AC-DC comparators for audio-frequency current and voltage measurements of high accuracy

This paper surveys recent developments in high-accuracy ac-dc comparators, the basic standards for ac current and voltage measurements at audio and higher frequencies. These instruments compare the unknown ac quantity with a dc reference. The ac-dc transfer characteristics of thermal converters, the most widely used comparators, can be evaluated to about 10 ppm at audio frequencies in national metrology laboratories. With recent developments even better accuracies, and greater convenience with automatic comparisons, are feasible.