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An ac-based capacitance transducer has been designed for use in electrical capacitance tomography (ECT) systems. The stray capacitance between the measurement electrodes and earth can be relatively large and may affect the performance of the transducer. This paper analyzes this effect, taking into account the ON-resistance of the CMOS switches in the circuit, the finite gain of op-amps and gain errors in the detection measuring unit, so that the overall measurement error can be estimated. It is shown that if the stray capacitance varies between 90 and 210 pF, the measurement error would be between +0.03% to -0.1%. This is negligibly small for ECT applications, confirming that this ac-based capacitance transducer is effectively stray-immune.