Evaluations of a Detector-Limited Digital Impedance Bridge

Abstract

We tested a simple digital impedance bridge using two nominally equal resistors to form a 1:1 ratio. We focused on resolution and stability of the detectors. Fluctuations of the source voltages were largely removed through postprocessing of the digitized data, and the measurement results were limited by the detector noise. This detector-limited operating condition was first demonstrated using three modified Keysight 3458A multimeters for measurements of the voltage ratios, achieving 0.01 μV/V type A uncertainty in less than 15 min at 1 kHz. In an effort to extend the applicable frequency range and develop a system with off-the-shelf components, we tested a system using three lock-in detectors for measuring small deviations from the perfect AC ratio of unity magnitude, achieving stabilities and resolutions of 0.1 μV/V in a few hours for each point from 1 kHz to 5 kHz.

Comparison of a 100-pF Capacitor With a 12 906-Ω Resistor Using a Digital Impedance Bridge

“…The detection system based on the DAQM909A for measuring ac voltage ratios compares favorably to the system based on the SR860 lock-in detectors which we evaluated previously [15]. We achieved a factor of 10 improvement in terms of the Allan deviations for the impedance ratio measurements over a comparable averaging window.…”

“…The IVD output (A) and the low-potential port (B) of Z2 form a 2TP potential loop of a digital bridge with two voltage detectors through a custom coaxial switching fixture, which was described previously [15]. The two detectors were periodically interchanged to minimize the effect of their gain drift.…”

“…The measurement accuracy of the DAQM909A for ac voltage ratios depends on not only the resolution of the digitizer but also the gain stability of the input amplifiers. To determine the limitations of the digitizer, we connected two input channels of a DAQM909A in parallel to the same sinewave voltage, with a rms value of 0.1 V at 1 kHz, similar to the tests described in the previous paper [15] when the two SR860 lock-in detectors were used to measure large ac signals. The best results were obtained when the two input channels, set at the 0.3 V input range, were periodically interchanged through the coaxial switching fixture, creating two virtually identical digitizing channels.…”