When I make a measurement using a scope and observe transducer voltage output on the scope screen, I want to eliminate the noise as much as possible which is due to device and scope coupling or common mode noise like 50 Hz and its harmonics or capacitive coupling ghost voltages. I want to be left with the transducer's own noise(intrinsic) as much as possible.

After reading this question I still am not sure about the common practice.

Is it better to use 3-prong or 2-prong(aka eliminating the earth) in many cases for eliminating ground loops or capacities coupling between the device to be measured and the scope?

Is the only reason to use 3-prong for a scope safety?

In instrumentation should the instrument be powered from an isolation amplifier to eliminate ground loops or coupling noise? Can you provide a diagram which illsutares some good practice?

  • 1
    \$\begingroup\$ Add some more details, scope, instrument, methods you used until now, isues. You should not remove the earth on your scope, this will not eliminate the noise, rather you get it even bigger. Using scope without erath is a shock hazzard, risk of electrocution. \$\endgroup\$ – Marko Buršič Sep 25 '18 at 12:30
  • \$\begingroup\$ I'm curious about this myself. I often disconnect the earth GND for various reasons even though everybody says not to. I'll bet I'm not the only one. \$\endgroup\$ – Drew Sep 25 '18 at 16:35
  • \$\begingroup\$ @DrewI agree Ground loops and capacitive coupling appears as noise \$\endgroup\$ – floppy380 Sep 25 '18 at 16:49

Yes, the only reason to use 3-prong is safety (but it’s an important reason, don‘t you think?)

And yes, the only reason to use 2-prong instead of 3-prong is to remove ground loops which could affect your measurement.

But with a differential probe, you can use 3-prong (safety) without ground loop -> that’s the way to go!


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