Unexpected differential voltage with differential probe (DP10007) - Page 1
Forum section: electronics / testgear

author: MattM
author_rank: Contributor
timestamp: June 06, 2022, 08:59:58 am
content: Dear All,I'm trying to probe the voltage across an LED using a differentialprobe 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 groundedThe driver is powered but set to output 0 current, I get ~ 2.3Vacross the differential probe. When I disconnect the probe andsimply use the DMM to measure the voltage across the LED it isas expected 0V.Am I overlooking something or doing something stupid?Thank you

author: IAmBack
author_rank: Regular Contributor
timestamp: June 06, 2022, 11:19:09 am
content: From the user's manual:"6) When a substantial change in temperature or other circumstances affectthe accuracy of the probe's zero point, a calibration is needed: short theinput terminals of the probe, then power the probe while simultaneouslypressing the Attenuation Range Buttons/Indicators for three seconds."Hope it helps

author: MattM
author_rank: Contributor
timestamp: June 06, 2022, 01:11:40 pm
content: 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.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: June 06, 2022, 01:31:53 pm
content: Have you tried measuring the voltage across the LEDs with a DMMwhilethe probe is connected?

author: MattM
author_rank: Contributor
timestamp: June 06, 2022, 02:06:24 pm
content: 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 mustbe 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.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2022, 02:17:56 pm
content: Quote from: MattM on June 06, 2022, 02:06:24 pmThe probe is not ground-isolated. It has 4M impedance to ground on each side. So try putting two 4M (or something in the ballpark) resistors to ground on either side of the LEDs (where you are connecting the probe and DMM) and then check with your DMM to see if that is enough to throw your circuit off in the same way.Many times--and perhaps this one--you need to completely isolate your DUT even when using a differential probe like this.

author: MattM
author_rank: Contributor
timestamp: June 06, 2022, 03:29:02 pm
content: Thank you bdunham7, that is the problem and what was wrong with my assumptions.Isolating the scope from mains ground got rid of the 'offset'/diff voltage. Also tested withthe resistors (1M) which resulted with ~ 3V differential voltage across the LED.Thanks again!

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2022, 03:46:53 pm
content: Quote from: MattM on June 06, 2022, 03:29:02 pmA word of caution here--usually you want to isolate the DUT not the scope, since in many situations isolating the scope can cause hazardous potentials to appear on the scope chassis--meaning you could be electrocuted simply by touching the scope or connections. This is almost universally true where the DUT is mains-powered.

author: MattM
author_rank: Contributor
timestamp: June 06, 2022, 06:18:31 pm
content: Of course, good point, thank you.