In a computer controlled impedance setup, the current sensitivity of the Solartron SI-1260 Impedance/Gain Phase Analyzer is improved by six orders of magnitude by the Keithley 428 Current Amplifier. Impedances up to the order of 10(13) Ohm can be measured at low frequencies with the ne signal of 30 mV rms. Spectra at constant temperatures are acquired in one of two arrangements : (i) electric oven with do power supply controlled by Eurotherm; (ii) closed cycle helium refrigerator with analog controller. Sample holders have an open circuit capacitance below 1 pF. The available frequency range is 1 mHz to 10 MHz. Current to voltage converters produce distortion of gain and phase at the high frequency end of their bandwidth, which decreases with increasing amplification. The measured impedance, Z(M), can be related to the actual impedance, Z(X), by : Z(M) = Z(X)(1 + eta(1)j omega - eta(2) omega(2) - eta(3)j omega(2)) The corrections are incorporated into the program for the nonlinear least-squares analysis of the impedance spectra. The parameters : eta(1), eta(2), eta(3), characteristic for a given amplification, can be estimated during fitting of an equivalent circuit. When corrections are used, the residual error of fit is much lower for data which are obtained with extended use of the more sensitive current ranges and exhibit distortions, than for data, which are obtained by the less sensitive current ranges and contain large scatter. Examples of impedance spectra and their simulation using equivalent circuits composed of resistors, capacitors and constant phase elements are presented for the oxygen ion conductor BICUVOX.