Might not be a bad idea to look up and review Bob Pease's article(s) on this. Grounding was a particular concern of his and he wrote several very good articles explaining the concepts.
Cheers!
Bruc
Quoting Dave McGuire <mcguire@...>:
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On 2/27/22 12:41, Jinxie wrote:
I'm not too sure what Harke means. I don't see what shorting the tip to the ground is going to achieve.
I'm probing A17 and A18 boards and there is simply /nowhere/ other than nearby aluminium chassis to clip the ground lead to. I appreciate what you say about keep the loop as short as possible, but the only alternative would be to pull the boards and solder fly-leads directly to the closest ground pours on the PCBs themselves. It's a pity HP didn't provide convenient, accessible ground pins for this purpose!
(It would be helpful to quote the email that you're replying to. Hint: This is a mailing list, not a web forum.)
I don't mean this the way it's going to sound, but most people don't see what it's going to achieve. Think of it this way.
Visualize the probe tip, ground clip, and ground wire as a "pickup loop", an antenna of sorts. Anything nearby can induce a current in that loop. "Ground" is a "virtual" entity in electronics; it means nothing other than "reference point of the moment". Everything is relative to "ground", but what "ground" means is relative. Your oscilloscope is showing a graph of voltage against "ground". Any voltage at the probe tip will cause Y-axis deflection on the oscilloscope, but variations in the voltage at the ground lead, with constant voltage at the probe tip, will also cause Y-axis deflection, because it's graphing the relative voltage between the two.
If current is induced in the ground wire, the oscilloscope's idea of "ground" changes, and you'll see Y deflection on the oscilloscope.
I'm not explaining things very well today, so I hope the concept is clear.
-Dave
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Dave McGuire, AK4HZ
New Kensington, PA
