Enhanced Limit-of-Detection of Current Sensor Based on Tunneling Magnetoresistive Effect With Multichips Differential Design

Accurately measuring weak electric currents in the order of microamperes in a noninvasive manner is crucial yet challenging for industrial electronics. In this work, we first fabricated low-noise magnetic field sensor chips based on the tunneling magnetoresistive (TMR) effect. A composite free layer and a three-step annealing process were used to improve the performance, resulting in a sensitivity of 30 V/V/T (3 mV/V/Oe), nonlinearity below 0.5%, and reduction of the noise density from 1.4 μV/V/√Hz to 0.7μV/V/ √ Hz at 1 Hz, in comparison with the traditional two-step annealing process. We then developed a weak-current sensing module with a differential design using two TMR sensor chips arranged antiparallelly. Equipped with a magnetic flux concentration structure and a low-noise signal processing circuit, we demonstrated transient measurement of weak current well below 1 mA with a wide bandwidth of dc-to-16 kHz, a dynamic range of ±150 mA (51 dB), and a rapid response time of ~30 μs. The limit-of-detection (LoD) of the sensing module for dc and ac current reached 800 and 300 μA (rms), respectively, which is about 30% lower than the single-chip design. This weak-current sensor based on differential TMR has the potential to be useful for industrial applications in smart grids and green energy.