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Problem with my scope? Reading positive versus negative average DC voltages.
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wobbly:
I have noticed a funny problem with my two-channel digital scope.  It looks like the two probes are not measuring voltages very well when one probe is measuring a voltage that is the opposite polarity to the other one!

The scope is a Keysight EDUX1052G, about 3 years old or so and has had an easy life without ever seeing voltages larger than +/- 30V DC.  Hasn't had many hours on it either, nor much of a temperature swing (in the UK).

To measure the average voltages, I'm not relying on my failing eyesight to read the waveform, I'm using the built-in "Measurement: Average Full Screen" or the "Analyse: DMM" features.  Both of which are pretty good when I'm using unipolar voltages.

Investigating further...

Lets say both probes are measuring the average voltage between GND and POSITIVE 10V; the scope reports the same results with a small discrepancy of perhaps +/-0.05V which I'm quite OK with.

Same thing goes for both probes are measuring the average voltage between GND and NEGATIVE 10V +/- 0.05V.

However, when I have one probe on +10V and one on -10V, there is suddenly a roughly 0.5V discrepancy between the measurements!  So when I'm expecting to see +10V and -10V respectively, I'm actually seeing +9.8V and -10.3V.  The same discrepancy exists when I swap the probes and measure the other way.

So I checked the obvious stuff...


* Made sure both probes were set to "X10" mode on the physical probe switches.
* Checked that the menu was also set to X10 for each probe.
* Ran the self probe check on both probes (they passed).  But this is using the positive-only DC square wave on the front of the scope.  Hmm.
* I'm using both GND clips on the probes connected to the same GND point.
* Three separate multimeters (Fluke 117, Aneng 8008, Uni-T Clamp meter) all agree that the scope is at fault!
Is my scope faulty?  Or is this something to do with the way scope front-ends work?  0.5V is close to a diode drop.  What's goin' on?
wobbly:
Captain's Log Supplemental:

I have only noticed this recently because I have been building a bipolar PSU (simple stuff: https://www.eevblog.com/forum/beginners/ua741-rail-splitter-with-complementary-darlington-emitter-follower/).

I was trying to use each scope channel to measure one voltage rail while I tuned the virtual GND to be exactly 0V, or half-way between the two rails.  I'd set the bench PSU to 20V for example and hook up CH1 to the nominal +10V rail, and CH2 to the -10V rail.

So after getting the scope's two readings to be exactly +9.97V and -9.98V (ish, given tolerances and losses) I was pretty happy it was set up right.  Only to verify with a multimeter to find it was still out by half a volt!

I don't have the experience to doubt a Keysight instrument's veracity, so I'm sure I'm doing something stupid.
Phil1977:
Did you check the voltages simultaneously with DMM and scope?

It seems that something shifts the GND-potential of the scope by 0.3V. Can you connect a DMM between the scope GND (e.g. at the signal generator output) and your GND point of the voltage source?

Is your +/- 10V voltage source floating or somehow connected to PE? I would nearly bet something small that there is a ground loop.
wobbly:

--- Quote from: Phil1977 on September 24, 2024, 08:02:53 pm ---Did you check the voltages simultaneously with DMM and scope?

It seems that something shifts the GND-potential of the scope by 0.3V. Can you connect a DMM between the scope GND (e.g. at the signal generator output) and your GND point of the voltage source?

Is your +/- 10V voltage source floating or somehow connected to PE? I would nearly bet something small that there is a ground loop.

--- End quote ---

Good idea.  My Fluke 117 reports 10.1V (stable) measured with Fluke's GND probe on the benchtop PSU's GND and the positive probe on the Scope's sig-gen GND BNC outer metalwork.  This checks out, since the Scope probes GND clips are sharing the virtual GND of the bipolar supply.

Does not look like there is any ground loop causing a voltage shift.  The bench PSU's GND output (Aim TTI PL303-P, which is also about 3 years old and I trust completely) is definitely floating.
Phil1977:
You definitely have very trustworthy equipment!  8)


--- Quote from: wobbly on September 24, 2024, 08:19:35 pm ---
Good idea.  My Fluke 117 reports 10.1V (stable) measured with Fluke's GND probe on the benchtop PSU's GND and the positive probe on the Scope's sig-gen GND BNC outer metalwork.  This checks out, since the Scope probes GND clips are sharing the virtual GND of the bipolar supply.

--- End quote ---

I hope it reports around 0V between both GND... But I think you measured the right things. Then I´ve got no idea about the reason, I agree that 3% deviation would be more than expected for a reputable scope.

Do you have such banana-->BNC adapters? You could measure with the DMM through the scope probe.
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