DIFFERENTIAL SYNCHRONOUS DEMODULATION FOR SMALL-SIGNAL AMPLITUDE ESTIMATION

A circuit for estimating the amplitude of small signals (in the range of microvolts to a few millivolts) developed initially for an electric al impedance tomography (EIT) data acquisition system is presented. Th e paper is focused on two points, the accurate demodulation of the dif ference of two sinewave signals and the common-mode correction of the measurement. The method is oriented for applications in measuring inst ruments, where a signal source (sinewave oscillator) is assumed to be incorporated. It uses a sample-and-hold circuit, suitably driven, as a n amplitude demodulator. One of the advantages of this method is that the realization of the synchronous demodulator (or phase-sensitive det ector) does not require a low-pass filter at the output, resulting in a smaller settling time, that is, in higher speed of measurement. Fina lly, to improve the accuracy, a self-calibration technique has been de veloped.