Avoiding scope errors (Part 4)

Part 1 tackles insufficient bandwidth and/or sample rate. Part 2 covers the selection of incorrect probe or its improper use. Part 3 covers inappropriate use of the averaging function. This final part of a four-part series covers choosing the wrong timebase.

One limitation of oscilloscope measurements is the linear timebase. The linear time scale does have its advantages, but it requires us to know where to look. The example below shows the same image taken with two different timebase settings and two different amplitude settings. If we look at the step load response of this POL at 2 mS/div, we see a very well-behaved device with a peak excursion of only 5 mV and a well-behaved recovery. In the second image, recorded at 200 mV/div and 200 mS/div, we can see the 5V output has peak excursions of nearly 1 V. The logic powered by this regulator will fail to operate at 4 V, because the voltage is too low, and could destroy the circuit at 6 V, which far exceeds the absolute maximum rating of 5.5 V.

Recommendation
Know what you are seeking, so that you can set the timebase apropriately. If you are uncertain, capture the same image at several timebase settings.

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

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