Why are oscilloscopes so inaccurate?  - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: August 10, 2019, 10:23:03 am
content: I realized how inaccurate my scope was when doing some DC measurements of a battery compared to my 6.5 digit DMM so I thought the unit needed calibration until I read the datasheet which shows a 2% voltage accuracy which is worse than the cheapest DMM on the market today.Why is this the case? Even Keysight's Infiniium UXR scopes which can cost up to 1M $ have the same accuracy as my cheap Rigol.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 10, 2019, 10:25:32 am
content: Because one uses a cheap DMM to measure a battery.

author: Rerouter
author_rank: Super Contributor
timestamp: August 10, 2019, 10:35:21 am
content: I would say that in general they are more concerned with AC performance, however if your DC is so far off, run a self cal on the scope, in most cases this should put you well under 1% for DC offset and for most should also reduce any gain errors, however I will warn that some scopes seem to only calibrate there AC coupled mode or there DC coupled mode, and do not store offsets for both (there are small offsets between these 2 on most front ends)The other one is , in most scopes your only dealing with an 8 bit ADC, now you can use the channel offset in a lot of cases to squeeze out extra precision, but in the end most scope measurements are relative, with repeatability being more valuable than absolute accuracy.If you want a faster, high precision ADC, that gets into purpose built test gear like keithley DAQ's, as to why the 1 million dollor scopes DC specs are poor, they are not intended to be used in that application, they needed the best bandwidth / sample rate and AC performance, how well it measured a slow DC signal was very much an afterthought.

author: tggzzz
author_rank: Super Contributor
timestamp: August 10, 2019, 10:48:37 am
content: Quote from: drummerdimitri on August 10, 2019, 10:23:03 amYou might as well ask why an oxy-acetylene torch is hotter than a soldering iron. Different tools for different jobs, each with different necessary strengths and allowable weaknesses.Choose the right tool for the job, work around the limitations of available tools.

author: mikerj
author_rank: Super Contributor
timestamp: August 10, 2019, 10:57:49 am
content: Why is my DMM useless for showing high frequency time domain behaviour?

author: Circlotron
author_rank: Super Contributor
timestamp: August 10, 2019, 10:58:25 am
content: Quote from: Rerouter on August 10, 2019, 10:35:21 amSo does that mean that if you shift the trace to the bottom of the screen and only use the upward portion of the trace, you are only getting 7-bit resolution?

author: wraper
author_rank: Unknown
timestamp: August 10, 2019, 10:59:15 am
content: Because even your 6.5 digit DMM is no longer 6.5 digit if you set faster sampling speed. There is trade-off between accuracy and speed. You cannot easily get both, especially at reasonable price. Probing also is an issue with high resolution, that's why multimeters set sampling speed as number of power line cycles (NPLC), to minimize power line AC interference impact on your measurements.Quote

author: soldar
author_rank: Super Contributor
timestamp: August 10, 2019, 11:02:37 am
content: How on earth do you measure a voltage on a scope screen with better than 1% accuracy? Does it involve a microscope?A 'scope is not meant for this kind of measurement, it is meant to visualize complex waves. Why would you use it to measure a DC voltage with any accuracy?A lawnmower and a nail clipper are both cutting tools but I would not use a lawnmower to trim my fingernails nor would I attempt to cut the grass using a nail clipper. Different tools for different purposes.

author: Rerouter
author_rank: Super Contributor
timestamp: August 10, 2019, 11:05:16 am
content: technically you would be measuring that voltage with the statistics menu built in to most DSO's,

author: Siwastaja
author_rank: Super Contributor
timestamp: August 10, 2019, 11:07:56 am
content: Optimization of bandwidth * voltage accuracy * price, for specific purposes they are supposed to serve.The most generic catch-all instrument would have massive BW, massive number of channels, with massively good voltaga gain and offset accuracy - but it would also be massively big, consume massive amount of power, and cost a massive sum of money.Out of these features, DC accuracy is easiest to sacrifice, since it's the least needed in typical use cases. Multimeters are great and abundant.The ADCs on scopes are almost always 8-bit, and the DC offset stability and gain accuracy specs of the analog frontend are not the #1 or #2 items in the optimization, even for mid-range (think $10k-ish) products.Note that product pricing is not a linear equation. If a scope BOM cost is $1000, you can probably sell the scope for $3000, but if the BOM cost is $2000, you can't sell it for $6000, because now at $6000 you would have much fewer than half the number of customers, so less total money in, and the NRE costs need to be calculated again per projected unit sales. Sometimes this ends up to totally ridiculous sums of money, and sometimes the result is that the market doesn't want the product.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 10, 2019, 11:09:01 am
content: Quote from: soldar on August 10, 2019, 11:02:37 amIt involves reading out the number from the screen. You know these digital storage oscillosscopes that really penetrated the market in the 1990's.

author: bd139
author_rank: Super Contributor
timestamp: August 10, 2019, 11:11:26 am
content: Scopes are shit at everything they do. They just do a lot of shit things at the same time which turns out is a lot more useful than things that do one thing really well.

author: magic
author_rank: Super Contributor
timestamp: August 10, 2019, 12:14:49 pm
content: The analog frontend could probably be fitted with a DC servo, frontends of cheap DSOs already use separate paths for high and low frequencies.Attenuators could be measured and calibrated digitally during self-calibration.Thermal stability of variable gain amplifier ICs and ADCs remains a question.Some scopes are so bad that they could use improvements even with zero hardware cost. My Hantek doesn't calibrate gain errors in various ranges in self-cal, they just don't match each other no matter what.

author: ogden
author_rank: Super Contributor
timestamp: August 10, 2019, 12:21:25 pm
content: Quote from: soldar on August 10, 2019, 11:02:37 amAnd on that bombshell we end this thread

author: 2N3055
author_rank: Super Contributor
timestamp: August 10, 2019, 12:56:05 pm
content: They are not. I have a scope that has 0,25% guaranteed accuracy. It is 16 bit.General purpose scopes are 8 bit. Their resolution is roughly 0,4%. That is smallest voltage step they are capable of. There is also DNL and INL in A/D converter, meaning, each step is not exactly 0,4%...To keep specs under control, best they can promise is 1 -1,5% (some manufacturers do), with everything being ideal. Which is not.Also it is really hard to make front end that has bandwidth from DC to 1GHz with max 1-2% total amplitude error.6,5 Digit DMMs have approximately 23-24 bit resolution. And rest of meter specialized and optimized for just that.

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2019, 01:11:57 pm
content: Quote from: Circlotron on August 10, 2019, 10:58:25 amNo, it's actually much worse than that...1. Unless your signal is just the right amplitude to fill the entire input range of the ADC (ie, just before clipping) then it won't be giving you the full 8 bits anyway.and2. Most DSO LCD screens don't have sufficient vertical pixels to display the full 256 levels coming from the ADC, especially when you have multiple channels displayed plus the general crap of annunciators, measurements etc. clogging up the margins. Check your LCD resolution and see how many bits of resolution it's actually displaying on a typical waveform - you will probably be surprised.EG. The Rigol 1054Z has 480 vertical screen pixels (480x800). Now split that between 4 traces and the surrounding crud and you've probably got less than 100 pixels per channel, ie. under 7 bits.

author: wraper
author_rank: Unknown
timestamp: August 10, 2019, 01:27:50 pm
content: Quote from: Gyro on August 10, 2019, 01:11:57 pmNot that you can accurately see actual voltage from the trace even if LCD resolution is high. But all modern scopes have measurement functions so you can see precise number in digits.

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2019, 01:31:01 pm
content: QuoteTrue, though you do have to remember the aforementioned 8 bit ADC limitation in your definition of precise.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: August 10, 2019, 01:50:28 pm
content: Lots of good info on here thanks for the clarification.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 10, 2019, 01:57:54 pm
content: Quote from: mikerj on August 10, 2019, 10:57:49 amHehe, indeed.The question was worth asking though - that's a very common "trap" for many beginners and even fresh engineers... I think it's already a good sign that the OP actually *noticed* the inaccuracy and asked the question. Many wouldn't even have cared.

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2019, 03:16:17 pm
content: The design constraints needed to make wideband amplifiers with a response down to DC are not compatible with DC precision. This is also why you cannot use an oscilloscope to directly measure settling time to high precision; instead, a rate of change or null measurement must be made.Further with an 8 bit instrument, once you throw away 1 bit for linearity, that is only 1 part in 128 so about 2% when 1 bit is sign.Higher resolution oscilloscopes and digitizers exist but for high accuracy, they have to compromise in other areas.Sampling oscilloscopes avoid these problems by omitting signal conditioning before sampling so they can achieve much higher resolution at high frequencies.Quote from: soldar on August 10, 2019, 11:02:37 amYou use an oscilloscope that includes a differential comparator.Quote from: magic on August 10, 2019, 12:14:49 pmThe problem with a DC servo is that in the event of overload, it ruins the overload recovery time. This is also a problem with two path designs unless special care is taken. There is a lot to be said for the performance and simplicity of the old stacked JFET input buffer.QuoteThermal balance is a big problem and especially so in integrated circuits. Even worse, the methods used to combat it in integrated circuits cannot be used at high frequencies because of parasitic coupling. Essentially the offset varies with operating point and time. Very high performance instruments compensate for this in the digital domain.A more obvious and less complex way to control FET mismatch-induced offset would utilize a matched dual monolithic FET. Readers are invited to speculate on why this approach has unacceptable high frequency error.- Jim Williams, Linear Technology application note 65, page 72.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 10, 2019, 04:48:45 pm
content: Quote from: David Hess on August 10, 2019, 03:16:17 pmSounds like a use case for a vintage analog scope...http://w140.com/tekwiki/wiki/1A5

author: DaJMasta
author_rank: Super Contributor
timestamp: August 10, 2019, 06:01:33 pm
content: It's been mentioned, but different instruments for different applications. It's a limitation of the practical design of the frontend and sampling system (low resolution, response down to DC and up being hard to keep linear, etc.), but it's also not something that's needed for the kinds of things scopes are usually known for.The scope's strength in measurement is time - its triggering and sample rate have to be very low jitter and tightly controlled to give you that nice smooth sinewave on the screen, but in essence, it's not intended to be a measurement tool - a scope is for visualization. A DMM, a power meter, a frequency counter... all of these are explicitly for measuring whereas the primary function of an oscilloscope is to show detail on a signal as a visualization tool, the measurement of a signal's parameters is sort of secondary addition (take a look at all-analog scopes, measurement was never a big priority until digitization started making it easier).

author: coppercone2
author_rank: Super Contributor
timestamp: August 10, 2019, 07:07:05 pm
content: look at bandwidth. Measurement usually gets ugly when you widen bandwidth.You have something that is widely considered perfectly linear (visible "DC", that is where you can track change with your eyeballs) and AC, in which every parameter follows 1/F tendancies. It's like looking at everything through a stroboscope (trying to visualize a waveform of 1MHz on a scope).

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 10, 2019, 07:44:40 pm
content: Quote from: bd139 on August 10, 2019, 11:11:26 amSame can be said about most people.

author: coppercone2
author_rank: Super Contributor
timestamp: August 10, 2019, 09:08:43 pm
content: scopes are good at telling you what is actually happening instead of giving too useful a measurement etcits like being able to look at something to see if its doing the right thing then trying to break out and measuring tapeANd I think high end unfriendly lecroys do a bit more then 'suck' (well they do suck balls but they are good)its like, being able to look at something and then decide where you put the ruler to measure it. The cabinet looks crooked (not lets measure every part of the cabinet to see if its good.. carpenters would go insane).

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2019, 04:02:37 am
content: Quote from: Alex Eisenhut on August 10, 2019, 04:48:45 pmOr something a couple years newer:http://w140.com/tekwiki/wiki/7A13Those differential comparators are ancestors to the ones now made by Preamble/LeCroy:https://teledynelecroy.com/probes/differential-amplifiersAn alternative method is to make an integrated measurement at a specific point in the waveform which gives the speed advantages of sampling with the low noise of integration:http://w140.com/tekwiki/wiki/7D12And that sort of thing went on to become gated integrated measurements in automated analog oscilloscopes like the Tektronix 4 channel 22xx series and maybe 24xx series.A DSO's averaging mode is effectively the same thing but is still limited by the precision of their signal conditioning and digitizer. In practice, very few applications require better accuracy so the alternatives are no longer economical. They were used in the past because there was no better alternative.

author: dom0
author_rank: Super Contributor
timestamp: August 11, 2019, 05:07:22 pm
content: As an owner and not-so-infrequent user of a 7A13 I can say that it is a surprisingly useful tool to have in the bag, just like the 7A22.Measurement functions of analog 'scopes also tend to be decent. For example, the DC VM in the 2246 has an accuracy spec of +- 0.5 % of reading. Peak measurements are implemented using precision rectifier, so relatively accurate as well. Only if you use manual cursors you are actually getting the full deflection and aberration errors.Edit: On second thought I am not so sure about the manual cursors. My guess would be that those are implemented using S&H circuits, so deflection and aberration errors are zero since cursor and measurement position are identical. So even those might actually be quite good. Have to check.

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2019, 07:32:40 pm
content: Quote from: dom0 on August 11, 2019, 05:07:22 pmI really like the 7A13's versatility but it comes at the cost of high noise. I am not sure there is anyway around that when a bootstrapped differential amplifier is involved.I do not know about Tektronix but HP makes low voltage probes which include the voltage comparison function. The offset controls of a DSO do the same thing but usually operate over only a very limited range unlike a 7A13 which is a very useful +/-10 volts.QuotePeak voltage measurements on the 22xx series are actually made using the B trigger which is how these oscilloscopes achieve full bandwidth peak voltage measurements. DC voltage measurements are made by inserting a very low pass trigger coupling filter which explains why they cannot be gated which is too bad.Signals are measured by using the B trigger level comparator as a successive approximation ADC to determine the peaks or DC level of the applied signal.QuoteI think that is correct; cursor measurements should be more accurate than graticule measurements for that reason.

author: HendriXML
author_rank: Super Contributor
timestamp: August 17, 2019, 09:22:53 pm
content: I had some fun using a DSO as a decent precision voltage meter.By doing some calibration steps, using the vertical offset (using multiple iterations narrowing down the measurement window) and a lot of averaging it is possible to get pretty descent precision.https://www.eevblog.com/forum/testgear/using-a-awg-and-a-scope-do-a-low-voltage-level-characterization-of-a-1n4005/msg2476305/#msg2476305

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: August 24, 2019, 03:21:42 pm
content: I've recently noticed that the voltage measurement of an AC signal on my scope change a lot with the time division setting.Is there a norm to set the time division to X seconds or show X amount of cycles for the most accurate readings?

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2019, 09:12:40 pm
content: Quote from: drummerdimitri on August 24, 2019, 03:21:42 pmThat should not happen unless the AC measurement is made over the entire record and only a fraction or small number of cycles are captured. Usually the default is to make the measurement over 1 or a whole number of cycles if they can be detected.

author: nctnico
author_rank: Super Contributor
timestamp: August 24, 2019, 09:35:11 pm
content: Quote from: drummerdimitri on August 24, 2019, 03:21:42 pmNo. Some oscilloscopes do measurements on a decimated (downsampled) set of data to speed up measurements. If you have such an oscilloscope: As a result the accuracy goes down if you have many cycles on the display. As a rule of thumb you should be able to see the shape of the signals for the measurement to be accurate. Ofcourse you should always have at least one full cycle on the display.

author: Bassman59
author_rank: Super Contributor
timestamp: August 26, 2019, 01:42:46 am
content: Quote from: Gyro on August 10, 2019, 01:11:57 pmWhat? No.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 28, 2019, 10:03:15 am
content: Another thing to keep in mind: Even if you have the most accurate scope in the world, have you stopped to consider the accuracy impact of your probes, connectors, coax, etc?If you want to measure to a fraction of a percent, a small amount of impedance mismatch here, a bit of parasitic inductance and capacitance there, and boom, you lost your fraction of a percent. Think about the fact that you have to manually go in and tweak that 10x probe. How on earth are you ever supposed to get accurate measurements out of that?This is a similar reason why things like spectrum analyzers can't measure to extreme accuracy.

author: magic
author_rank: Super Contributor
timestamp: August 28, 2019, 12:02:28 pm
content: You will sooner reach limitations of the scope than the probe.Particularly at DC. Scope probes are actually usable with DMMs in a pinch if you calibrate them beforehand. I did it once to measure something that needed not to be disturbed by extra capacitance and noise, though I was only after 1% or so accuracy.A scope I wouldn't trust to get even 1% right.

author: tggzzz
author_rank: Super Contributor
timestamp: August 28, 2019, 12:38:03 pm
content: Quote from: magic on August 28, 2019, 12:02:28 pmDC voltage maybe, maybe not.Otherwise poor probing technique frequently dominates measurement validity.

author: SilverSolder
author_rank: Super Contributor
timestamp: September 11, 2019, 02:22:07 pm
content: Playing around with a ratio transformer the other day, I decided to check a 54622D scope, just for fun. Fed an accurately sourced 1V rms signal at 50Hz via a 10:1 probe, the Agilent was able to achieve ~0.7% accuracy on the calculated RMS value it displayed on the screen.This level of accuracy (better than the spec of 2%) was only achieved right after running the self calibration function. The accuracy was a lot worse before running user cal - several percent out!Overall better than expected performance, and a good demonstration of the value of the Auto Cal function.