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Measuring Voltage Balance Using a Switching Scheme

Svetlana Avramov-Zamurovic1, Brian Waltrip2, Andrew Koffman2 and George Piper1 1 United States Naval Academy, Annapolis, Maryland, USA avramov@usna.edu 2 National Institute of Standards and Technology†, Gaithersburg, Maryland, USA

Introduction Most precise electrical impedance measurement procedures involve a voltage balance. A voltage balance is defined as the condition in which the difference between two voltages is driven to zero by independent injections. The uncertainty of the measured value directly depends on how close to zero the balance was driven in magnitude and phase. The uncertainty level required by current state-of-the art impedance experiments is on the order of 1×10-6 (one part per million). Traditionally, voltage balance has been achieved by using inductive voltage dividers in a bridge configuration [1], [2]. The inductive voltage dividers are used because they can provide acceptable uncertainty. An attempt is made to explore the possibility of using electronics instead of inductive voltage dividers in a generalized bridge configuration.

This paper will describe in detail a measurement system that requires a precise voltage balance. The measurements involve a switching scheme that aides the iterative process of driving the voltage difference to zero. The mathematical model of the system will be described and the balance voltage derived. The validity of the assumptions will be checked using software simulation. This is preliminary work that requires experimental confirmation. Based on the results of the proposed algorithm an experiment may be planned in which the system will be constructed and the suggested measurement procedure tested.

In the Systems Engineering Department at the U.S. Naval Academy, we strongly emphasize capstone design projects. The proposed research presents a very good opportunity to teach students the full design cycle. This experiment is well defined using mathematical tools and in that sense it is a straightforward task to develop a software simulation. The actual construction of the measurement system is rather complex but students will benefit from using it once it has been completed. Making measurements to verify the model increases our understanding of the developed system.

Measurement System In the field of precise measurements, it is often desired to balance voltage differences. In the example that will be presented in this paper, a generalized circuit that operates in the audio frequency range is given (see Fig. 1.) Voltages V1, V2 V3, and V4 are node voltages derived from an impedance bridge. These voltages

† Electronics and Electrical Engineering Laboratory, Technology Administration, U.S. Department of Commerce. Contribution of the U.S. Government. Not subject to copyright in the U.S.

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Avramov-Zamurovic, S. (2005, June), Measuring A Voltage Balance Using A Switching Scheme Paper presented at 2005 Annual Conference, Portland, Oregon. 10.18260/1-2--15124