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| Unexpected differential voltage with differential probe (DP10007) |
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| MattM:
Dear All, I'm trying to probe the voltage across an LED using a differential probe DP10007. The schematic is attached. Notes: - DP10007 X10, connected to Rigol scope, powered from Rigol scope (also tried powering from notebook 'isolated' from mains ground) - nothing else connected to the scope - DUT grounded The driver is powered but set to output 0 current, I get ~ 2.3V across the differential probe. When I disconnect the probe and simply use the DMM to measure the voltage across the LED it is as expected 0V. Am I overlooking something or doing something stupid? Thank you |
| IAmBack:
From the user's manual: "6) When a substantial change in temperature or other circumstances affect the accuracy of the probe's zero point, a calibration is needed: short the input terminals of the probe, then power the probe while simultaneously pressing the Attenuation Range Buttons/Indicators for three seconds." Hope it helps |
| MattM:
Thanks for the idea but when the diff probe is not connected and its leads are shorted the offset is pretty much zero, i.e. the 'offset' is only present when the probe is connected to DUT. |
| Caliaxy:
Have you tried measuring the voltage across the LEDs with a DMM while the probe is connected? |
| MattM:
Yes, got the same voltage that I see on the scope so connecting the probe clearly does something to the circuit. I just can't figure out what and why. I assume(d) that diff probe is high-impedance ground isolated but I must be overlooking something. I tried connecting 1M resistor (biggest I currently have on hand) in parallel with the LED (diff probe not connected) to 'simulate' the probe but that doesn't have the effect that the diff probe has. |
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