Analysis of Non-Ideal Characteristics of an AC-Based Capacitance Transducer for Tomography

J C Gamio1, W Q Yang2 and A L Stott2

1 Instituto Mexicano del Petroleo, Eje Central Nte·#152, Mexico City, DF, Mexico, jgamio@imp.mx

2 Department of Electrical Engineering and Electronics, UMIST, PO Box 88, Manchester M60 1QD, UK, w.yang@umist.ac.uk

ABSTRACT

An AC-based capacitance transducer has been designed for use in electrical capacitance tomography (ECT) systems. It has a high signal-to-noise ratio (SNR) and good stray-immunity, compared with a charge/discharge transducer designed for the same purpose. However, the non-ideal characteristics of amplifiers and CMOS switches limit the performance of the AC-based transducer. This paper analyses the effects of the frequency-dependent gain of the amplifiers and the finite “ON” resistance of CMOS switches. It is shown that the non-ideal characteristics restrict the frequency response of the AC-based transducer by introducing extra poles, which depend not only on the parameters of the devices but also on the stray capacitance. Mathematical expressions quantifying these effects are presented.

Keywords Capacitance measurement, Frequency analysis, Operational amplifier, Tomography