Electronic interface design considerations for biomagnetic sensing using TMR sensors

We present a comprehensive design methodology for the electronic interface of a tunneling magnetoresistance (TMR) sensor, which plays a crucial role in determining the detectivity of biomagnetic measurement systems. A theoretical noise model is developed that links sensor detectivity to key design parameters such as the Wheatstone bridge configuration, sensor biasing, and analog front-end (AFE) noise performance. The model is based on a detailed characterization of the TMR sensor and accurately predicts the influence of bias voltage and resistance mismatches on the power supply rejection ratio (PSRR). It shows that the full Wheatstone bridge configuration achieves superior detectivity and that the PSRR can degrade from near-infinite values to approximately 28 dB under a background magnetic field of 5μ T. Guided by this model, the sensor system is optimized in terms of bridge configuration, bias conditions, and AFE design. Experimental validation confirms a detectivity of 7.4 pT/ Hz at 1 Hz and an integrated rms noise of 20 pT within the 5-100 Hz band, under both near-zero and 5μ T magnetic fields. These results surpass those of previously reported TMR-based sensor systems operating in similar frequency bands and under comparable working conditions. The findings highlight the importance of a systematic and integrated approach to electronic interface design for TMR sensors, which supports the development of biomagnetic sensing systems capable of operating without extensive magnetic shielding.