Question

Can you explain the meaning behind the amplitude linearity specification found on Endevco® accelerometer data sheets?

Answer

Amplitude linearity is a measure of how linear the output of an accelerometer is over its specified amplitude range. It is sometimes called amplitude non-linearity, since it specifies the deviation from perfect linearity. Ideally, an accelerometer would have exactly the same sensitivity at any amplitude point within its specified amplitude range. In actuality, however, such an ideal does not reflect the actual operation of a typical accelerometer. Thus, as it relates to the real-life operating conditions of an actual accelerometer, an amplitude linearity specification is simply a means of identification of the limitations placed on how far accelerometer output deviates from perfect linearity.

There are several industry recognized means of properly specifying amplitude linearity. The most restrictive is to specify percentage of reading, typically ±1% over the entire full scale range. This is a close tolerance specification, as it means that accelerometer sensitivity cannot vary by more than ±1% at any point in the amplitude range. A much less restrictive way is to specify linearity in a piecewise manner, such as can be found in this example: 1% per 500 g, 0 to 2000 g. The significance of this specification is simply, that at the top end of the amplitude range, sensitivity can vary by as much as 4% from the low end of the amplitude range.

In environments where multiple vibration frequencies are present, intermodulation distortion, also commonly known as IMD or intermod, can result from amplitude linearity errors, creating frequencies in the instrumentation that were not present mechanically at the point of accelerometer "input." A basic definition of this term is the condition by which undesirable amplitude modulation of accelerometer signals with multiple unique frequencies are present within a system with non-linearity. A detailed discussion of intermodulation distortion is beyond the scope of this article. However, it is important to have a basic understanding of this phenomenon, as amplitude linearity errors can be a cause of signal distortion, particularly when measuring high-amplitude accelerations.