Introduction

Strain measurements must often be made in the presence of electric and/or magnetic fields which can superimpose electrical noise on the measurement signals. If not controlled, the noise can lead to inaccurate results and incorrect interpretation of the strain signals; and, in severe cases, can obscure the strain signals altogether. In order to control the noise level, and maximize the signal-to-noise ratio, it is necessary first to understand the types and characteristics of electrical noise, as well as the sources of such noise. With this understanding, it is then possible to apply the most effective noise-reduction measures to any particular instrumentation problem.

This publication identifies some of the more common noise sources, and describes the routes by which the noise is induced into strain gage circuits. It should be noted that the treatment here is limited to noise from external electrical and magnetic sources. This note does not cover effects from nuclear or thermal sources, nor does it consider the effects of variable wiring or contact resistance caused by slip rings, connectors, switches, etc. Following the discussion of noise sources, specific methods are given, varying with the noise-coupling mechanism, for noise avoidance. The information in this technical note is equally applicable to both analog and digital systems employing dc amplifiers. It also applies to systems using carrier excitation and carrier amplifiers.

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