A digitally-assisted current comparator for audio frequency impedance scaling

A current comparator bridge, suitable for the comparison of four terminal-pair impedance standards in the audio frequency range at 1:1 and 10:1 nominal ratios is here presented. The bridge is digitally-assisted: its accuracy is granted by electromagnetic devices, but voltages and current required to reach principal and auxiliary equilibria are given by a polyphase direct-digital-synthesis generator. The measurement is semi-automated and equilibrium can be achieved in a few minutes. Measurements performed on calculable resistors give a base accuracy of a few parts in 10⁷, with the potential for further improvements.