High frequency and high resolution capacitance measuring circuitfor process tomography

摘要

A stray-immune AC capacitance measuring circuit has been developednfor electrical capacitance tomography. For this application a highnexcitation frequency is essential to achieve high sensitivity and fastndata collection rates, and also to reduce the effect of any conductivencomponent in parallel with the measured capacitance. A high excitationnfrequency has been made possible by using some novel methods: (a) a highnfrequency digital signal generator; (b) parameter-optimised ACnamplifiers and (c) a phase-sensitive demodulator utilising CMOSnswitches. With a 500 kHz excitation signal the circuit has goodnlinearity and stability, and a resolution of 0.035 ff