Ammeter influence on the frequency response measurement of a dielectric

Abstract

Determination of dielectric spectra by current-voltage measurements is investigated in order to point out the role of the ammeter. Conditions where its presence may be ignored in results analysis are obtained by a complete resolution of the modelled circuit equations and through a thorough examination of curves resulting from microcomputer simulations. The more prominent aspects are brought out for a Debye-like dielectric. The initial transient regime, in the framework of the model considered, proceeds only from dielectric relaxation. As a result, paradoxical behaviour occurs: for frequencies higher than the relaxation frequency, the transient regime's initial amplitude is a decreasing function of the voltage setting. A 'waiting time' ta is defined and studied as the time necessary for measurements to be free from initial perturbation. It is shown that ta is at most of the order of magnitude of the involved period, giving a lower boundary for the measuring time of slow phenomena at very low frequencies. A comparison between the effectively measured impedance Zm and the impedance Ze obtained by ignoring the ammeter reveals a drastic limitation in the frequency interval over which Zm approximately Ze. This limitation is described in terms of the capacitive ratio and a compatibility index Lambda =log ( tau 0/ tau m).