Problem with my scope?  Reading positive versus negative average DC voltages. - Page 1
Forum section: electronics / testgear

author: wobbly
author_rank: Regular Contributor
timestamp: September 24, 2024, 07:46:50 pm
content: 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 aKeysight 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 andPOSITIVE 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 andNEGATIVE 10V+/- 0.05V.However, when I have one probe on+10Vand one on-10V, there is suddenly a roughly0.5Vdiscrepancy 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?

author: wobbly
author_rank: Regular Contributor
timestamp: September 24, 2024, 08:00:54 pm
content: 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.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 24, 2024, 08:02:53 pm
content: 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.

author: wobbly
author_rank: Regular Contributor
timestamp: September 24, 2024, 08:19:35 pm
content: Quote from: Phil1977 on September 24, 2024, 08:02:53 pmGood 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.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 24, 2024, 09:04:25 pm
content: You definitely have very trustworthy equipment!Quote from: wobbly on September 24, 2024, 08:19:35 pmI 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.

author: tautech
author_rank: Super Contributor
timestamp: September 24, 2024, 09:12:20 pm
content: Quote from: Phil1977 on September 24, 2024, 09:04:25 pmCough.+3% vertical accuracy has been industry standard for decades but now improved near 10x with the advent of cheap 12bit DSO's.Spec from the datasheet for the DSO OP uses is:DC vertical gain accuracy:+3% full scale (≥ 10 mV/div)+4% full scale (< 10 mV/div)

author: wobbly
author_rank: Regular Contributor
timestamp: September 24, 2024, 09:18:21 pm
content: Quote from: Phil1977 on September 24, 2024, 09:04:25 pmMight be able to scrounge something together at work. Can you go into more detail about what you mean?

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 24, 2024, 09:23:56 pm
content: Quote from: tautech on September 24, 2024, 09:12:20 pmA good rule of thumb for industrial machinery is if you specify 3% tolerance then your standard deviation should be a tenth of it - 0.3%.It may be in spec, but are the scopes really that bad? I must admit I rarely ever used a scope for something the DMM should measure.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 24, 2024, 09:25:27 pm
content: Quote from: wobbly on September 24, 2024, 09:18:21 pmJust an adapter so that your DMM can measure what comes out of the scope probe.

author: tautech
author_rank: Super Contributor
timestamp: September 24, 2024, 09:27:07 pm
content: Quote from: Phil1977 on September 24, 2024, 09:23:56 pm8bit, yes but it has never stopped anyone from doing valuable work while they are aware of the limitations of their instrument.

author: bdunham7
author_rank: Super Contributor
timestamp: September 25, 2024, 12:15:32 am
content: Quote from: wobbly on September 24, 2024, 08:19:35 pmThis is not adding up for me. Are the ground clips of both scope probes connected to the PSU GND connection? If not, can you quickly draw out a schematic of exactly how you have everything connected?

author: Fungus
author_rank: Super Contributor
timestamp: September 25, 2024, 12:24:31 am
content: Quote from: Phil1977 on September 24, 2024, 09:23:56 pmYes. Scopes are accurate horizontally, not vertically.I expect them to be linear but vertical numbers on 'scopes are a ballpark figure.

author: Fungus
author_rank: Super Contributor
timestamp: September 25, 2024, 12:36:00 am
content: Quote from: tautech on September 24, 2024, 09:12:20 pmWhat's the spec for crosstalk between channels?

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2024, 12:40:11 am
content: Quote from: Fungus on September 25, 2024, 12:36:00 amDunno, a while ago closed the datasheet tab.Easy to find though.....

author: Fungus
author_rank: Super Contributor
timestamp: September 25, 2024, 01:49:10 am
content: Quote from: tautech on September 25, 2024, 12:40:11 amNot THAT easy...I found one and it isn't specified.https://eu.mouser.com/datasheet/2/878/5992_3484-1759582.pdfI suspect that's what this might be though.

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2024, 02:03:29 am
content: Quote from: Fungus on September 25, 2024, 01:49:10 am0.5V over 20V is far more than one would expect from channel crosstalk but within accuracy spec.

author: Fungus
author_rank: Super Contributor
timestamp: September 25, 2024, 06:02:30 am
content: Quote from: tautech on September 25, 2024, 02:03:29 amYeah, I guess.The next question would be does the reading change on a channel between when they're both connected to +10V and you connect the other channel to ground, not -10V.

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2024, 07:11:02 am
content: Quote from: Fungus on September 25, 2024, 06:02:30 amRevisiting the OP, what one thing has the OP not done ?

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 25, 2024, 07:11:33 am
content: I´ve just checked the Rigol 1054 out of curiosity: (without probe, DC directly fed into BNC, normal sampling mode, AVG-measurement, all tests with 1V/div)It´s not good, but it seems to follow the 1/10th-of-the-tolerance rule. I checked for DC-crosstalk with a negative voltage on Ch2 and there was nothing.@wobbly: Is it possible your trigger is tricking you? Sometimes the 50/60Hz-hum mixes into the triggering and then it looks on the screen like a DC-offset for fast horizontal settings. May you try to set the timebase to something slower or use roll-mode?PS: Is there a good way to copy tables from libreoffice (<=> excel) into the forum without using images?

author: wobbly
author_rank: Regular Contributor
timestamp: September 25, 2024, 07:36:30 am
content: I just tried this experiment at work (with completely different equipment) and got almost the exact same result!The work scope is an old Tek 3034.I guess I just wasn't expecting it to be such a large discrepancy. If that's just the way I things are then so be it!Shame it's not something that can be adjusted by the user.

author: Fungus
author_rank: Super Contributor
timestamp: September 25, 2024, 07:43:52 am
content: Quote from: wobbly on September 25, 2024, 07:36:30 amIt really is...

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2024, 07:51:45 am
content: Quote from: tautech on September 25, 2024, 07:11:02 amNot worked it out yet ?

author: Fungus
author_rank: Super Contributor
timestamp: September 26, 2024, 01:51:52 am
content: Self-cal?

author: Aldo22
author_rank: Super Contributor
timestamp: September 26, 2024, 06:38:27 am
content: With my cheap scopes I have found that the positive DC measurement is quite good (definitely within 3%, usually better) but negative DC polarities can be off by 5-6%.Is this a general observation or is it a “special feature” of cheap scopes?Yes, I know, scopes are not made for that...

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 26, 2024, 06:52:32 am
content: And please check if the voltage levels change when playing with the trigger settings. I´ve seen it for too often that Auto-mode was expected to be free-running but was in phase with something unexpected. This exactly looks like a DC-offset and would affect the "average full screen" in the same way.

author: tautech
author_rank: Super Contributor
timestamp: September 26, 2024, 08:46:46 am
content: Quote from: Fungus on September 26, 2024, 01:51:52 amYes, the only thing the OP had not said he'd done.

author: Fungus
author_rank: Super Contributor
timestamp: September 26, 2024, 01:43:03 pm
content: Quote from: tautech on September 26, 2024, 08:46:46 amYeah, but he said it read OK if both probes were on the same place.