How to eliminate strong electrical coupling interference in power FTUs?

Strong electrical coupling interference couples to the FTU's low-voltage circuitry via electric and magnetic fields, affecting sampling accuracy. The solution employs multiple layers of isolation and shielding. For electric field coupling, a 0.8mm thick metal shield is added between the strong and low-voltage circuits and grounded at a single point. For magnetic field coupling, a high-permeability μ125 magnetic ring is used on the strong-voltage cable, while the low-voltage signal line uses twisted-pair shielded cable with the shielding layer grounded. In the circuit, the sampling input uses a CMZ20212A-900T common-mode inductor (90μH) for filtering, coupled with ESDULC5V0D9B ultra-low capacitance ESD protection. Power isolation uses a WRB2405S-3W DC-DC module. The PCB layout separates the strong and low-voltage circuits with a spacing greater than 20mm and a grounding guard ring in between. This solution reduces strong electrical coupling interference by more than 50dB, with a sampling error of less than 0.1%, meeting the IEC 61000-4-3 radiated immunity 10V/m test requirements.