Probe Safety Demystified: Dynamic Range and Voltage Swing

One of the most basic things to know when using any probe is “what is the maximum voltage the device can safely measure?” The answer isn’t as straightforward as you might imagine, it requires understanding several key probe specifications as well as the nature of your signal.

Single-ended Range

Figure 1. Single-ended range is
measured voltage input to ground.

Everyone is pretty familiar with single-ended range: that's the maximum safe voltage input to ground, shown in Figure 1. Ground is directly tied to oscilloscope ground, which is tied to building ground. Therefore, when measuring voltage within this range using a single-ended probe, the ground connection cannot be a floating voltage, or you could damage the probe, the DUT, the oscilloscope...maybe yourself, as well. Single-ended voltage must be a grounded voltage on your board or something that could be tied to ground.

Probe Safety Demystified: CAT Ratings

Figure 1. CAT ratings required to
safely test different electrical sources.
(Sourced from “Measurement Categories,”
Wikipedia, Oct. 28, 2019.)

Any voltage probe will have several published specifications that are meant to indicate under what circumstances that probe is safe to use. They answer questions such as “What is the maximum voltage I can safely run through this probe?” and “Can I safely hold this probe while using it, or does it need to be mounted somewhere far away?” In this post, we’ll explain what the CAT rating is telling you.

CAT ratings are standardized ratings used to categorize the suitability of a voltage measurement device based on the source impedance of what it is used to measure. They are issued by the International Electrotechnical Commission (IEC).