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A signal conditioning circuit based on a relaxation oscillator is presented for use with resistive transducers. Both the frequency and the duty-cycle of the output signal carry independent information coming from a pair of different sensors. Namely, the frequency of the output signal changes linearly with the resistance deviations detected by a Wheatstone bridge, while the duty-cycle is dependent on the resistance of a second sensor.

A new microprocessor-based measurement system using an LC resonance locking technique for capacitance measurement is presented. The measuring circuit automatically finds the resonance frequency and tracks its changes, which depend on capacitance changes. The instrument has high sensitivity and resolution, wide measurement range, fast response time and is insensitive to stray capacitances, due to the shield and the wiring, and to their variations. Although the measuring system can be used in different fields such as laboratory and industrial applications, we designed it as a conditioning circuit for industrial capacitive transducers, such as capacitive pressure or displacement sensors. The circuit showed a response time of the order of 2 ms, a resolution of about 0.1 fF for an 8 pF transducer capacitance, with stray capacitances ranging from 4 to 20 pF, and a sensitivity of about in the range from 6 to 16 pF. The measurement technique and the circuit are described in detail in the paper.