How to suppress common-mode interference after isolating I/O modules?

Even if the I/O module employs signal or power isolation, common-mode interference still needs to be suppressed after isolation, as high-frequency noise can couple through the parasitic capacitance of the isolation device. On the system side of the isolator, i.e., the receiving side, a common-mode choke, such as the CML series, is still required to suppress incoming common-mode noise. Small-value Y capacitors can be connected in parallel between the signal line and local ground on both sides of the isolation barrier to provide a low-impedance local discharge path for high-frequency noise, preventing it from entering subsequent circuitry. Provide independent, low-noise power supplies for the circuitry on both sides of the isolator and ensure adequate decoupling.

On the PCB layout, the circuitry on the isolator's system side should be kept away from other noise sources and reference a clean ground plane. For isolated power supplies, the shielding layer between the primary and secondary sides of the transformer should be grounded to bypass common-mode noise. If common-mode interference is particularly severe, consider using double-isolated or reinforced-insulation isolation devices with lower coupling capacitance. The common-mode rejection effect can be evaluated by measuring the high-frequency noise voltage between the grounds on both sides of the isolator, and filter parameters can be adjusted accordingly.