Electrical impedance measurements by synchronous sampling applied to square waveforms

Abstract

In electrical impedance imaging we inject a current (frequency f/sub c/) and detect a drop in voltage whose amplitude is modulated by the impedance sensed. Amplitude demodulation is carried out by phase-sensitive (or coherent) detection by using as a reference either a sinus wave (homodyne detection) or a square wave (switching detector) having a frequency f/sub r/=f/sub c/. We have previously shown that a phase-sensitive detector can also be implemented by synchronous sampling. In synchronous sampling we sample the modulated signal at a frequency f/sub r/=f/sub c//k (k being any integer). By so doing, a simple sample and hold (S&H) circuit performs a coherent demodulation. Furthermore, synchronous sampling allows us to demodulate before converting the signal from differential to single-ended mode, thus resulting in a very large CMRR. Our demodulator is based on a floating capacitor whose input is inherently differential. Synchronous sampling, however, opens many spectral windows to noise because the reference signal is a train of pulses. This effect can be mitigated by band-pass filtering the signal to be demodulated. In order to keep the CMRR very high, we use differential filters (i.e., filters with differential input and differential output). By inserting a first-order, differential, band-pass filter, we can achieve a signal-to-noise ratio determined only by the low-pass filter placed at the output of the demodulator.