Analysis of non-ideal characteristics of an ac-based capacitance transducer for tomography

An ac-based capacitance transducer has been designed for use in electrical capacitance tomography systems. It has a high signal-to-noise ratio 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 the CMOS switches. It is shown that the non-ideal characteristics restrict the frequency response of the 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.