Synchronous Sampling and Demodulation in an Instrument for Multifrequency Bioimpedance Measurement

Direct sampling of known carriers is the preferred digital method for measuring biomodulation of tissue impedance. Due to limited resolution and conversion rate of analog-to-digital converters and limited processing power of available digital processors and/or lack of energy resources, conventional discrete-Fourier-transform-based algorithms are not efficient in small medical devices. Knowing exactly the frequencies of carriers (and excitations), an energy-saving fast signal processing method can be developed and implemented. When sampling synchronously with a carrier, it is possible to minimize the complexity of calculations and to introduce a digital-to-analog feedback for enhancement of resolution by digitizing only the small variations between adjacent samples. The proposed system is qualified on proprietary hardware.