Self-Compensating Networks for Four-Terminal-Pair Impedance Definition in Current Comparator Bridges

The four-terminal-pair (4TP) definition of impedance standards allows reaching ultimate accuracy in impedance metrology. In general, the 4TP definition requires dedicated circuitry to be included in the bridge network and the attainment of auxiliary bridge balances during the measurement. A careful choice of the network topology allows the bridge to be self-compensating: the 4TP definition is in large part achieved by the behavior of the network itself without any adjustment. The additional circuitry required for impedance definition can therefore be simpler, the balancing procedure becomes easier, and a more robust 4TP can be achieved. This paper analyzes a current comparator bridge network with a topology featuring self-compensation. A digitally assisted test bridge has been implemented; test measurements on ac resistance comparison in the 25–100-
$\Omega $
range at kilohertz frequencies are reported.