Method using bilinear transformation for measurement of impedance parameters of a multi-element two-terminal network

For many years, impedance spectroscopy is one of basic research methods for objects which can be modeled by a multi-element two-terminal network (e.g. in electrochemistry for testing of parameters of anticorrosion coatings, in biomedicine for research of pathological changes on the basis of impedance parameters of skin, biological liquids). It consists in the measurement of the object impedance Zm in a wide frequency range and an analysis of the obtained frequency profile of the impedance. Using the obtained impedance spectrum and the information about the nature of physico-chemical phenomena appearing in the object under test, it is possible to construct its equivalent circuit in the form of a multi-element two-terminal network, its elements describe the tested phenomena. The last stage of the research process is an identification of the values of components of the two-terminal network. The most commonly used tool for fitting of the parameters of the equivalent circuit to empirical data is the CNLS (Complex Nonlinear Least Squares) iterative method. The literature describes many computer programs performing parametric identification based on the CNLS method. The following programs can be listed: LEVM, Zview/Zplot, EQUIVCRT.