How to shield against power frequency interference from a concentrator?

When the concentrator is in a strong power frequency magnetic field environment (such as near a power transformer), a 50Hz magnetic field will induce an error voltage in the acquisition loop. Shielding against power frequency interference requires magnetic shielding using high-permeability materials. For sensitive circuits (such as current sampling CTs and metering chips), a permalloy (μ>10000) shield can be used to completely enclose the circuit. The shield thickness is typically 0.5-1mm, and good grounding should be ensured to simultaneously shield the electric field.

During PCB layout, minimize the area of the sampling loop, such as placing the sampling resistor and operational amplifier input pins close together. Differential sampling technology should be used, utilizing the high common-mode rejection ratio (CMRR) of the instrumentation amplifier to cancel the common-mode voltage induced by the magnetic field. For external connection lines, twisted-pair cables should be used to reduce the magnetic field induction area. Through comprehensive shielding, the power frequency magnetic field induced voltage can be suppressed to below 10μV, ensuring that the concentrator's metering error variation is less than 0.1% in a 100A/m power frequency magnetic field (IEC 61000-4-8 Level 5).