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 electrical impedance tomography (EIT) data acquisition system is presented. The paper is focused on two points, the accurate demodulation of the difference of two sinewave signals and the common-mode correction of the measurement. The method is oriented for applications in measuring instruments, where a signal source (sinewave oscillator) is assumed to be incorporated. It uses a sample-and-hold circuit, suitably driven, as an amplitude demodulator. One of the advantages of this method is that the realization of the synchronous demodulator (or phase-sensitive detector) does not require a low-pass filter at the output, resulting in a smaller settling time, that is, in higher speed of measurement. Finally, to improve the accuracy, a self-calibration technique has been developed.