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In order to extract the impedance information at different frequencies in our multi-frequency electrical impedance tomography (EIT) system, a digital demodulation method was employed. Theoretically, this method can improve the SNR by √(N/2) times, if we sampling N point per period. Moreover, as very little system resources been needed in this method, all the calculating operations can be finished in a single Field Programmable Gate Arrays (FPGA) device instead of in a personal computer. And the final data acquisition system based on this method was established and the preliminary imaging results were obtained.