A high-accuracy, high-speed interface circuit for differential-capacitance transducer

A novel high-accuracy and high-speed interface circuit based on lock-in detector is presented for differential-capacitance transducer. In the interface, the feedback capacitance rather than feedback resistance is used in the current detector to reduce the noise due to large resistance. Capacitor–array circuit proposed makes possible to cover a wide range of capacitance changes by choosing feedback capacitor close to the measurand, which results in transfer coefficient of the current sensing circuit about −1. A precision full-wave rectifier based on phase-sensitive-detector (PSD) incorporated in demodulator eliminates the effect due to threshold voltage of diode to improve the accuracy and suppress noise. A second-order low-pass filter is incorporated in demodulator, which allows high-speed and high-accuracy interface for differential-capacitance transducers. Moreover, the circuit is immune to parasitic capacitance, resistances and exciting source. It is capable of detecting the capacitance change as small as 0.005% of the total capacitance up to 16 pF with the speed higher than 5 ksps. A capacitance diaphragm pressure transducer and a standard pressure source (RUSCA 2465 piston gauge) are connected to the circuit to confirm the analysis.