An Improved ac Bridge Circuit for Use in Four-Terminal Resistance Thermometry

Details are given of an improved version of the ac Kelvin bridge first described by Ekin and Wagner [Rev. Sci. Instrum. 41, 1109 (1970)]. Battery-operated operational amplifiers with highly superior noise performance at 33 Hz were used for the voltage followers required to make the bridge compatible with four-leaded germanium resistance thermometers. A discussion of bridge operation, accuracy limitations, and final performance characteristics is presented, including a confirmation of the design objective to accurately measure a ΔT of 0.5 mK at 4.2 K, with a 1 mV germanium sensor excitation and a 1 Hz detector bandwidth.