A high-speed electrical impedance measurement circuit based on information-filtering demodulation

In the paper, an information-filtering demodulation method is proposed and a high-speed ac-based electrical impedance measurement circuit with a simple configuration is presented. As a crucial part of the ac-based impedance measurement circuit, the information-filtering demodulator can output a preliminary demodulation result by processing only a small number of sampling data within a signal period and the signal-to-noise ratio (SNR) can be further improved by involving more sampling data. Compared with other digital demodulators requiring integer multiples of the signal's period, the information-filtering demodulator is more advantageous in flexibility. Moreover, compared with the recursive least-squares-based demodulator, the proposed demodulator is of relatively low computation complexity and suitable to be implemented on a field programmable gate array. Using this demodulator, the ac impedance measurement circuit based on the ac self-balancing bridge can achieve a high measurement speed. Experimental results showed that one measurement can be accomplished in 17μs, corresponding to one-third of the signal period, at an excitation frequency of 20kHz, and the demodulation SNR can reach up to 65dB. If the data of a complete signal period are used for demodulation like other widely used digital demodulators, the SNR of amplitude demodulation will be higher than 75dB and the standard deviation of the demodulated phase is below 0.012°, which validates the good performance of both the new demodulator and the impedance measurement circuit.