A low cost, high resolution interface circuit for differential capacitance transducer

For accurately detecting capacitance changes of the differential-capacitance transducer, which is widely used in environmental and chemical engineering, a low-cost interface circuit was developed. The circuit uses a "lock-in detection" circuit with feedback loop containing a DC amplifier and a modulator to yield such advantages over the others that it reduces the sensitivity to element parameter variations in the "lock-in detection" circuit, that it decreases the sensitivity to the disturbances within closed-loop, and that it is immune to parasitic capacitance, resistance, amplitude, and frequency of the excitation voltage source. The circuit has the capability to detect a capacitance change as small as 0.0078% of the total capacitance, up to 80 pF. A capacitance diaphragm pressure transducer and a standard pressure source (Rusca 2465 piston gauge) are connected to the interface to confirm the analysis.