Comparison of Low DC Current Traceability Methods and Gas Capacitors AC–DC Dependence

Abstract

This article compares two instruments for the traceable measurement of dc low currents, a custom capacitance–voltage ( $C\text{-} V$ ) source and the ultrastable low-current amplifier (ULCA), a commercial precision transresistance amplifier. The instruments are calibrated through independent traceability routes. The comparison base relative accuracy is in the $10^{-6}\!-\!10^{-5}$ range. Differences between the two instrument readings, in the $10^{-5}$ range, are interpreted as an effect of the frequency dependence of the capacitor employed in the $C\text{-} V$ source. Such frequency dependence can also affect primary metrology experiments in other fields.