APPENDIX A WHY SO MANY TERMINALS ? ( Understanding impedance measurement techniques )

Automatic test systems that check components “in-circuit” have reawakened interest in multiterminal techniques for measuring impedance, initially developed for laborato y applications. State-of-theart impedance measurement methods, incorporated in complex bridge configurations, use as many as ‘four-pair” (or eight-terminals “for determining a single component value. Starting with “two-terminal: an working up to ‘eight-terminal” techniques, this article explores multi-terminal methods for measuring impedance, indicating types of errors and typical applications for each method. To illustrate these methods, the following discussion uses ideal voltmeters and ammeters (i.e.-the voltmeter does not draw current and the ammeter does not develop voltage). Commercial instruments using either bridge or meter techniques are not ideal and will contribute additional errors. Negligible in most applications, inaccuracies introduced by these meters can often be measured and correctedfor in the equations.