EMC differences between non-isolated and isolated I/O modules

There is a fundamental difference in EMC performance between non-isolated and isolated I/O module designs. Non-isolated modules connect their circuit ground directly to the system reference ground. Their advantages include no need for an isolated power supply, low cost, and small size. However, they are prone to ground loops, have poor suppression of common-mode interference introduced by ground potential differences, and a surge or fault in one port can affect the entire system. Isolated modules electrically isolate the field side from the system side using isolation devices, effectively breaking ground loops and providing high common-mode voltage withstand capability, such as withstanding transient voltages of several kilovolts.

Isolated designs typically require an additional isolated power supply, resulting in higher cost and larger size. In EMC testing, isolated modules generally perform better in immunity tests such as EFT, surge, and power frequency magnetic fields. However, isolation is not a panacea; high-frequency noise can still couple through the parasitic capacitance of the isolation device, and the isolated power supply itself may generate switching noise. Therefore, for applications with complex ground noise, high safety requirements, or long-distance connections, isolated designs are necessary; for low-voltage, short-distance applications with clean ground environments and cost sensitivity, non-isolated designs may be sufficient.