How to shield power frequency magnetic field interference from FTU?

Power frequency magnetic field interference (50Hz) mainly originates from transformers, motors, etc., affecting the current sampling accuracy of the FTU. The solution employs high-permeability shielding materials and sensor optimization. The housing uses 1mm thick electrical pure iron DT4C with a relative permeability exceeding 5000, providing a shielding effectiveness of over 20dB against power frequency magnetic fields. The secondary side of the current transformer uses a CMZ3225A-201T common-mode inductor (200μH) for filtering, coupled with ESDLC5V0D3B low-capacitance ESD protection. The sampling line uses twisted-pair shielded cable with a 20mm twist pitch, and the shielding layer is grounded at a single point. The sampling loop area on the PCB is minimized, employing a differential input architecture with an AD8629 low-noise operational amplifier. The software uses a digital filtering algorithm with a 40dB notch depth at 50Hz. This solution can reduce power frequency magnetic field interference to below 0.1%FS, meeting the IEC 61000-4-8 100A/m power frequency magnetic field immunity test requirements.