Temporal Resolution of the Skin Impedance Measurement in Frequency-Domain Method

The temporal-resolution of the frequency-domain method for the identification of the impedance locus depends on the basis frequency used in the current waveform construction, i.e., the higher basis frequency provides the better temporal resolution. The impedance locus can be characterized by the impedance parameters Z₀, beta, and tau_m. The frequency distribution of limited number of data in the impedance locus would affect the accuracy of the estimated impedance parameters. Therefore, we investigated the relationship between the estimation accuracy of the impedance parameters and the frequency coverage of the impedance locus in relatively low to high impedance conditions (dc impedance Z₀=51 kOmega-45 MOmega). As the basis frequency, 100 Hz was enough for the usual impedance with Z₀ less than 203 kOmega. On the other hand, 10 Hz and 1 Hz were required for the medium-level (Z₀=517 kOmega), and high-level (Z₀=45 MOmega) impedance, respectively. The required basis frequency, accordingly the temporal resolution, depended much on the central relaxation time tau_m which affects the frequency distribution on the impedance locus. The results of this study are expected to serve as the reference of the frequency selection in the frequency-domain analysis of the skin impedance