Novel automatic digital calibration techniques for GMR sensors

Simple and innovative automatic techniques for digital calibration of Giant Magnetoresistive (GMR) sensor bridges are presented, which allow compensating the main problems of these devices: temperature dependence, sensitivity variations, nonlinearity, electrical offset and mechanical misalignments. They are based on obtaining the best fit of the GMR bridge output with a sinusoid using the Levenberg-Marquardt algorithm, and thus generating the optimal gain, electrical offset and mechanical offset compensation. Gain and offset temperature drifts are also digitally compensated by sensing temperature from the GMR bridge itself. Nonlinearity is removed using a very simple logic that exploits the symmetry and weak nonlinearity of the GMR output. The digital calibration techniques have been used in a contactless angular detection system developed by the authors. Measurement results show that digital calibration reduces maximum errors from 7.2° to less than 1° in an angular range of 110°.