A Self-Balancing Digital Differential Voltmeter Based on the Direct-Current Comparator

The differential voltmeter described is based on the dc comparator, so that the voltage adjustment is made by varying turns and not by selecting taps on a resistive divider. The linearity errors are, therefore, permanently less than one least significant digit. One, ten, hundred, and thousand volt ranges with impedances of 1000 Ω/V are provided; also provided are high-impedance ranges of 1-10 V. Either positive or negative voltages may be measured. On all except the 10-V high-impedance range, there is a 60-percent overrange capability. The method of automatically converting the analog-input signal to a digital display, the main sources of error, and the techniques and components used to keep the error less than one least significant digit are discussed.