A New Wideband True Rms-to-Dc Converter

A new wideband true rms-to-dc converter is described, covering the ranges of 10 Hz to 100 kHz and 30 mV to 1 kV full scale, with crest factors in excess of 7 to 1. Operation is based on use of two precision dual-heater thermistors in an automatic double bridge, by means of which the temperatures of both thermistors are maintained constant to better than 0.002°C. Converter linearity is 0.02 percent over a four-to-one input range, with response typically 2.5 seconds to settle within 0.1 percent of final value. Applications range from ac signal conditioning for digital voltmeters to 0.01 percent ac-ac transfer work-with nonsinusoidal wave forms as well as the quasi-sinusoids typical in field measurement situations.