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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. © 2001 American Institute of Physics.