Incorrect voltage readings -- Rigol DS1054z Oscilloscope - Page 1
Forum section: electronics / testgear

author: Ejohnson
author_rank: Newbie
timestamp: October 06, 2015, 03:20:33 am
content: Hey EEVbloggers!I have a new Rigol DS1054z that isn't correctly displaying / reading voltage. Am I doing something wrong?I hooked up the output of a TEMS device (one of those zap zap medical things) to my DS1054z input. For some reason it says the channel is receiving more than 30 volts. However, my Fluke 87V says: Mean voltage = 2.5 VAC, Min voltage = 1.560 VAC, Max voltage = 3.457 VAC. The current is very low (on the order of 10^-2 A). The oscilloscope leads were on the 1X setting.I trust the readings from my Fluke. I literally trust my life to that meter.Is there a setting in the Rigol that is scaling the potential? I've scaled the *.csv data set to work with it in R - but I'd much rather just get good information without any scaling! In fact the *.csv output has a high value of "284," which is really confusing.Thank you very much for your input!Yay EEVBlog!!E

author: coppice
author_rank: Super Contributor
timestamp: October 06, 2015, 03:30:11 am
content: Since the scope is saying about 10x what you expect it to, I guess you only changed the switch in the probe to x1, and didn't change the software setting on the scope to match this.You would be better off changing the probe to x10. Unless you want to look at a signal so small you need the added gain of a x1 probe, x10 is almost always the better choice. To see why, look through Dave's videos. He has one on this topic.

author: Ejohnson
author_rank: Newbie
timestamp: October 06, 2015, 03:31:13 am
content: Ah! Fantastic! I'll try that.Thank you for the reply. : )

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 06, 2015, 04:01:46 am
content: That's right. The DS1054Z cannot auto-sense the probe's attenuation setting, and defaults to the 10x setting in the Channel menu. For most normal use, one would use the Probe's switch set to 10x and also in the Channel menu, one selects 10x for the Probe attenuation.The Rigol's Channel>Probe menu has a big selection of possible probe settings.

author: coppice
author_rank: Super Contributor
timestamp: October 06, 2015, 04:30:14 am
content: Quote from: alsetalokin4017 on October 06, 2015, 04:01:46 amEven if a scope does support auto-sense you need to check very carefully if its working. Typically you need to be using exactly the right model of probe with the right model of scope for it to function.

author: SteveLy
author_rank: Regular Contributor
timestamp: October 06, 2015, 07:05:45 am
content: In addition to getting the 1x vs 10x probe setting right on the scope, keep in mind that scopes typically report peak or peak-to-peak voltage, while DMMs show RMS;2*V(RMS) = 1.414 * V(peak) = 0.707 * V(peak-to-peak).

author: nugglix
author_rank: Regular Contributor
timestamp: October 06, 2015, 07:12:39 am
content: Quote from: SteveLy on October 06, 2015, 07:05:45 am*cough*Seehttps://en.wikipedia.org/wiki/Root_mean_square

author: Performa01
author_rank: Super Contributor
timestamp: October 06, 2015, 07:33:24 am
content: Quote from: SteveLy on October 06, 2015, 07:05:45 amScopes typically 'report' nothing at all by default. The user deliberately adds the measurements of interest and in case of voltage has the opportunity to select Vpp, Vmean and/or Vrms from the measurement menu.One should keep in mind that the ADC on a typical scope is only 8 bits. Therefore the resolution and in turn the accuracy of measurements is rather poor, particularly when compared to a decent DMM. On the other hand, a DSO allows measurements at frequencies, where any DMM has long given up...

author: SteveLy
author_rank: Regular Contributor
timestamp: October 06, 2015, 08:44:44 am
content: Quote from: nugglix on October 06, 2015, 07:12:39 amGood catch. 2*V(pp) = V(p) = sqrt(2)*V(rms)

author: Performa01
author_rank: Super Contributor
timestamp: October 06, 2015, 09:14:15 am
content: Quote from: SteveLy on October 06, 2015, 08:44:44 amNot quite correct, but you slowly get there...Try it this way:Vpp = 2 * Vp;Vp = sqrt(2) * Vrms; --> FOR PURE SINE WAVES ONLY!

author: SteveLy
author_rank: Regular Contributor
timestamp: October 06, 2015, 10:52:11 am
content: LOL! Clearly not my day for tackling advanced mathematics like multiplying and dividing by 2, let alone square root of 2.

author: Performa01
author_rank: Super Contributor
timestamp: October 06, 2015, 11:09:41 am
content: Quote from: SteveLy on October 06, 2015, 10:52:11 amThat's the beauty of the community - with combined efforts we're getting it right, eventually!

author: jwm_
author_rank: Frequent Contributor
timestamp: October 06, 2015, 11:14:11 am
content: Quote from: alsetalokin4017 on October 06, 2015, 04:01:46 amI always thought a good feature for a scope would be to use the probe calibration port to set the probe multiplier properly. so it would be probe independent. Just take the probe, stick it to the calibration point and press and the scope will find the channel with the cal signal on it and see exactly what it is being attenuated by and set the multiplier accordingly.

author: Fungus
author_rank: Super Contributor
timestamp: October 06, 2015, 11:31:43 am
content: Quote from: jwm_ on October 06, 2015, 11:14:11 am??Lots of 'scopes already have probes that tell the 'scope what the attenuation is automatically. Just not the DS1054Z.

author: jwm_
author_rank: Frequent Contributor
timestamp: October 06, 2015, 11:42:50 am
content: Quote from: Fungus on October 06, 2015, 11:31:43 amYes, but only with probes specifically designed for that scope generally from the same manufacturer.This method would work with any probes (including homemade ones) and be a pure software thing so no added expense in fancy BNC plugs that have extra sensing contacts and can be added to inexpensive scopes.

author: Salas
author_rank: Frequent Contributor
timestamp: October 06, 2015, 11:55:46 am
content: As an aside lets mention that a scope's probe Cal signal is usually a non zero crossing pulse i.e. it has positive DC component. A scope set to DC coupling will measure its RMS correctly when even a True RMS DMM without DC+AC True RMS ability will measure it wrongly.

author: Ejohnson
author_rank: Newbie
timestamp: October 11, 2015, 05:30:10 am
content: This is a great site and community. I really appreciate all of the great comments that came from my question!Thanks again everyone!