Sensors & Transducers Journal
(ISSN 1726- 5479)
Vol. 80, Issue 6, June 2007, pp. 1246-1256
Further Analysis of Charge/Discharge Capacitance Measuring Circuit Used with Tomography Sensors
Xiaohui HU, Michael KATSOUROS, Wuqiang YANG
School of Electrical and Electronic Engineering, the University of Manchester, Manchester M60 1QD, UK
Schlumberger Cambridge Research, High Cross, Madingley Road,
Cambridge CM3 0EL, UK
Received: 19 April 2007 /Accepted: 19 June 2007 /Published: 25 June 2007
Abstract: Although the charge/discharge capacitance measuring circuit has been used for many applications, in particular for electrical capacitance tomography (ECT), its detailed dynamic performance has not been analysed and hence the circuit has not been optimised. This paper presents a further study of the charge/discharge circuit. To investigate the transient processes of the charging and discharging phases and the steady-state output of the current detector, the circuit has been analysed theoretically in the time and frequency domains and by PSpice simulation to confirm the analytical results. It is found that the non-ideal characteristics of the circuit, including the stray capacitance and ON resistance of CMOS switches used to control the charge/discharge processes, dominate the performance of the circuit. The values of the feedback capacitor and resistor in the current detector need to be carefully selected to achieve an optimised sampling rate and accuracy. The offset error of op-amps can be reduced by using differential configuration and the error due to charge injection can be compensated by frequent system calibration.
Keywords: Electrical capacitance tomography, Charge/discharge circuit, Transient process, Laplace transform
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