How does the isolation impedance of I/O modules affect EMC?

The isolation impedance parameters of isolation devices in I/O modules directly affect the EMC performance of the system. Isolation impedance mainly includes insulation resistance and isolation capacitance. High insulation resistance can effectively suppress the conduction of low-frequency common-mode current and reduce ground loop interference. Low isolation capacitance can reduce high-frequency common-mode noise from crossing the isolation barrier through capacitive coupling, improving immunity to high-frequency transients such as EFT. Therefore, when selecting isolators, their typical isolation capacitance value should be considered; for example, the isolation capacitance of some high-performance digital isolators can be as low as 0.5pF.

In actual circuits, even when isolators are used, their parasitic capacitance will still form a path with externally connected Y capacitors, cable capacitors, etc., so filtering and grounding strategies need to be configured appropriately on both sides of the isolator. For example, on the field side of the isolator, a small-value Y capacitor can be used to guide noise to the chassis ground, which can reduce the common-mode voltage acting on the isolation capacitor. During the design, a simplified model can be established using isolation impedance parameters, and the coupling path of high-frequency noise can be analyzed through simulation to optimize the parameters of filtering components. Finally, the design needs to be verified through common-mode transient immunity testing.