New method for extracting signals generated by magnetoresistive sensors

A new AC detection technique which reduces the influence of offset, temperature drift and remanence of magnetoresistive sensors is proposed. The technique relies on nonlinearity of the magnetoresistance, permitting mixing of excitation fields of close frequencies and detection of the sensor output signal at the difference frequency. The difference frequency component appears in the output signal spectrum only when an external DC field is applied along the excitation direction. This frequency is located further apart from the excitation frequencies, and this relatively large gap between the frequencies reduces undesirable coupling between the output and excitation circuits and alleviates the filtering problem. Another advantage of the method is due to its potentially better sensitivity (up to nearly two orders), since it enables measuring fields aligned along the sensor easy axis, for which the demagnetizing field is smaller