How to design the EMC of the PMS (Power Measurement System) transformer interface?

The interface between the PMS and the external current/voltage transformer (CT/VT) is a weak point for electromagnetic interference (EMI), and EMC design must balance signal transmission accuracy and immunity. The interface circuit should include protection, filtering, and impedance matching. For protection, a bidirectional TVS diode (such as SMBJ6.5CA) or a varistor (such as 7D390K) should be connected in parallel across the secondary side of the transformer to clamp overvoltage. For filtering, an RC low-pass filter (such as 100Ω + 1nF) should be used before the signal enters the ADC, with the cutoff frequency set according to the signal bandwidth (usually 2kHz).

For high-precision metering, differential reception should be used, and a small resistor (such as 10Ω) can be added between the differential lines to dampen ringing. In terms of PCB layout, the interface terminals, protection devices, filtering components, and ADC input pins should be compactly arranged to form a local independent area. Cables connecting the transformer must use shielded twisted-pair cables, with the shield connected to the housing ground (PE) at a single point at the PMS terminal. If the transformer itself has a metal casing, it should also be grounded. Furthermore, to prevent ground loops, the secondary side of the current transformer should not be grounded at the sensor terminal. A well-designed interface can withstand the IEC 61000-4-5 combined surge test (1kV) and ensure metering accuracy.