Research on Analog Demodulation Techniques of Bio-electrical Impedance Measurements

The bio electrical impedance measurement is the basis of the impedance plethysmography or electrical impedance tomography(EIT). In order to measure the bio electrical impedance, a safe AC stimulus signal is applied to the measured object, and the amplitude or/and phase of the response signal to the stimulus must be demodulated. The common used hardware demodulation techniques are Rectifiers, Switching demodulators, and Multipliers. All of them need a low pass filter(LPF) so that the stimulus frequency or its twice frequency signals can be filtered out. The LPF may worsen the dynamic response of the bio impedance measurement system, and the above demodulation techniques are not suitable for the high speed and multi channel measurements. So a new high speed demodulation method based on Sample and Hold (S/H) techniques was designed, the LPF is not needed in this method. It can demodulate the impedance information in a single signal cycle and has the features of high speed and phase sensitive demodulation. The technique greatly improves the dynamic response of the measurement system.