A readout circuit for tunnel magnetoresistive sensors employing an ultra-low-noise current source

We present a tunneling magnetoresistive (TMR) sensing microsystem consisting of a low flicker noise TMR sensor and a custom integrated readout frontend. The proposed sensor readout circuit introduces a novel ultra-low-noise current biasing scheme for the TMR sensor, which achieves a very low current noise floor of 2.2 pA√Hz for a 1 mA biasing current. The TMR output voltage is processed by a differential readout scheme to improve the baseline-to-signal ratio. The microsystem also features an on-chip 10-bit current DAC that allows compensating for the large process variations in the TMR base resistance value. The readout chip is manufactured in a 180 nm SOI CMOS technology and heterogeneously integrated with the TMR sensor. The readout chain provides a thermal noise floor of 4nV/√Hz, while, together with the biasing scheme, consuming a total power of 38 mW. The complete sensor system consisting of the TMR and the readout circuit provides a state-of-the-art magnetic field noise floor of 120 pT/√Hz.