The required frequency range of electrical bioimpedance (EBI) measurements depends on the dielectric properties of the object and covers about three to four decades typically within the kHz to MHz range. The accuracy of fitting of the equivalent circuit-based impedance model with the measurement results depends on the number of frequency components used. In general, the use of more frequencies improves the fitting accuracy but complicates the measurements. However, the fitting accuracy also depends on the signal-to-noise ratio (SNR) of measurement signals. If covering the frequency range with a simultaneous multifrequency excitation, the root mean square values of its frequency components are decreasing when their number increases. It follows that the number of used frequency components has two contradictory effects on the accuracy of estimation of parameters of the equivalent circuit model. This chapter deals with the analysis of typical EBI model-fitting situations and shows how the number of frequency components influences the fitting error. Our research shows that in the case of multifrequency excitation the accuracy of fitting of the typical equivalent circuit-based EBI model may not increase when the number of frequencies is increasing. The results described here could be useful in other implementations of impedance spectroscopy.
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