Demodulation of a magnetoresistive sensor signal to achieve a low-cost, stable and high-resolution vector magnetometer

A technique for demodulating a simple a.c, biased magnetoresistor's signal is presented. The application highlighted is the measurement of power-line frequency magnetic fields for the study of their effects on human health. The circuit enables very low magnetic fields to be resolved whilst maintaining a wide d.c, magnetic-field dynamic range with the aid of d.c, feedback in both hard and easy axes of the sensor. The easy-axis feedback field maintains constant sensitivity even under the influence of large easy-axis external fields. Long-term stability is typically better than 20 nT.