Differential Probe Measurement Error???

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endowaudio
Newbie
Posts: 1
Joined: Thu May 23, 2019 3:16 pm

Differential Probe Measurement Error???

Post by endowaudio » Thu May 23, 2019 3:26 pm

Hello, just learning how to use the 441 1:1 differential probe with a 4444 scope. The signals being measured did not make sense, so in an attempt to isolate the problem we tried many things including having the red lead of one channel and the black lead of another channel connected to the same node on the DUT. No other leads are attached; so no channel has both a red and black lead connected to the DUT. But, there is a signal displayed by the scope on both channels. The channel with the black lead is inverted relative to the channel with the red lead and both are a 500 Hz sine wave with an amplitude of about 1/2 volt. How is it possible to have a signal from the differential probe with only one lead connected per channel? Tried connected the alligator ground lead to the ground terminal on the 4444 chassis but that did change anything. Any advice is greatly appreciated!!!

Gerry
PICO STAFF
PICO STAFF
Posts: 471
Joined: Mon Aug 11, 2014 11:14 am

Re: Differential Probe Measurement Error???

Post by Gerry » Thu May 30, 2019 10:07 am

Hi endowaudio,

As long as there is a return path via ground you can pick up a signal (even without the ground clip connected. there will be a return path back to mains via the picoscope earth, and then back to the DUT via the power supply to it, unless it is running on batteries, and the fact that connecting the ground clip made no difference confirms that there is another connection to ground).

The differential probe still has a common mode of operation, so any voltage appearing on one probe will generate a difference in potential with respect to ground. The idea behind a differential probe is that it can reject common mode voltage when you use both probes. If you want to understand it's operation better you could look at our guide on active differential probes here: https://www.picotech.com/download/manuals/choosing-the-right-Pico-Technology-active-differential-probe.pdf, most of which also applies to passive differential probes.

Regards,

Gerry
Gerry
Technical Specialist

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