Electrical Impedance Principles Measurement and Applications By Luca Callegaro

Devices and appliances of interest in impedance measurement

Common practice methods

Going digital

Some applications of impedance measurement

Metrology: traceability and uncertainty

Metrology: standards

Metrology: realization and reproduction

Motivation The interest in the accurate measurement of electrical impedance is shared by scientists and engineers from different backgrounds. Impedance measurements can be performed on an impedance standard, to perform a calibration and issue a calibration certificate. Electromechanical appliances and electronic components can be characterized by impedance measurement to identify the parameters of their equivalent electrical model. Properties such as resistivity, permittivity, and permeability of material samples can be derived from impedance measurements conducted with proper electrical fixtures.

 Biological quantities related to a tissue, or even to a living being, can be related to their impedance. Sensors of many physical quantities can have electrical impedance as their output. Impedance spectroscopy permits to follow the evolution of an ongoing electrochemical reaction; impedance tomography is an imaging technique. Despite such broad range of existing applications, and the potential for new ones, high school and university courses show a marginal interest to the subject of impedance measurement. Often, impedance measurement techniques are described as minor variations of the corresponding dc resistance measurements; recent publications may cite obsolete measurement techniques.

Difficulties When performing an impedance measurement, the experimenter faces conceptual and practical difficulties that are not encountered in resis tance measurements. Voltages and currents become geometry-dependent quantities, and different parts of the measurement circuit can interact in unexpected ways because of mutual capacitances and inductances. Even commercial impedance meters ask for careful wiring techniques, which may involve a number of conductors. The measurement result can be expressed in a variety of representations, related by non-trivial mathematical transformations, prone to be misinterpreted.