| Electronics > Repair |
| Weird USB oscilloscope ground issue? |
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| Alex_Baker:
--- Quote from: shakalnokturn on June 19, 2024, 05:11:46 pm ---There is a ground loop sketch it out, you'll see... Probe ground, USB ground and PC ground are all common now if you connect the probe ground to another length of wire leading to the PC's PSU inlet ground connection you create a loop which is also an additional return path for the supply current to the Picoscope. This alternate return path runs through the Pico's frontend ground tracks and probe ground wire which also have resistance (say approx. 1.3 \$\Omega\$) so measuring anything is offset by the voltage drop across the probe's ground wire. --- End quote --- What puzzles me is that I have never ran into this problem before. I have used the pico dozens of times over the last several years and never noticed any funny business before. So If I do create a ground loop when connecting the pico to an earthed BNC, is that normal? if so how do I avoid it in the future? |
| shakalnokturn:
Creating a ground loop in this situation is normal and unavoidable without additional hardware, why it should only show effects now is another problem... The most obvious to me would be higher resistance in the conductors or connections somewhere between probe ground clip and PC PSU. (Probe, BNC connector, Pico PCB, Pico USB connector, USB cable,PC USB connector, optional USB to mainboard connector, mainboard to PSU connector...) Try hooking everything up so it shows the offset then wriggle things around to see effects on offset. Out of curiosity with Pico connected to unplugged PC what resistance measurement do you get between probe ground clip and PC's PSU earth terminal? I'd look for a high speed USB data isolator and a (ungrounded) linear regulated power supply for the Pico. Still I'd first try to understand why this issue is showing now if it hadn't previously... |
| 2N3055:
--- Quote from: shakalnokturn on June 19, 2024, 05:11:46 pm --- --- Quote from: 2N3055 on June 19, 2024, 09:38:08 am ---This is not ground loop question. Something is wrong. 240mV voltage drop with 180mA current suggest 1.3 Ω resistance. --- End quote --- I don't often disagree with you but here I'll have to, not on the fact that the resistance is rounded down from 4/3 \$\Omega\$ ;D There is a ground loop sketch it out, you'll see... Probe ground, USB ground and PC ground are all common now if you connect the probe ground to another length of wire leading to the PC's PSU inlet ground connection you create a loop which is also an additional return path for the supply current to the Picoscope. This alternate return path runs through the Pico's frontend ground tracks and probe ground wire which also have resistance (say approx. 1.3 \$\Omega\$) so measuring anything is offset by the voltage drop across the probe's ground wire. Edit: OTOH you weren't saying there wasn't a ground loop, rather it wasn't the problem. It still looks the most plausible problem to me considering it's DC. --- End quote --- My English. Sorry. There is a wire loop. Always is when you measure with any scope. Problem is that voltage is DC and is huge. That is what I meant. Ground loop is not source of the problem, it is what allows us to measure that something is injecting current to ground. And it should not. And even if it does, impedance of grounding should be lower. 10-30mV is normal. 250mv means something is wrong. All the stuff on your desk should be connected to same grounding point. Connected with short cables, there should be order of magnitude less potential difference in what essentially aspires to be equipotential net. Something is not right with grounding and/or power distribution wiring. In addition to DC current being injected into ground from somewhere. Also not right. |
| shakalnokturn:
--- Quote from: 2N3055 on June 20, 2024, 06:23:13 am ---In addition to DC current being injected into ground from somewhere. Also not right. --- End quote --- Yes that is another possibility of course DC current in a shared ground wire could give similar symptoms and be potential safety hazard if ground continuity is lost at any time. Once power mains (line) power distribution ground conductors have been confirmed to have sufficiently low resistance (which is one of the requirements for a safe power distribution) a couple of quick measurements should be able to reveal if cause is DC current flowing in the ground conductor. The easiest to me still seems to be a comparative test: Take a random remote ground point that shows DC voltage / current WRT to Picoscope probe ground as per OP's issue, take the same random point and repeat measurements WRT to PC's casing. If the same voltages / currents exist in both cases 2N3055 and wraper's explanation fits here, if not mine applies. It looks like I missed this along the way: --- Quote from: Alex_Baker on June 19, 2024, 03:40:49 pm ---This ground "leakage" problem has nothing to do with the scope, I can replicate this stray current flow from any GND point on the computer, not just the scope. --- End quote --- So I could be wrong about the origins of the problem in the end.... Then OP should get very worried about the leakage source. |
| m k:
--- Quote from: Alex_Baker on June 19, 2024, 03:40:49 pm ---This ground "leakage" problem has nothing to do with the scope, I can replicate this stray current flow from any GND point on the computer, not just the scope. --- End quote --- When measuring those resistances, did you include the power cord? Move the tower to the bathroom. Disconnecting mass storage will speed things up. What say your GFCI? If nothing check earth connection. |
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