Understanding Noise Specifications

Among the numerous parameters specified on accelerometer data sheets are the unit's noise specifications. Is this parameter important and when should it be of concern? The answer is that it is of little concern when making high level vibration measurements or for most high level shock applications. The noise specification is very important when making seismic measurements and other low-level and generally low frequency measurements.

Common terms used to specify the noise floor of an accelerometer are residual noise, threshold (resolution) and noise spectral density. This article will briefly describe these terms and their importance.

The residual noise is the overall rms noise level measured over a wide frequency range, which is specified on the data sheet. Residual noise is usually expressed in millivolts or microvolts rms.

A more useful term for noise is the threshold or resolution of the accelerometer which is expressed in g's. This month's technical paper has more information on this figure and how it is calculated. The threshold gives the user an indication as to the minimum discernable signal that can be detected. As a rule-of-thumb, most engineers find that 10 times the threshold yields the lowest level signal that can be comfortably observed.

Another useful and often misunderstood specification is noise spectral density. Data sheets give the actual power spectrum (in Volts2 / √ Hz) over specified bandwidths. In some cases an actual spectral plot is provided.

The noise spectral data is important in that it provides an indication as to the noise present within bands of interest. As an example, a user may be interested in collecting data over a bandwidth of 1 - 10Hz and plans to filter out higher frequencies in the measurement electronics. It can be seen that only the very low frequency noise spectral density would be of interest.

Noise information is rarely specified for charge-mode piezoelectric accelerometers. Since most of the noise will be generated as a result of the electronics and cable capacitance thus the noise level of the entire measurement chain is of paramount importance. The piezoelectric material, of the accelerometer generates a very insignificant amount of noise. Noise data can be found in Isotron (IEPE), piezoresistive and variable capacitance accelerometer specifications.

For more information, please refer to Endevco's technical paper on Practical Considerations of Accelerometer Noise.


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