33.1 Introduction

Electrical impedance may be defined as an apparent opposition to the flow of current in an electric circuit. Quantitatively, impedance is the ratio of the voltage across a circuit to the current flowing in the circuit (a generalized form of Ohm s law):

(33.1)

where <i class="emphasis">Z</i>=impedance, <span class="unicode">?</span><i class="emphasis">          V</i>=voltage across <i class="emphasis">Z</i><i class="emphasis">           I</i>=current through <i class="emphasis">Z</i>

An understanding of the impedance concept is important for proper use of nearly all electrical measuring instruments and the correct interpretation of the measured values. Whenever a measuring instrument is connected to another device, attention should be given to the effect of the added loading (or change in the total impedance attached to the device) on the operation of the device, particularly with respect to the accuracy of the measured values.

Ideally, the application of a test instrument to a device under test would not disturb the operation of the device, and the measured values would be identical to those values in the device when the instrument was not connected. Unfortunately, test instruments require extraction of energy from the device under test (by drawing a current) and thus modify the operation to varying degrees depending on the impedance levels of both the test instrument and the device under test. For example, almost all electronic circuits will...