Error correction algorithm for high accuracy bio-impedance measurement in wearable healthcare applications

High accuracy in measurement of physiological signals like skin impedance is critical and is more difficult to achieve in biomedical systems with very limited power budget. A new method for accurate bio-impedance measurement using square wave demodulation is proposed in this paper. An error correction algorithm has been derived from the analysis of a generic impedance model with multiple poles, which reduces the estimation error in square wave demodulation from 9.3% to 1.3% for a simple tissue model. Simulation results in Matlab using ideal RC values show an accuracy of <;0.14% for single pole and <;0.67% for two pole RC networks. Measurements from saline phantom solution gives an accuracy <;0.72%. The results indicate that the algorithm can be used as a postprocessing technique for error correction or even incorporated into wearable signal monitoring SoCs since modulation and demodulation using square waves consumes less power compared to sine waves.