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-w ave output whose frequency is related to the bridge unbalance, and duty-cyc le is a function of the overall sensor bridge resistance, hence of the sens or operating temperature. In this way, two measurement values are simultane ously and independently carried on the same output signal. The circuit make s use of a constant current bridge excitation which enables the connection of remote sensors without accuracy degradation, and moreover, for silicon p iezoresistive sensors, provides a first-order temperature compensation. A f requency-doubling output stage significantly reduces the nonlinearity due t o switching delay times, at the parity of output center frequency and span, Experimental results are reported on the characterization of both the circ uit alone and interfaced to a silicon pressure sensor.