How to design the EMC of the current transformer interface for the concentrator?

The interface between the concentrator and the external current/voltage transformer (CT/VT) is a weak point for electromagnetic interference (EMI), and EMC design must balance signal accuracy and immunity. The interface circuit includes protection, filtering, and impedance matching. For protection, a bidirectional TVS diode (such as SMBJ6.5CA) or a varistor (7D390K) is connected in parallel across the secondary side of the transformer to clamp overvoltage. For filtering, an RC low-pass filter (100Ω + 1nF) is used before the signal enters the ADC, with the cutoff frequency set according to the signal bandwidth. For high-precision metering, differential reception is used, and a small-value resistor (10Ω) can be added between the differential lines to dampen ringing. On the PCB layout, the interface terminals, protection devices, filtering components, and ADC input pins are compactly arranged. Cables connecting the transformers must use shielded twisted-pair cables, with the shield grounded at a single point at the concentrator end. The transformer's metal casing should also be grounded. This design can withstand the IEC 61000-4-5 combined surge test (1kV).