Cs. Koukourlis et al., DIFFERENTIAL SYNCHRONOUS DEMODULATION FOR SMALL-SIGNAL AMPLITUDE ESTIMATION, IEEE transactions on instrumentation and measurement, 42(5), 1993, pp. 926-931
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.