High-Precision Thermal EMF Comparator for Automatic AC-DC Transfer Measurements

A high-sensitivity EMF comparator, suitable for automatic comparison of standard voltage or current thermal converters is described. One of the two compared EMF´s and their difference are simultaneously balanced to null with two potentiometric voltages produced on low-value resistors by controlled currents. Any direct switching or resistance adjustment in the circuit involving the EMF´s is avoided. Also the sensitivity control of the two light-beam-coupled nanovolt preamplifiers, utilized as null detectors, is performed only by feedback on the potentiometric voltages. Tracking between these voltages and one of the EMF´s reduces the balance instability due to random source variations and drifts. Additional amplitude stabilization is obtained directly on the ac and dc sources by deriving a feedback signal from the comparator. A first implementation of the system allows to obtain balances below the nanovolt level and exhibits a proportional standard deviation of 5 × 10-8 in a series of ten successive intercomparison measures of two thermal voltage converters with output EMF´s in the 10-mV range.