Are 8 bits really enough ? - Page 1
Forum section: electronics / testgear

author: theoldwizard1
author_rank: Regular Contributor
timestamp: March 31, 2016, 04:04:52 am
content: I am a retired EFI systems engineer so I know my way around a 'scope, both analog and digital(although the digital ones from my day were WAY more expensive !)As digital electronics advances, the low end of the market place keeps looking a lot more "high end", with larger displays and deeper memory and more channels. The thing that does not seem to be advancing in the low-end, is the number of bits in the ADC. Yes, I understand that more bit is $$$, especially since you would have to upgrade the entire analog front end ! Adding 2 or 4 more bits really would not cost that much on the "digital side".Now you can argue that the 'scopes I am referring do not really need more "accuracy". They are aimed at hobbyists, and "in-the field" measurements(so why aren't they battery operated ?)and no professional engineer would have one as the primary scope on his bench. Still as a hobbyist, I would "feel better" with a 10 bit 'scope than with an 8 bit.How about some feedback, because I am retired and some what "out-of-touch".Quote

author: Fungus
author_rank: Super Contributor
timestamp: March 31, 2016, 04:30:32 am
content: More bits has all sorts of effects further down the pipeline. The conversion is slower, the internal RAM now needs to be 10-bits wide, the DSP that processes the signals for display needs to work with-10 bit numbers instead of 8 bits (which probably doubles the processing power needed), etc.And all for very little real benefit. The voltage reading on screen might be 1% more accurate.PS: My DS1054Z can switch to 12-bit mode by pressing a button. It goes slower, etc., ... but it can do it.

author: LPaul
author_rank: Contributor
timestamp: March 31, 2016, 07:20:17 am
content: Quote from: blueskull on March 31, 2016, 04:12:08 amWhat is EAR? didn't know of any restriction on test gear based upon performance.

author: coppice
author_rank: Super Contributor
timestamp: March 31, 2016, 09:44:55 am
content: Quote from: LPaul on March 31, 2016, 07:20:17 amGoogle "Wassenaar Arrangement". EAR is the US way to implement technology export controls, in accordance with the Wassenaar Arrangement and their own local policies.

author: coppice
author_rank: Super Contributor
timestamp: March 31, 2016, 10:00:15 am
content: The primary use of an oscilloscope is to display waveforms on its screen. If the screen is, say, 1024x768 pixels (most are lower than that), and you display 2 waveforms, one above the other, plus some annotations, you are down to about 256 pixels per waveform. So, you can't benefit from more than 8 ADC bits. If you are using the scope as more of a signal capture tool for later analysis, or greatly value supplementary functions of the scope, like spectral analysis, more bits could have real value. Some high end scopes have more bits for that reason.The real issue is not the 8 bit limitation. Its that in many scopes the limitations of the ADC and front end amp noise means you don't usually get more that 6 to 6.5 bits. That is somewhat coarse.

author: LPaul
author_rank: Contributor
timestamp: March 31, 2016, 10:03:08 am
content: Just read a bit about export control. Yet can't really see how a 14bit scop would be more helpful to produce restricted devices than any other test instrument?

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: March 31, 2016, 12:08:55 pm
content: Quote from: LPaul on March 31, 2016, 10:03:08 amIt's less about using the scope to produce something, it's more about the ADCs being used in military applications (i.e. Electronic Warfare).

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: March 31, 2016, 12:34:00 pm
content: Quote from: theoldwizard1 on March 31, 2016, 04:04:52 amAs others explained it's quite some effort. 12bit 14 bit scopes have existed for over 20 years but they were all pretty much low sample rate scopes. Building a fast ADC with 12 bits and high sample rates is a challenge, and also to the remaining processing infrastructure which now has to process 50% more data.It's also pointless when a scope is used as an instrument to *look* at waveforms, which is what most entry level scopes are used at. Higher resolutions make sense when you go away from staring at waveforms and using a scope as a waveform analyzer, but that's the realm of upper mid-range and high-end scopes then.QuoteThen have a look at the OWON XDS, which is the only true 12bit entry-level scope I know:http://www.owon.com.hk/products_info.asp?ProID=182There should be some discussions of these scopes around in the forum.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 31, 2016, 01:04:22 pm
content: Are 8 bits really enough?Thats like asking for a part recommendation like " any good diodes?"8 bits is plenty, depending on what you are doing or what you are measuring. Sometimes it is not enough and 12-14 or even 16 bits would be sweet. And yes we have the technology to do it today.In a scope design, there are multiple tradeoffs between cost, sample rate, waveform update rate, accuracy, bandwidth, noise, processor, memory, display, etc. The more bits the better, obviously, but designers have to consider the tradeoffs and there always are tradeoffs. As all these technologies evolve you will start to see more and more bits used in acqusition systems. I am both a hardware design engineer and a consumer and have faced those challenges. There is alot more in developing functional and performance specs and hardware than just implementing the widest ADC available.Remember the early IBM PC and the coarse Hercules graphics adapter and monochrome monitors? Sure, a retina 4k resolution display and driver would have been preferred, but took 25 years of evolution to achieve it.

author: Howardlong
author_rank: Super Contributor
timestamp: March 31, 2016, 01:53:28 pm
content: Also consider the noise: you will need to use pretty careful probing techniques especially when not band-limited at typical low voltage levels used with scopes, and trust that the front end of the scope is up to feeding a 14 bit ADC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2016, 02:35:03 pm
content: Well, how much do you need?A typical DSO is only a few hundred pixels tall on the screen, so you can't *see* anything finer than about 8 bits (more like 9 bits these days, i.e. 512 pixels height, or even 10 bits for fancier or PC-based scopes). If you need to zoom in without taking additional readings, you might need more, but you run into problems with dynamic range vs. offset, and bandwidth capability.Most scopes also average down the data (either by default, or as an option like Hi-Res Mode), so that an 8 bit instrument (at full speed) will record more bits at lower sweep rates. The tradeoff is lower visible sample rate, which again is fine for a display of some hundreds of pixels across.For radio applications, high dynamic range is extremely important, but high quantization noise (i.e., 8 bits is ~65dB dynamic range) needn't be a problem, for the same reason: a radio might be  tuned to a very narrow bandwidth, perhaps a MHz out of a 1Gsps total, giving a Lg(1G/1M) ~= 10 bits improvement! It's doubtful that the 8 bit range is even smooth enough and repeatable enough to obtain an actual 18 bits, but it will improve.By "digital tuning", I mean processing the sampled signal (which itself might be downconverted from some higher RF, e.g. satellite comms in the 10s of GHz; or simply baseband, whatever's picked up from an antenna plugged right into the ADC), applying a digital filter of sufficient scale (lots of FIR length / IIR poles) to bandpass the digital signal to a frequency range of interest (a channel some kHz or MHz wide, with say >60dB attenuation/rejection to nearby channels), and further processing that (converting the digital IF to detected modulation in whatever format: various kinds of AM, FM, QAM..).The neat thing about SDR is, you can do a TRF or hetrodyne design in whatever combination you like, without being limited by physical components and their adjustability; filter coefficients can be varied continuously over huge spans (decades). Though calculating them might not be as easy (in general, solving for filter coefficients is solving for the zeros of a large polynomial). All it comes down to is having unlimited hardware and computational power...Tim

author: TAMHAN
author_rank: Frequent Contributor
timestamp: March 31, 2016, 02:52:11 pm
content: AFAIK, high precision has always been the domain of Nicolet Gould (dead) and Yokogawa (not exactly famous round here). So it IMHO, at least to some small extent, is a case where the market has spoken.BTW: I do see the need for higher resolution in theory. But in my lab work, it never arose so far...as Wuerstchen already said...

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 31, 2016, 03:07:59 pm
content: Quote from: T3sl4co1l on March 31, 2016, 02:35:03 pmTim-Well said! Indeed your experience and sage perspective clearly underscores the salient points of this topic.@grouchobyte

author: Howardlong
author_rank: Super Contributor
timestamp: March 31, 2016, 05:04:56 pm
content: Quote from: T3sl4co1l on March 31, 2016, 02:35:03 pm1000 times oversampling gives you about 5 more bits.Double the sampling rate, +3dB, or half a bit.Just sayin'. The stylus screeched off my mental gramophone and my nerd alert switched in.Edit: and I assume you're quadrature sampling at 1Gsps otherwise you lose another 3dB of procesing gain thanks to Nyquist.

author: jeffsf
author_rank: Contributor
timestamp: March 31, 2016, 09:47:06 pm
content: Since you're asking, I'm guessing you're not using the 'scope for high-precision or commercial work.An 8-bit ADC will give you nice traces on the screen.If you expect to use the 'scope as a waveform-capture device for later processing (for example, estimating spectral information through DFT), you may find that an 8-bit ADC is only giving you 6-7 bits of resolution between stepped gain/attenuation and quantization noise. You're not going to be seeing signals -40 dBc with that capture.All that said, I'm very happy with my Rigol 1054 for hobby use, especially for its price.

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2016, 10:36:22 pm
content: Quote from: Fungus on March 31, 2016, 04:30:32 amWhat setting enables that?Presumably it's doing oversampling, as it only has an 8-bit ADC.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 31, 2016, 10:40:30 pm
content: It's never been a problem for me. I did attempt to over sample with one of my old LeCroy's just for fun.

author: Fungus
author_rank: Super Contributor
timestamp: March 31, 2016, 11:05:11 pm
content: Quote from: Hero999 on March 31, 2016, 10:36:22 pmAcquire->Mode->High Res.This mode uses a kind of ultra-sample technique to average the neighboring pointsof the sample waveform to reduce the random noise on the input signal and generatemuch smoother waveforms on the screen. This is generally used when the samplerate of the digital converter is higher than the storage rate of the acquisition memory.Note: “Average” and “High Res” modes use different averaging methods. Theformer uses “Multi-sample Average” and the latter uses “Single Sample Average”.

author: Fungus
author_rank: Super Contributor
timestamp: April 01, 2016, 12:49:00 am
content: Quote from: grouchobyte on March 31, 2016, 03:07:59 pm

author: bson
author_rank: Unknown
timestamp: April 01, 2016, 02:05:25 am
content: Quote from: T3sl4co1l on March 31, 2016, 02:35:03 pmThis ^Purely from a bang for buck angle it's better to increase the sampling rate than to add bits. Decimation with a sinc filter will add precision, and having extra samples permits "digital phosphor" type presentations (including heat traces which analog scopes can't do) where multiple samples make up one column on the screen. In general scopes aren't used primarily for measurements and aren't all that precise to begin with, and it's all about waveform visualization. HD scopes that are more suited for measurement will also either have vastly lower sample rates or you can add a zero to the price.

author: Berni
author_rank: Super Contributor
timestamp: April 01, 2016, 11:44:10 am
content: First of all its difficult to make a 12bit ADC running at 1GS/s that has low enough noise to actually make the 12 bits useful. There are a few scopes out there that do have 10 or 12 bit ADCs, but it tends to come at a substantial price premium for what makes not a whole lot of difference on the screen in a lot of cases.What scopes usually do is use a nice cheep tried and tested 8bit ADC but give you processing features to give you more resolution for when you need it. This feature is sometimes called "10bit mode" or "12bit mode" or something.But these 12bit modes are not just a marketing gimmick. They are useful for getting rid of high frequency crap on a signal when you are not interested in it and give you some extra resolution in software. Especially useful in scopes with very fast ADCs well above 1GS/s where you might not need all of the speed all the time.

author: Fungus
author_rank: Super Contributor
timestamp: April 01, 2016, 12:35:12 pm
content: Quote from: Berni on April 01, 2016, 11:44:10 amThe difference on screen in 12-bit mode is very noticeable. It really looks like a much higher resolution ADC.

author: Berni
author_rank: Super Contributor
timestamp: April 01, 2016, 12:45:13 pm
content: Quote from: Fungus on April 01, 2016, 12:35:12 pmIt certainly is noticeable, especially on scope with a higher than VGA resolution. Just saying that a 12bit ADC doesn't provide a whole lot of benefit as using high resolution mode tends to do the job just as well.Only limitation is that you can't look at signals close to the scopes bandwidth when in this high resolution mode, but that's usually not that bad of a limitation in day to day use.

author: coppice
author_rank: Super Contributor
timestamp: April 01, 2016, 12:57:30 pm
content: Quote from: Fungus on April 01, 2016, 12:35:12 pmThat's rarely because you get 12 bits. Its because the noise is integrated down to the point were you are really getting at least 8 bits, and not the 6 bits or so you get with all the front end noise. Anything much more than 8 bits is just not going to be visible with the resolution of the typical scope LCD panel.

author: krivx
author_rank: Frequent Contributor
timestamp: April 01, 2016, 01:26:02 pm
content: Quote from: T3sl4co1l on March 31, 2016, 02:35:03 pmI can't see the detail, but won't the onscreen measurements benefit from higher bit depth?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2016, 01:44:18 pm
content: Quote from: krivx on April 01, 2016, 01:26:02 pmNot really: most of the ones I've seen, display about three digits more than they represent, anyway! It would be very nice if they put a sigma on those as well, sadly most don't (with I think Lecroy being one of the few who do, unless it's simply standard only on the embedded-PC-based pricey ones).A simple measure of accuracy is how many digits are dancing around in a given steady state display.Errors arise from the signal itself (voltage and phase noise), trigger noise (and delay if shifted from the trigger point), quantization noise (including time quantization*) and thresholding.*Which depends on if it's a "deep memory" scope, or if it's reading by various methods: decimated to screen resolution, averaged to screen resolution (Hi-Res), histogram (DPO or heat map style); and how the measuring algorithm operates on that data (does it operate on what's on screen, or on that segment of the buffer, or the total buffer?).From what little I know of the usual measuring algorithms, many of them have threshold limitations: for example, to measure "top" of a square wave, how do you determine which points are in the "top"? What if you try measuring this on a wave that's decidedly not square: like a sine, or a switching waveform with ringing, or white noise? There should be some measure of confidence corresponding to that measurement, which is relayed to the user. But this is rarely provided.(More broadly, signal and image processing needs to take these things into account. It's amusing when you have a picture of a fire hydrant identified as "dog" by some AI; it's less amusing when your digital communications channel is fading out and it makes bad assumptions about what it thinks the signal is, rather than reducing bandwidth to a more confident level.)In any case, a hard threshold is the worst possible signal processing method, and has noise equal to any given sample. Ideally, you'd like the measurement to have 1/sqrt(N) noise, for N samples in the measurement span (whether it's piled on top as a histogram or stretched out in a very long buffer).And yes, when you have megs of buffer, random errors can be very well refined indeed (1e6 points --> 1e3 reduction in noise!), leaving you with only the systematic errors (like the nonuniformity of bit sizes in the ADC). If the measurement algorithm harnesses this resource, then it can be as accurate as the systematic limitations; my concern is that most will not go to this length, and worst of all, you have no way to know because they rarely tell you the uncertainty.Tim

author: Fungus
author_rank: Super Contributor
timestamp: April 01, 2016, 01:45:27 pm
content: Quote from: krivx on April 01, 2016, 01:26:02 pmBy how much? Half a percent?I'm not sure how important the accuracy of the on-screen measurements are in an oscilloscope. Opinions will obviously differ (especially around here) but "a couple of percent" seems fine to me.I wouldn't pay a lot more to go from (eg.) 2% to 0.5% and I see no reason at all for the 0.1% accuracy I demand from a multimeter. I'm much more interested in what's going on generally, looking at the shape of the waves, the rising edges, etc.YMMV.

author: Fungus
author_rank: Super Contributor
timestamp: April 01, 2016, 01:47:11 pm
content: Quote from: T3sl4co1l on April 01, 2016, 01:44:18 pmMy DS1054Z dances around quite a lot in 8-bit mode. In 12-bit mode it's rock steady.(on the calibration signal)

author: coppice
author_rank: Super Contributor
timestamp: April 01, 2016, 01:58:44 pm
content: Quote from: Fungus on April 01, 2016, 01:47:11 pmThat's all about the front end noise. You don't need a 12 bit ADC. You need an 8 bit front end, to match the 8 bit ADC.

author: theoldwizard1
author_rank: Regular Contributor
timestamp: April 01, 2016, 02:41:01 pm
content: Quote from: jeffsf on March 31, 2016, 09:47:06 pmMany havecorrectlystated the issue of why increase the "accuracy" when the display is the limiting factor and this is an excellent point.But as the on-board computing capability continues to increase(look at Red Pitaya)as does display technology technology(look at the latest iPads)and 8 bit ADC is going to look like the engine out of an old Ford Pinto in a new Mustang(both 2.3L, although the later is turbocharger).I was extremely happy to see Rigol's latest 'scopes move to a larger screen. What is practical ? 8" - 10" ?? There are several battery operated, automotive directed 'scopes with 8" screen on the market these days (think really thick tablet with a heavy rubber case, all ridiculously over priced).Quote from: jeffsf on March 31, 2016, 09:47:06 pmAnd that is where I am going. That "fancy" post processing software is become more like "standard equipment" on these new 'scopes (and a market Red Pitaya and SmartScope are hoping to tap). Or at least, a potentially profitable option (the manufacturer only pays for software once !)Quote from: jeffsf on March 31, 2016, 09:47:06 pmThe new "low end" 'scopes are AMAZING bang for the buck !FYI, My thoughts were, "How much would it cost to go from an 8 bit to a 10 bit 'scope ? Maybe even a 12 bit 'scope " 14 bit is crazy for this marketplace.

author: Fungus
author_rank: Super Contributor
timestamp: April 01, 2016, 03:01:36 pm
content: Quote from: theoldwizard1 on April 01, 2016, 02:41:01 pmProbably quite a lot. Everything downstream from the ADC would have to get wider. The CPU would probably have to switch to 16-bit processing instead of 8-bit. Do they make 10-bit (or 12-bit) RAM chips? If not you're going to have to use multiple chips instead of the neat 8-bit wide RAM chips they're using at the moment. It all adds up to more money.

author: theoldwizard1
author_rank: Regular Contributor
timestamp: April 01, 2016, 07:03:53 pm
content: Quote from: Fungus on April 01, 2016, 03:01:36 pmYou are out of touch with reality ! The Red Pitaya has multiple processors, one of them is 64 bits wide !Quote from: Fungus on April 01, 2016, 03:01:36 pmOf course they do and they have for years !You could easily make one using part of one of the FPGAs, and some external "instrument grade" OP amps and laser trimmed resistors. I suspect no one has done this because to calibrate a "discrete" design to less than 1mV and KEEP IT IN CALIBRATION while running, is to too difficult.

author: Howardlong
author_rank: Super Contributor
timestamp: April 01, 2016, 07:35:43 pm
content: Quote from: theoldwizard1 on April 01, 2016, 07:03:53 pmIt's just a shame the Red Pitaya has yet to show itself as anything other than a toy in the TE arena due to the extremely rudimentary software and 100MSa/s sampling rate.The Red Pitaya has some niche marketsnd uses, I know, I am using them in very low volume OEM devices, but a piece of test equipment it ain't.QuoteYour idea of easy and mine are somewhat different I fear. How about doing it and educating us all how easy it is?

author: Andreas
author_rank: Super Contributor
timestamp: April 01, 2016, 09:07:03 pm
content: Quote from: theoldwizard1 on April 01, 2016, 02:41:01 pmAnd how about 16 Bits. (20 Bits with oversampling ?).https://www.picotech.com/oscilloscope/5000/flexible-resolution-oscilloscopeIf you ever worked with a >= 12 Bit scope you never want to go back.And with a PC screen you can at least use 10 bits at a glance and can easily zoom in at details of complex waveforms.With best regardsAndreas

author: theoldwizard1
author_rank: Regular Contributor
timestamp: April 01, 2016, 11:16:20 pm
content: Quote from: Andreas on April 01, 2016, 09:07:03 pmI guess that is where I am coming from ! "Back in my day" (>10+yo) the few DSO scopes we had were >= 12 bits, but I don't recall the sample rate.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2016, 11:23:11 am
content: With the small screen found on normal DSOs there is little need for more than 8 Bits resolution. Especially as you get a little more resolution due to oversampling if the sampling rate is lower. So users usually prefer the faster 8 Bit ADC over a 10 Bit ADC with lower speed. Often it is also more the input amplifier or the probe that limits the noise and not so much the ADC. So it is not only about having a better ADC and more memory (not a big deal any more) a higher resolution scope should also have a lower noise input stage.A lower noise input stage could make the scope more useful - even with only 8 Bit resolution.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: April 02, 2016, 03:32:09 pm
content: Quote from: Andreas on April 01, 2016, 09:07:03 pmBut look at the specs:Flexible Hardware Resolution, from 8 to 16 bits200 MHz analog bandwidth1 GS/s sampling at 8 bit resolution500 MS/s sampling at 12 bit resolution62.5 MS/s sampling at 16 bit resolution512 MS buffer memory100,000 waveforms per second200 MS/s Arbitrary Waveform GeneratorSerial decoding and mask testing as standardWindows, Linux and Mac software"I'm not sure how useful a scope is that at 16bit is limited to a 30MHz sine wave (and much lower frequencies for non-sine waveforms). It's also not exactly cheap, at £699 for the 2ch 60MHz variant and £1695 for the 200MHz 4ch version with AWG.PicoScopes are great for a few specific tasks but they make for very poor general purpose scopes. Even our engineer that love them wouldn't want them as their only scope.

author: rf-loop
author_rank: Super Contributor
timestamp: April 02, 2016, 03:47:17 pm
content: Quote from: Kleinstein on April 02, 2016, 11:23:11 am(Entry level)Owon XDS3000(A) have 1GSa 8bit ADC and 500MSa/s 12bit ADC.Both are enough for full 100MHz BW. If look some scope what have "high resolution" up to 12bit useable frequency response is very low.Screen resolution (800x600) is not enough for 12bit but of course it is useable when scope is topped and do vertical zoom for details. Also of course full resolution is available in various file formats.There is somewhere thread where it was looked with bit more details/tests.

author: Fungus
author_rank: Super Contributor
timestamp: April 02, 2016, 04:14:19 pm
content: Quote from: Kleinstein on April 02, 2016, 11:23:11 amThat's expensive to do in hardware.In software you can do averaging to get the same result.The DS1054Z (for example) has an 8-bit averaging mode to reduce noise and a "12-bit" oversampling mode to give pseudo-higher resolution.

author: rf-loop
author_rank: Super Contributor
timestamp: April 02, 2016, 05:16:33 pm
content: Quote from: Fungus on April 02, 2016, 04:14:19 pmAnd frequency response with12bit high resolution is? Is it 2MHz or what?Btw, do you know what settings need use it really is 12bit least "in theory". I have not seen any real data about it.Can you get 12bit resolution data out from scope, example as .CSV file or is is just only for "nice image"Example some Tektronix have normally 8bit data available but with software baased high resolution it give out double bytes so that this improved accuracy (done using box car averaging) is also available.

author: nctnico
author_rank: Super Contributor
timestamp: April 02, 2016, 05:34:00 pm
content: Quote from: Wuerstchenhund on April 02, 2016, 03:32:09 pmI don't buy their explaination. As usual Picoscope pulls 16 bits out of their arses. Generally you can't get 16 bits from an 8 bit ADC. Non-linearity will kill you long before you get to 16 bits not to mention clock jitter. Also there is the noise requirement to be able to upsample at all; basically the lowest 8 bits of the 16 bit result must be swamped in noise to be able to increase resolution. If you have a very steady, low noise DC level you won't be able to digitize that with 16 bit accuracy using an 8 bit converter because the result will always be stuck at a constant value from the ADC. Oh and please note Picoscope uses the word resolution which is something completely different than accuracy.