I'm performing some measurements with a differential probe and I'm not sure what is the correct attenuation ratio I should use (measuring about 300 V voltage drop on a varistor, which is a surge protection device), so I ended up trying them both. I got the more reasonable results using 50:1 ratio, but the thing is I got a measurement of 316 V while the probe's datasheet indicates that the max voltage I should use in 50:1 is 140 V, but while trying 500:1 ratio my results didn't make any sense.

Is there an explanation for that?


1 Answer 1


I suspect you are using ( but you forgot to mention) this probe

Allowable differential voltage
(between + and – terminals)
±140 V (DC + ACpeak) at 50:1 attenuation  (DO NOT USE)
±1400 V (DC + ACpeak) at 500:1 attenuation  ( OK to use)

If your CMRR noise is high then when you put both probes on same line, it will not be a flat line. Use this as your calibration to prevent saturation or to calibrate each probe.

ACtive FET diff probes are also very ESD sensitive so be careful.

There are other tricks such as using twisted pairs to source, keeping leads short, adding Ferrite CM chokes but these will affect transient response so test on a high BW DSO for best results calibrated with a flat line on same square wave.


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