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 excita tion frequency is essential to achieve high sensitivity and fast data collection rates, and also to reduce the effect of any conductive comp onent in parallel with the measured capacitance. A high excitation fre quency has been made possible by using some novel methods: (a) a high frequency digital signal generator; (b) parameter-optimised AC amplifi ers and (c) a phase-sensitive demodulator utilising CMOS switches. Wit h a 500 kHz excitation signal the circuit has good linearity and stabi lity, and a resolution of 0.035 fF.