An ac-based electrical capacitance tomography ECT system has been developed at UMIST, with
sinusoidal excitation and phase-sensitive demodulation PSD . In this article the overall system
design is briefly described and then discussed in detail are the key design features, needed to achieve
a high signal-to-noise ratio and a high data acquisition rate, including transient process analysis of
the first stage, design of an analog Butterworth low-pass filter and optimization of the data
acquisition process. The experiments show that the ac-based system has signal variance 19 times
better than the previous design, the charge/discharge ECT system, and that a data acquisition rate of
140 frames per second is achieved for a 12-electrode system
