How to ensure EMC of the 4-20mA circuit of the I/O module?

Ensuring the EMC performance of the 4-20mA current loop of the I/O module requires comprehensive protection. Firstly, at the receiving end, the precision sampling resistor must be of the low inductance and low temperature drift type, with a TVS diode such as SMBJ6.0CA connected in parallel across its ends to limit transient high voltage. Before the signal enters the conditioning circuit, it should pass through a common-mode choke, such as CMZ3225A-101T, to suppress common-mode interference on the line. For the power supply circuit of a two-wire transmitter, an LC filter needs to be added to the output of the power supply, and a PWR series power inductor can be used.

In the PCB layout, the current loop area should be minimized, and the sampling resistor traces should use the Kelvin connection method for accurate measurement. The entire current loop should use shielded twisted-pair cable, with the shield grounded at a single point at the receiving end. If the environment is harsh, an isolated signal conditioner can be considered to isolate the field side from the system side. For long outdoor lines that may be susceptible to lightning strikes, a gas discharge tube should be installed at the entry point as primary protection. Ultimately, the circuit needs to pass immunity tests such as EFT and surge tests to verify that it can still transmit current signals stably and accurately in noisy environments.