When connecting multiple oscilloscope probes to a circuit, does each probe need to connect to ground?
The short answer is yes!
Why? The long answer is kind because of the ground loop. Remember, a circuit needs a closed path. And while on DC circuits we may rarely think about the distance of that path, it absolutely matters when there is an AC or frequency component.
When you do not connect each probe’s ground, the signal path because enormous since it must connect to the circuit’s ground through another probe. (See the animation in the video above.)
Long story short, you pretty much always need to connect all the grounds!
At the end of the episode, I showed these two images. Same Arduino, code, oscilloscope, and settings. Neither has the probe’s ground connected. So why are the waveforms very different? Astute viewers noticed the boards are powered differently, but why does that matter?
Hint: if the probe’s ground is connected to either measurement, the waveform is clean.
For full information, check out the explanation on this element14 Community notes page.