A 2 MHz Bandwidth TMR-Based Contactless Current Sensor With Ping-Pong Auto-Zeroing and SAR-Assisted Offset Calibration

Abstract

This article presents a tunnel magnetoresistance (TMR)-based magnetic sensor for contactless current sensing. The TMR readout circuit utilizes a current-balancing instrumentation amplifier (CBIA) with ping-pong auto-zeroing (PPAZ), achieving an integrated magnetic noise of 206 nTrms in a wide bandwidth of 2 MHz. Compared with chopping amplifiers, an auto-zeroed CBIA provides a ripple-free output, not only extending the available bandwidth but also ensuring a system linearity of 0.24%. In the gain configuration of 42 dB, the input magnetic field range is ±0.08 mT, corresponding to a dynamic range (DR) of 58 dB. The maximum offset of the TMR sensor is reduced to 311 nT (ten samples) through an SAR-assisted offset calibration scheme. Moreover, after compensating the temperature coefficient (TC) of the TMR bias generator, the sensitivity drift of the TMR sensor is reduced by $18{\times }$ . Overall, the proposed TMR sensor readout, including the sensor bias circuit, achieves a state-of-the-art power efficiency in terms of figure-of-merit (FoM) of 2.5 fW/Hz or 146 dB. As a general bridge sensor readout, this work achieves 5.4 nV/ $\surd $ Hz input-referred noise (IRN), $1~{\mu }$ V input offset, and bandwidth efficiency of $I_{\text {supply}}$ /bandwidth $ {=} 0.44$ .