Low-noise AC current measurement with a single-element magnetoresistive sensor and lock-in amplification

In this paper, a magnetoresistive (MR) sensor implemented with a single MR element, instead of the usual bridge configuration, has been developed for alternating (AC) current measurement, including the design of its low-noise signal conditioning circuit. The sensor is based on a tunneling magnetoresistive effect (TMR) and is placed on a copper bar carrying the high current under measurement. The circuit incorporates a voltage-to-current (v-i) driver to bias the MR element, as well as a lock-in amplifier for extracting the sensor signal with improved signal-to-noise ratio. The v-i driver is based on a full Wilson current mirror with emitter degeneration, which also compensates the sensor output at zero current (offset). A lock-in amplifier is added to extract small sensor signals from noise by synchronous detection, enabling precise measurement of sensor resistance variations. The v-i driver was analysed to evaluate the accuracy of the mirror current under realistic transistor mismatch conditions, and the boundary design conditions for its operation as a current mirror were established both by simulation and experimentally. The design and analysis of the proposed circuit are verified experimentally with a MR element submitted up to 125 A at 50 Hz, analysing immunity to interferences.