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.
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Min et al. (2007) studied this question.
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