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In this paper, we present an integrated ac current sensor based on sensitivity-optimized horizontal Hall effect devices and a differential readout chain. This microsystem has been designed for 5-A rms nominal ac current measurement with 5-kV galvanic isolation and 0.5% accuracy after calibration from 250 mA to 5 A, over 1.5-kHz bandwidth, which allows up to 30th (25th) harmonic detection in 50-Hz (60-Hz) applications. This qualifies the current sensor microsystem for class 1 products according to international norms for energy meters (IEC44-1 and IEC1036). From the sensing element throughout the instrumental chain's output, the signal conditioning is exclusively performed by low-noise standard CMOS analog blocks. Moreover, the whole microsystem features a mixed-signal structure dedicated to auto-balancing.