How to design the equipotential of I/O modules?

The equipotential design of I/O modules aims to reduce the potential difference within the module and between the module and external devices, thereby suppressing common-mode interference. First, the ground reference point of all circuits within the module should be connected to the same star grounding center point via a low-impedance path, typically the ground of the power input filter capacitor. For multi-board systems, the backplane should provide a low-impedance ground plane, with each board connected at multiple points via gold fingers or connectors. The module's metal casing should achieve high-frequency equipotential with the internal circuit grounds, which can be achieved by placing a row of grounding vias along the PCB edge to connect to the chassis, or by using a metal shield.

At signal interfaces, balanced transmission techniques such as differential signals are used, as they are inherently insensitive to common-mode potential fluctuations. For unbalanced interfaces, common-mode chokes can be used between the signal line and the local ground to suppress common-mode current caused by ground potential differences. For long-distance connections between devices, isolators can be used to break ground loops, while high-frequency equipotential is maintained through isolation capacitors or varistors. After design, the voltage difference between the grounds of each interface of the module must be tested at different frequencies to ensure it is below the threshold required for immunity testing.