An AC-based capacitance measuring circuit for tomography systems and its silicon chip design

The electrical capacitance tomography (ECT) system developed at UMIST employs the charge/discharge capacitance measuring circuit. The problem with this circuit is that a lot of CMOS switches are used to control charge/discharge operation and to select excitation and detection electrodes, and these switches cause charge injection problems. This paper describes an AC-based capacitance measuring circuit designed for ECT systems. In this circuit there are no CMOS switches directly connected to measurement electrodes, and therefore there is no charge injection problem. A silicon chip has been designed for this circuit using the standard cells in the MIETEC CMOS library. Simulation results show that the silicon chip will perform well at 500 kHz.