TABLE OF CONTENTS Yoh Narimatsu
Agilent Technologies
Kobe, Japan
Impedance (admittance for its reciprocal value) is a fundamental parameter associated with electronic materials, components, and circuits (these are inclusively called devices in the later sections). When a current flows through a device, the device gives an opposition to the current flow; the degree of opposition is called resistance if the current is a direct current (dc), and impedance if it is an alternating current (ac). Ohm s law governs the relationship between the resistance, the current, and the resultant voltage drop across the device under test (DUT) for dc cases, and can be mathematically extended to deal with ac cases. The DUT impedance can, therefore, be expressed by the ratio of the voltage across the DUT to the current flowing through it. However, impedance is a more fundamental property inherent to a device; it can be accurately determined and maintained.
The many impedance- or admittance-measuring methods vary in DUT type, cost, accuracy, and test conditions. For many years, a variety of bridges, most of which were manually operated, were used for medium- to high-accuracy measurements. Nowadays, however, digital impedance meters (names vary widely), which automatically perform impedance measurements, have also been widely accepted, owing to their ease of use (less susceptible to human error), relatively high accuracy, and convenience for systemization.
When a direct current is applied to a DUT, the relationship between parameters is expressed by Ohm s law:
