HIGH-FREQUENCY AND HIGH-RESOLUTION CAPACITANCE MEASURING CIRCUIT FOR PROCESS TOMOGRAPHY

A stray-immune AC capacitance measuring circuit has been developed for electrical capacitance tomography. For this application a high excitation frequency is essential to achieve high sensitivity and fast data collection rates, and also to reduce the effect of any conductive component in parallel with the measured capacitance. A high excitation frequency has been made possible by using some novel methods: (a) a high frequency digital signal generator; (b) parameter-optimised AC amplifiers and (c) a phase-sensitive demodulator utilising CMOS switches. With a 500 kHz excitation signal the circuit has good linearity and stability, and a resolution of 0.035 fF.