Oscilloscope vs Multimeter Voltage difference (Fluke 175 vs TDS2024B) - Page 1
Forum section: electronics / testgear

author: steaky1212
author_rank: Regular Contributor
timestamp: February 19, 2014, 10:16:31 am
content: Hi,I've got a Tektronix TDS2024B 'scope and Fluke 175 DMM and I have noticed a variance in voltage measurements.I'm measuring a battery under load and the DMM measures 12.78V, where the 'scope is measuring 13.0V (min), 13.1V (mean), 13.2V (max) on 2.00V/div.The Fluke goes off for a yearly calibration, but I don't think the scope ever has. I've run the self-cal utility and probe check.I'm using P2220 probes on 10X attenuation but when using 1X attenuation, the scope measures 12.9V (min), 12.9V (mean), 13.0V (max). The voltages are comparable across different probes on different inputs into the 'scope. (If I disconnect the probe BNC the 'scope measures 0.00V(min), 58.4mV(mean), 160mV(max)).I've checked with another Fluke 175 and a Fluke 287 and the voltages are within 0.01V.I've connected a Tektronix TDS2012 on 10X and measure 12.9V (min), 13.1V (mean), 13.3V (max). I've got access to a Tektronix MSO2024 but just not at the minute.I know that DMMs are better for steady state voltages, but we're having an issue with voltage levels when firing solenoids so I need to see what the voltage is like during the firing period hence the 'scope.Is this just an artifact of the 'scope measurement, am I missing something, or does it sound like the 'scopes need sending off for calibration?

author: EEVblog
author_rank: Unknown
timestamp: February 19, 2014, 10:26:36 am
content: Most scopes are speced for only a few % accuracy. So the scope value could certainly be within spec.Scopes are not designed for accurate voltage measurements, they only have 8 bit resolution for starters.I checked:http://www.testequipmentconnection.com/specs/TEKTRONIX_TDS2024B_Digital_Storage_Oscilloscope.PDF3% accuracy, so your scope is well within spec.

author: steaky1212
author_rank: Regular Contributor
timestamp: February 19, 2014, 10:44:16 am
content: Dave,Thanks for that. I was just looking through the spec sheets for both devices. At the voltages I've got, the accuracy is +-0.588V for the scope and +-0.039V for the DMM.Well, that's today's lesson learnt then.Dawgie,The issue is we are monitoring the voltage downstream of an ORed battery and mains input and have set a threshold of 14V for the power fail alarm. By the sounds of it either we just dont monitor the power whilst firing, or adjust the threshold.

author: jpb
author_rank: Super Contributor
timestamp: February 19, 2014, 10:45:16 am
content: You might be able to improve your scope accuracy a bit by setting the dc offset and using a narrow range (this will help with the 8 bit limit).Though, as Dave points out, your scope is within the 3% (which is nearly 0.4V for a nominal 13V) your results are a bit disappointing. When I tested my 7 year old WaveJet it was in very good agreement with my DMM and there was no obvious offset, it is specced at 2% (actually 1.5% + 0.5% of full scale) but was within 1%.If you have a variable power supply it might be worth doing a calibration in the sense of plotting scope V vs DMM V for say 1V to 15V then you can see if there is a consistent off set and perhaps a scale factor which would allow some crude error correction.It also might be worth setting the band limit (if it is appropriate) to minimize noise.I posted my WaveJet measurements on this thread :https://www.eevblog.com/forum/testgear/lecroy-waverunner-lt-224-review-and-(partial)-teardown/msg205402/#msg205402I measured the dc gain and found it well within spec, in very good agreement with my 5 1/2 digit dvm:DVM WaveJet96.199mV 96.5mV494.17mV 494mV0.99643V 997mV1.9948V 1.99V4.9936V 4.99V9.9903V 9.99V14.9865V 15.0V19.9905V 20.0V29.9852V 30.0VNote the WaveJet results look rather better than their accuracy allows as (I've given 3sf whereas 8 bits can only range from 1 to 256, though averaging improves things) but I think it illustrates that the offset you're seeing is not very good.

author: BravoV
author_rank: Super Contributor
timestamp: February 19, 2014, 11:12:39 am
content: This problem brings back nostalgic moment when I was playing with scope for the 1st time, my mentor made a really excellent analogies ...Scope => video cameraDmm => photography cameraFor static scene, camera always beats video even the video scene is freezed properly without blur. The detail and scene accuracy from the freezed frame simply can not be compared with a photo generated by a photo camera.Your case here measuring a steady DC signal is equal to a static scene.For dynamic scene, ... well, you should be getting it by now.Hence, thats why you never see a video say that has a resolution of a modern camera like 16 Mega pixels in full 30 fps.