How to improve EMC performance of I/O module optocoupler isolation?

Improving the EMC performance of optocoupler isolation circuits in I/O modules requires optimizing both the optocoupler itself and its surrounding circuitry. Choose optocouplers with high common-mode transient rejection capabilities; their internal shielding structure better withstands high-voltage transients. On the input side of the optocoupler, connect a resistor in series to limit LED current and connect a TVS diode (such as ESD5V0D3B) in parallel to protect the LED from reverse voltage breakdown. On the output side, the collector or emitter pull-up resistor value should balance switching speed and anti-interference capability; a small capacitor can be connected in parallel to ground at the output to filter high-frequency noise.

Provide independent power supplies for the input and output sections of the optocoupler, or at least isolate them with ferrite beads on the power path to prevent noise coupling through the power supply. In PCB layout, the optocoupler should be connected across the gap between the input and output grounds; the input and output traces should not be parallel or cross to avoid capacitive coupling. For high-speed optocouplers, pay attention to the control signal return path to avoid forming large loops. Increasing the creepage distance between the input and output pins of the optocoupler can improve its tolerance. Finally, EFT and surge tests need to be performed on the optocoupler isolation circuit to verify its isolation effect.