Why an Oscilloscope Probe Derating Curve Matters

The derating curve cannot be ignored!

If you don’t follow your probe’s derating curve, you could damage your probe leading to flawed measurements.

In this episode of Probing Pitfalls, Melissa explains the dangers of ignoring the derating curve of your passive probes. She’ll also explain how to read and follow derating curves. You’ll leave the video with an understanding of what the derating curve is, how to determine safe measurement constraints for the probe you are using, and the consequences of measuring beyond the derated specifications of the probe.

She will explain:
• Frequency changes the capacitive reactance of a probe – as frequency increases, the probe’s capacitors are more susceptible to being damaged with respect to voltage
• Power, resistance, and dissipation factor effect the voltage derating of a probe in addition to frequency and capacitance
• The derating curve specific to your probe can be found in its datasheet

Stay tuned for future Probing Pitfalls episodes covering common probing mistakes and how to avoid them to ensure you are making accurate measurements.

Probing Pitfalls is hosted by Erin East and Melissa Spencer. The video series covers common oscilloscope probe pitfalls, the impact they have on your measurements, and how avoiding them can improve your test results. You’ll learn proper probing practices that will save you time in validating and debugging your designs, plus improve your overall knowledge and skill in the test lab.

#oscilloscope #oscilloscopes #deratingcurve #probeproblems #probing #oscilloscopeprobing #probe #oscilloscopeprobes

[CNe]

This is why whenever you get anything in life, you read the documentation (manuals, brochures, and labels) that came with it or seek it out if it got lost along the way. Regardless of whether you have a firm education on the subject matter or not. You could either be missing out on features or warnings like this. Young hams, telecomms, and radar workers, this one is for you.

Today's lesson is somewhat a combination of "Ohm's law" and the fact that a capactor basically becomes "like a wire" or "short to ground" with AC and RF signals. Think I*Z=V. There are a whole bunch of other things to talk about and its much more complex than this but basically the best simplification I can think of at the moment.

So far, as a personal rule of thumb, as long as your signal is under 1W along with your probe's resistance (part of Z) you should be fine. Again check the specs or at least the label b/c you don't know if its a 50ohm, 1megohm, 1k or other ohm probe.

About 300V @ 1GHz signals, I wouldn't even go near the source of it unless required to do so with protection/preventative measures. You might as well imagine putting yourself into a microwave oven. RF burns aren't pretty either. Insidious too since you can't even feel it. Don't worry though because a ~300V @ ~1GHz (even MHz) signal is ultra rare unless you are lucky enough to get a hand-me-down part from a telecomm or radar company. That or purposely mess around with a microwave oven.

You and your equipment can still get fried though. A typical offender personally witnessed are those working with a powerful (25W or more @50ohms) transceiver/radioset as licensed hams. This too is rare since they know and should take proper precautions (don't get be that careless person). ;)

[JamesLebihan]

Very well explained. Thank you for doing these videos, they are very helpful.

[AXUNSAS]

Good morning, thanks for this video. I think there is mistake in the formula. Explanation : P=U²/R -> U=sqrt(P*R). Knowing that ESR = DF/(2pifC) -> U=sqrt(P*DF/(2pifc)).
In other words the d=DF=Dissipation factor should have been place on the top of the division and not under the disition. Roland.

[RobeenaShepherd]

Last video I was scared of believing measurements, now I'm scared to even measure a circuit! Okay not really, but this is lots more I never thought about, especially the limit in maximum voltage vs frequency.

[ronshaw]

Hummm...good information. How do I find the curves for my older probes? I got them used, so no documentation was present as far as derating curves are concerned.