A relaxation-oscillator-based interface for high-accuracy ratiometric signal processing of differential-capacitance transducers

For high-accuracy signal processing of differential-capacitance transducers, an interface circuitry is developed based on a relaxation oscillator. Two capacitors of the transducer are multiplexed by diode switches to form the op-amp-based integrator. The duty ratio of the oscillator output then measures the ratio of two capacitances. A circuit analysis shows that the interface can detect the capacitance change as small as 0.01% of the total capacitance. Experimental results are also given to confirm the analysis.