which inherently have common-mode rejection capabilities, the common-mode voltage range of the interface chip is limited; therefore, terminating resistors and common-mode chokes are still required at both ends of the bus.

Isolation techniques, such as optocouplers or digital isolators, can completely cut off the conduction path of common-mode current. During wiring, digital signal lines should use twisted-pair or shielded twisted-pair cables, with the shield grounded at a single point on the module side. Internally, the power supply for digital circuits should be provided through common-mode inductors or isolated DC-DC converters to prevent common-mode noise from power lines from entering. Measuring the common-mode voltage of the signal lines to ground can assess the interference level and verify the effectiveness of the elimination measures.