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What is the effect of connecting the tip of an oscilloscope probe to a ground lead? Here is one of the few links that I've found that shows the set-up: The Shorted Scope Probe. What are the possible uses of such set-up and what is the cause for the behaviour one might observe when shorting the probe (reduced susceptibility to environment noise, etc.)?

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I always do this when measuring any fast signals or when the circuit is noisy. Putting the tip to the ground clip should give you zero volts to the scope, but often it doesn't. That is because the potential at the ground point of the scope can be moving relative to earth (or the local reference point of the supply feeding the scope). Even if your circuit is "floating" there will be capacitance to ground. As rapid transient occur at the ground clip point, currents will flow in the probe ground wire. This will in turn generate voltage transients between the ground clip and the actual scope ground. These can be measured by placing the tip onto the ground clip. Any waveform that you observe when doing this will be superimposed on the real signal that you are trying to measure when you connect the tip to some point varying with respect to the ground clip.

If you see high speed transients when using a scope probe, try this test to check if they are due to this affect. This also applies to differential probes, connect the +ve probe to the same point as the -ve one to check that the differential probe is rejecting all the common mode signal.

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  • \$\begingroup\$ Great answer! There's a lot of concepts for me to comprehend so I will keep going back to it over time :) There is one question that I have right off the bat, however - when you refer to the set-up for detecting superimposed signal, do you mean that the probe is shorted to the ground wire and is measuring the signal at the same time? In this case the rapid transient would be passed through both the tip and the ground wires, right? But because of the capacitance that you have mentioned, the spike will stick around for longer in the ground wire, correct? Thanks again! \$\endgroup\$ – fandor Feb 15 '16 at 23:49
  • \$\begingroup\$ Glad you found it useful. What I meant about superimposed transients and signal is than when you move the probe tip off ground to measure the signal that you are interested in, you will also see the transients that you observed when looking at the ground clip. But not you will recognize them and know that they are not really appearing between the ground clip and probe tip. \$\endgroup\$ – user1582568 Feb 16 '16 at 9:17
  • \$\begingroup\$ As an example, suppose you put the ground clip on the 0V end of a 1uF capacitor decoupling 3.3V rail. You put the tip on the 3.3V end. You see 3.3V but with fast transient spikes. You are suspicious that they are really acorss the capacitor. You would put the tip also on the 0V end of the capacitor. If you now see 0V average but still see the spikes then you know that you are seeing a ground current effect and the transients are not really across the supply. The loop antenna thing mentioned in other answers is a good trick too! \$\endgroup\$ – user1582568 Feb 16 '16 at 9:22
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By forming a loop with the ground lead, you form an antenna. This can be useful for observing EMI issues with a circuit, or for triggering on an EMI spike. I have used a scope probe connected as shown to trigger on an ESD test pulse (connecting directly to the DUT is risky, as the ESD could damage the scope). Additionally, you can use the probe to find an area of a circuit that that is radiating excessively (set scope to show FFT, wave the probe around over the circuit).

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  • \$\begingroup\$ Thank you for your reply! This is a very interesting application. I knew there had to be something useful that this set-up is used for :) \$\endgroup\$ – fandor Feb 15 '16 at 23:55
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What Doug Smith wrote is accurate and complete. A simplified answer is it makes a loop antenna for "sniffing" electromagnetic fields.

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  • \$\begingroup\$ Glad you pointed this out - it went completely over my head that Doug actually did address the purpose of such set-up. He seems to iterate the point about external noise possibly being bigger in magnitude than any signal coming from the circuit. Thank you for the response! \$\endgroup\$ – fandor Feb 15 '16 at 23:52

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