A wideband high common mode rejection ratio amplifier and phase-locked loop demodulator for multifrequency impedance measurement

Design considerations and implementation of a multifrequency measuring chan nel for application in the field of bio-impedance measurement are discussed in this paper. The input amplifier has a differential configuration which is electrically isolated from the remaining circuits. Transformer coupling provides improved common mode rejection when compared to non-isolated input stages. The frequency characteristic of the section between input and demo dulator is flat within +/-0.1 dB between 4kHz and 1024kHz. The synchronous demodulator is based on a wideband switched video amplifier. In contrast to commonly used lock-in techniques, the carrier for demodulation is recovere d from the input signal by means of a phase-locked loop. This method ensure s zero phase shift with respect to the input signal and improves the accura cy of measurement. The system has been developed primarily for thoracic imp edance cardiography (TIC) but has also succesfully been applied in the fiel d of total body bio-impedance analysis (BIA). At present an electrical impe dance tomograph is under development based on the instrumentation described . Results regarding the measurement range and accuracy are given and some r ecordings of patient data are shown.