Oscillator-based signal conditioning with improved linearity for resistive sensors

A signal conditioning circuit for resistive sensors is presented. It is based on a relaxation oscillator in which both the frequency and the duty-cycle of the square-wave output signal simultaneously carry information from a pair of different sensors. The output frequency is linearly related to the resistive unbalance of a Wheatstone bridge, while the duty-cycle is independently controlled by a second sensor. The latter can be a thermoresistor used for thermal compensation of the primary sensor. The design, analysis, and experimental characterization of the circuit and its application to a thick-film pressure sensor are reported. A method for compensating the accuracy degradation caused by the finite switching delays is illustrated, and results on its experimental validation are given.