A Switched-Capacitor Digital Capacitance Bridge

Abstract

A switched-capacitor bridge has been developed for capacitance measurements. It consists of four arms connected between the low-impedance output and virtual ground nodes of an op-amp, and hence is insensitive to parasitic capacitances. The capacitance to be measured is first given a proportional charge. This charge is then compared successively with charges quantized by means of a programmable binary-weighted capacitor array, until a balance is reached. This digital balance operation makes it possible to accomodate an automatic calibration scheme, which affords, in conjunction with the parasitic-insensitive configuration, an accurate measurement. Error analysis has shown that a 10-bit quantization accuracy and a relative error as small as 0.1 percent are attainable when the bridge is fabricated in LSI circuit form using present MOS technologies. A prototype bridge built using discrete components has confirmed the principles of operation. Examples of measurement are also given.