A Dual-Channel Automated Comparator for AC-DC Difference Measurements

An automated ac-dc difference calibration system is described. The system incorporates a new electronic comparator which determines ac-dc differences of thermal voltage converters (TVC's) by simultaneously measuring the difference between nearly equal ac and dc voltages with both the test and standard instruments. The comparator consists essentially of two practically identical channels each containing a digital-to-analog converter balancing circuit, an operational amplifier to amplify voltage imbalance, and an integrator circuit in which a capacitor is charged during an accurately controlled 10-s period. The difference between the ac and dc voltages applied to the test and standard TVC's is computed from differences in capacitor voltages, and the ac-dc difference is derived from the variation between the test and standard channel indications of this voltage difference. Measurements are made in about half the time required for our manual procedures.