Impedance Comparison Using Unbalanced Bridge With Digital Sine Wave Voltage Sources

This paper presents a digital automatic unbalanced bridge designed for comparing the impedances of R-R, R-C, and C-C. The measurement principle uses initial balancing of the bridge using two digital sine wave sources, then measuring the ratio of voltage drops on the compared impedances as well as the finite bridge unbalance voltage. The present method is novel because it is not necessary to obtain a complete voltage nulling and also because it is readily implemented with a commercial data acquisition (DAQ) card. An NI USB6281 DAQ card was used to obtain the results; this card combines the two 16-bit digital-to-analog converters with one multiplexed 18-bit analog-to-digital converter. Such a measurement system, which is relatively simple and inexpensive, can compare impedances with an error below 10 ppm. Some considerations concerning the relationship between the measurement accuracy of unbalanced voltage and the level of the bridge unbalance are given.