A Simple Digitizer for GMR-Based Magnetic Field Sensor With Some Key Practical Considerations

In this article, a simple and accurate digitizer is presented for a giant magneto-resistance (GMR)-based magnetic field sensor. The digitizer uses differential excitation for the sensor to reduce the possible measurement error/non-linearity due to a mismatch in sensor resistances. Further, the digitizer employs a modified dual-slope-based technique that aids in reducing the overall measurement time, without affecting the measurement resolution and accuracy. The final output of the digitizer varies linearly with respect to the applied magnetic field. A simple calibration method and a compensation algorithm are proposed to minimize the non-linearity gain and offset errors in the output, owing to one major sensor non-ideality. The merits of differential excitation, measurement methodology, digitizer operation, and factors affecting the output are described in detail. The digitizer can also be suitably adapted for other types of resistive sensors. Several tests are carried out in simulation and emulation environments for preliminary validation of the methodology and circuit operation. The worst-case non-linearity obtained for an ideal GMR sensor is less than 0.071%. Later, actual experimental studies are conducted with the AA004-02 GMR sensor IC. The final transfer characteristic of the complete instrumentation system is found linear, with output non-linearity lying close to that of the GMR sensor. The performance parameters of the developed digitizer are found better than or equal to that of the existing interfacing schemes for the GMR sensor.