Charge injection compensation for charge/discharge capacitance measuring circuits used in tomography systems

The charge/discharge capacitance measuring circuits used in electrical capacitance tomography (ECT) systems employ CMOS switches to control the charge/discharge operation. The gate-channel capacitances of these switches inject additional charges into the measuring circuits, reducing system accuracy. This paper describes a method for compensating these charge injection signals and for obtaining absolute capacitance measurements. The system errors are analysed and the measurement accuracy is assessed. Experimental results show that the charge/discharge circuit with charge injection compensation can measure small capacitance values, down to a few femtofarads in magnitude.