Résumé
Bioimpedance spectroscopy consists in measuring the complex impedance of biological tissues over a large frequency domain. This method is convenient in particular for studying body composition, blood characterization and even cancer detection. This wide range of applications makes it suitable as a part of health monitoring systems. Today's self-monitoring devices tend to be portable, wearable or even implantable. Next generation bioimpedance sensing systems thus require to be implemented with power and resource savings in mind. Impedance measurement methods are divided into two main categories. Some are based on "single-tone" signals while the others use "multi-tone" signals. First methods use a pure frequency sine wave to make the measurement. They benefit from a very simple analysis that may consist in synchronous demodulation. However, the operation must be repeated for each frequency over the domain of interest. Due to this necessary frequency sweep, the total measurement may take long. On the other hand, generating a multi-frequency signal allows the analysis to cover the whole frequency range simultaneously. This is at the cost of a more complex analysis algorithm (discrete cosine transform-DCT, typically). Unfortunately both methods result in excess power consumption: long time of measurement for single-tone frequency sweep, hardware and computational resources for multi-tone. That make both approaches hardly suitable for embedded applications. In this paper, we propose an intermediate approach that combines the speed of multi-tone systems with a much simpler analysis algorithm than DCT or FFT. Using specific properties of the bioimpedance, we even show that we can get rid of any multiplier. This results in a minimal implementation using only adders and synchronous ADC. For optimal performances, this small footprint digital processing can be easily synthesized and embedded on a mixed-mode ASIC together with the analog front-end.