Oscillator-based interface for measurand-plus-temperature readout from resistive bridge sensors

A signal conditioning circuit based on a relaxation oscillator is proposed for use with resistive bridge sensors. The circuit provides a rectangular-wave output whose frequency is related to the bridge imbalance and the duty-cycle is a function of the overall sensor bridge resistance, and hence of its operating temperature. In this way, two segments of information are simultaneously and independently carried on the same output signal. The circuit makes use of a constant current bridge excitation which enables the connection of remote sensors without accuracy degradation, and, for silicon piezoresistive sensors, provides a first order temperature compensation. A frequency-doubling output stage significantly reduces the nonlinearity due to switching delay times, at parity of output center frequency and span. Experimental results are reported on the characterization of both the circuit alone and interfaced to a silicon pressure sensor