A self-balance-type capacitance-to-DC-voltage converter for measuring small capacitance

A simple self-balance-type capacitance-to-dc-voltage (CV) converter is proposed for the measurement of small capacitance. The balancing loop operates so as to drive the capacitor to be measured and the ac-dc converter, which is used to detect the signal corresponding to the capacitance, with an arbitrary constant voltage. Therefore, good conversion linearity can be expected. Using a prototype system, very good conversion linearity has been obtained in measuring small capacitance. The error has been estimated as less than ±0.1 percent when measuring a capacitance smaller than 100 pF. Also, a dispersion of less than ±0.001 pF and a temperature coefficient of less than +50 ppm/°C have been observed. These results show that the converter is convenient to operate in capacitive sensor applications.