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Avoiding scope errors (Part 3)

Posted: 07 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:probe? point of load? voltage regulator? oscilloscope? measurement?

Part 1 tackles insufficient bandwidth and/or sample rate. Part 2 covers the selection of incorrect probe or its improper use. The third part of this four-part series covers inappropriate use of the averaging function.

The oscilloscope averaging function is an easy way to filter the noise in the measurement. Unfortunately, it is equally capable of filtering out the signal. Consider the case of a point of load (POL) voltage regulator. The lower trace is a load current step from approximately 100 mA to 500 mA. The oscilloscope is triggered on the rising edge of this load step. The upper display shows three captures of the output voltage in response to the step. The excursion is dependent on where in the switching cycle the step is applied. One capture (M4) shows a minimum excursion case, while another capture (M1) shows a maximum excursion. Capture M2 is the result of averaging 50 measurements.

Figure 1: The transient response of a POL. The transient is not consistent. It depends on where in the switching cycle the load is applied. The average (M2) result is neither the minimum nor the maximum excursion.

Figure 2: The same transient response using a colour persistence display.

The averaging function is useful in cases where the measurement is very consistent and the noise you wish to eliminate is not time correlated with the measurement. If you are uncertain, do not use averaging. Consider bandwidth-limiting eRes to active filtering, which, for example, is programmable in the Teledyne LeCroy Waverunner 6Zi oscilloscopes.

About the author
Steve Sandler is the founder of AEi Systems.

To download the PDF version of this article, click here.

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