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Understanding Terms - Crest Factor

Some multimeters will give the crest factor in the specifications.  Why would we want to know anything about crest factor parameters when choosing a digital multimeter?  A lot of the time you probably wouldn’t, because you are only measuring DC or a pure sign wave AC.

However, sometimes it is necessary to measure a wave form that is not DC or a pure AC sine wave, for example, a pulse width wave output from a digital source, or a wave from an inductor that is cycling a charging and collapsing magnetic field. 

What is Crest Factor?

Crest factor is the ratio between the RMS (Root mean square) value and the peak value of a wave form.  It is calculated by dividing the peak value by the RMS value.  If you are not familiar with RMS, see our guide on RMS 

The graph below shows a pure sign wave with the RMS at 0.707 and the peak at 1. The crest factor here would be 1 divided by 0.707. That is 1.414

 Sine wave with RMS value

If a pulse width digital wave output is on for a short time, then off for an extended time in its cycle, the RMS (equivalent DC effect) will be low compared to the peak, hence the ratio between the RMS and the peak will be high. That is, the crest factor will be high. The graph below shows this.  Here the crest factor would be about 3 (Peak/RMS).

 Low RMS wave 

If the opposite is occurring and the output is on for a longer time and off for a shorter time, then the RMS value will be closer to the peak value hence the ratio will be lower.  That is, the crest factor will be lower.  In the example below the crest factor would be a little bit more than 1.

 High RMS wave 

A collapsing magnetic field on an inductor can cause large spikes in the wave peaks, but the RMS value can be rather low in comparison to these peaks.  If you have cause to be measuring such wave forms, you need to choose a DMM that is designed to measure a wave form with the high expected crest factor.

If the crest factor is outside the specifications of the multimeter, then the measurements will become inaccurate.

In conclusion, if the type of measuring you are doing only involves DC or pure sine wave AC, then crest factor will not be a major consideration in regards to the multimeter specifications.

If however you are measuring other wave forms, and expecting larger variables in crest factor, then you need to choose a multimeter with appropriate crest factor specifications.