A Comparatorless Dual-Slope Digital Voltmeter

This paper discusses the design and operation of a digital voltmeter employing a new technique in the analog-to-time converter. The circuit is based on dual-slope integration, which eliminates the necessity of a highly stable clock. Any long term drift in the clock is canceled, so that a simple two-transistor multivibrator clock is used. With the aid of a special diode circuit, the ramp-generating integrator also serves as an accurate comparator, so that no separate comparator is needed for analog-to-digital conversion. Finally, the new circuit requires only a single up counter, and no special polarity switching circuits for automatic polarity indication are needed. The new voltmeter, developed for the University of Arizona's new Locust Computer, has full-scale ranges of ±1 volt and ±10 volts. Tests on the instruments have shown its accuracy to be ±0.02 percent of half-scale. The sampling rate can be varied from 80 ms to 1 second, and the input impedance is 2 MU on the 10-volt range.