An Automatic RMS/DC Comparator

The design of an instrument for the automatic comparison of an ac voltage with a stable dc source is described. A differential multijunction thermal converter is used as an rms/dc converter with an FET-switched input amplifier for ac/dc substitution. The output voltages of the rms/dc converter with ac and dc input voltages are sampled and stored, and the difference amplified and displayed on a panel meter or chart recorder. Accuracy is ±20 ppm of input ranges of 10-200 V at frequencies of 50 Hz-1 kHz, and maximum full scale deflection sensitivity is 0.01 percent of input range. The instrument may be used either as an rms comparator with a linear voltage scale or as a mean-square comparator with a linear power scale.