On Thursday, 16 September 2021 at 18:39:10 UTC+1, John Larkin wrote:> On Thu, 16 Sep 2021 17:15:50 GMT, Jan Panteltje > <pNaonSt...@yahoo.com> wrote: > > >On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened > >jla...@highlandsniptechnology.com wrote in > ><i0o6kgpiu14mo0g7e...@4ax.com>: > > > >>On Thu, 16 Sep 2021 14:48:40 GMT, Jan Panteltje > >><pNaonSt...@yahoo.com> wrote: > >> > >>>On a sunny day (Thu, 16 Sep 2021 07:23:38 -0700) it happened > >>>jla...@highlandsniptechnology.com wrote in > >>><iuj6kgd5abi888ppa...@4ax.com>: > >>> > >>>>I've been researching this and see a lot of papers and appnotes that > >>>>are mostly the same. > >>>> > >>>>We conjecture that some capacitive-DAC pipeline ADCs do not act as if > >>>>they have a single sample-and-hold in the front end, but are more > >>>>complex and have, essentially, multiple s/h elements. One consequence > >>>>is that the presence of very high frequency components of the signal > >>>>can cause missing codes if the sample aperatures are not absolutely > >>>>identical. I think we may be seeing this happen. > >>>> > >>>>Sometimes we deliberately add dither noise to improve ADC histograms, > >>>>but it could be that very high frequency noise has the opposite > >>>>effect. > >>>> > >>>>I've seen data sheets and appnotes that suggest adding series > >>>>resistors or RCs to fast ADC inputs. I assumed that was to offset some > >>>>charge injection thing, but maybe not. Those might be lowpass filters. > >>>> > >>>>Has anybody run into this effect? > >>> > >>>Well you sample at 2f and have a nyquist filter at 1f? > >>> https://en.wikipedia.org/wiki/Anti-aliasing_filter > >>> > >>>Scope ADCs do not use a filter ADFAIK... > >> > >>The issue isn't aliasing, it's missing codes. > >> > >>In fact we're not volating Nyquist with our main signal, but we think > >>a little very-HF noise is making the ADC miss codes. > > > >How can you have RF noise if you use a Nyquist filter? > Nothing's perfect.However, the datasheet is very specific in its claim that there are no missing codes. So either the devices are broken and don't meet their specification or something about the design is provoking the problem. I don't see how the DNL plots in the data sheet could have been obtained with a device behaving as you describe. John
pipeline ADC missing codes
Started by ●September 16, 2021
Reply by ●September 16, 20212021-09-16
Reply by ●September 16, 20212021-09-16
On a sunny day (Thu, 16 Sep 2021 10:38:09 -0700) it happened John Larkin <jlarkin@highland_atwork_technology.com> wrote in <vov6kghibf6iklg5v4t23coipj4kb6o6jp@4ax.com>:>> >>How many missing codes are there? > >Maybe 1/3 or 1/2 of the 1024. It's bad. Sometimes two adjacent bins >are missing.Reference decoupling? Supply decoupling?>Don't know. It's an ADC10080. > >We didn't have time to investigate this in detail; the resistor fix >was good enough to let us ship. I plan to get back to this when I have >time. I was just wondering if anyone else had opinions.I am not sure about that topograghy, but personally I would go for a FLASH type converter maybe like the AD9410, more in my video way of thinking
Reply by ●September 16, 20212021-09-16
John Larkin wrote:> On Thu, 16 Sep 2021 12:57:00 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> jlarkin@highlandsniptechnology.com wrote: >>> On Thu, 16 Sep 2021 11:59:14 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> jlarkin@highlandsniptechnology.com wrote: >>>>> On Thu, 16 Sep 2021 14:48:40 GMT, Jan Panteltje >>>>> <pNaonStpealmtje@yahoo.com> wrote: >>>>> >>>>>> On a sunny day (Thu, 16 Sep 2021 07:23:38 -0700) it happened >>>>>> jlarkin@highlandsniptechnology.com wrote in >>>>>> <iuj6kgd5abi888ppa5dj46nflto41lm5pk@4ax.com>: >>>>>> >>>>>>> I've been researching this and see a lot of papers and appnotes that >>>>>>> are mostly the same. >>>>>>> >>>>>>> We conjecture that some capacitive-DAC pipeline ADCs do not act as if >>>>>>> they have a single sample-and-hold in the front end, but are more >>>>>>> complex and have, essentially, multiple s/h elements. One consequence >>>>>>> is that the presence of very high frequency components of the signal >>>>>>> can cause missing codes if the sample aperatures are not absolutely >>>>>>> identical. I think we may be seeing this happen. >>>>>>> >>>>>>> Sometimes we deliberately add dither noise to improve ADC histograms, >>>>>>> but it could be that very high frequency noise has the opposite >>>>>>> effect. >>>>>>> >>>>>>> I've seen data sheets and appnotes that suggest adding series >>>>>>> resistors or RCs to fast ADC inputs. I assumed that was to offset some >>>>>>> charge injection thing, but maybe not. Those might be lowpass filters. >>>>>>> >>>>>>> Has anybody run into this effect? >>>>>> >>>>>> Well you sample at 2f and have a nyquist filter at 1f? >>>>>> https://en.wikipedia.org/wiki/Anti-aliasing_filter >>>>>> >>>>>> Scope ADCs do not use a filter ADFAIK... >>>>> >>>>> The issue isn't aliasing, it's missing codes. >>>>> >>>>> In fact we're not volating Nyquist with our main signal, but we think >>>>> a little very-HF noise is making the ADC miss codes. >>>>> >>>>> Just wondering if anyone has seen this. >>>>> >>>>> >>>>> >>>> So your conjecture is that the sampled charge is being passed down the >>>> pipeline from capacitor to capacitor as the SAR process goes along, till >>>> nothing is left at the end, and that capacitance variations between >>>> stages might make the more significant bits get decided wrong? >>> >>> No, I was thinking that at the very input, there are multiple s+h >>> elements and they don't sample at exactly the same time. >>> >>> I have seen diagrams that imply, in some ADCs, the same capacitive >>> ladder that is used for digitizing is also the s+h. >>> >>> And even if the first stage is pure flash, the first few bits could >>> follow a different path from the residual part. >>> >>> I'm imagining mechanisms, but the missing codes are very real. >>> >>>> >>>> It's a plausible mechanism, but it seems like that would get taken care >>>> of in the self-calibration step because it ought to be a fixed-pattern >>>> effect. >>>> >>>> If it's not fixed-pattern, I'd be more inclined to suspect capacitive >>>> feedthrough in the sampling switch, or maybe junk getting into the >>>> reference path, maybe via ground inductance. >>>> >>>> It might conceivably be the interaction of the ADC input's kickout spike >>>> with the instantaneous bias condition of the op amp--lots of amps with >>>> boosted output stages have lower open-loop Zout during rapid slewing, IIUC. >>>> >>> >>> We can't imagine any of the classic noise mechanisms causing missing >>> codes. Some bins have a million hits and an adjacent bin has 14. This >>> is an 80 MHz 10-bit ADC being clocked at 40 MHz. >> >> Yikes. If there are a bunch of samplers all connected to the input >> pins, they can't all be sampling at once or the pipeline wouldn't be >> doing anything useful. >> >> How many missing codes are there? > > Maybe 1/3 or 1/2 of the 1024. It's bad. Sometimes two adjacent bins > are missing. > >> >> Are they at the MSB carries? > > > They are scattered all over the code space. If we digitize a linear > ramp, it's worse at low codes, which might suggest some oscillation in > the signal source, or something inside the ADC. > >> >> Do they go away if you put caps on the inputs? > > > There is a cap on the input! Adding a series resistor seems to help. > > >> >> Is the part self-calibrating? > > Don't know. It's an ADC10080. > > We didn't have time to investigate this in detail; the resistor fix > was good enough to let us ship. I plan to get back to this when I have > time. I was just wondering if anyone else had opinions. >Yikes, so noted. One thing that plagues some ADCs is internal slew limiting. To get the I_Q headline spec, they starve internal circuitry of bias current so that it goes all nonlinear on fast edges. I'll be very interested to hear the forensic results. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 16, 20212021-09-16
On Thursday, September 16, 2021 at 3:21:47 PM UTC-4, John Walliker wrote:> On Thursday, 16 September 2021 at 18:39:10 UTC+1, John Larkin wrote: > > On Thu, 16 Sep 2021 17:15:50 GMT, Jan Panteltje > > <pNaonSt...@yahoo.com> wrote: > > > > >On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened > > >jla...@highlandsniptechnology.com wrote in > > ><i0o6kgpiu14mo0g7e...@4ax.com>: > > > > > >>In fact we're not volating Nyquist with our main signal, but we think > > >>a little very-HF noise is making the ADC miss codes. > > > > > >How can you have RF noise if you use a Nyquist filter? > > Nothing's perfect. > However, the datasheet is very specific in its claim that there are no missing codes. > So either the devices are broken and don't meet their specification or something > about the design is provoking the problem. I don't see how the DNL plots in the > data sheet could have been obtained with a device behaving as you describe.There are a million ways to screw up a design. There are much fewer ways to get it right. It could be something as simple as power decoupling... or something else all together. Without investigating fully they are likely to never know. -- Rick C. + Get 1,000 miles of free Supercharging + Tesla referral code - https://ts.la/richard11209
Reply by ●September 16, 20212021-09-16
torsdag den 16. september 2021 kl. 19.16.28 UTC+2 skrev Jan Panteltje:> On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened > jla...@highlandsniptechnology.com wrote in > <i0o6kgpiu14mo0g7e...@4ax.com>: > >On Thu, 16 Sep 2021 14:48:40 GMT, Jan Panteltje > ><pNaonSt...@yahoo.com> wrote: > > > >>On a sunny day (Thu, 16 Sep 2021 07:23:38 -0700) it happened > >>jla...@highlandsniptechnology.com wrote in > >><iuj6kgd5abi888ppa...@4ax.com>: > >> > >>>I've been researching this and see a lot of papers and appnotes that > >>>are mostly the same. > >>> > >>>We conjecture that some capacitive-DAC pipeline ADCs do not act as if > >>>they have a single sample-and-hold in the front end, but are more > >>>complex and have, essentially, multiple s/h elements. One consequence > >>>is that the presence of very high frequency components of the signal > >>>can cause missing codes if the sample aperatures are not absolutely > >>>identical. I think we may be seeing this happen. > >>> > >>>Sometimes we deliberately add dither noise to improve ADC histograms, > >>>but it could be that very high frequency noise has the opposite > >>>effect. > >>> > >>>I've seen data sheets and appnotes that suggest adding series > >>>resistors or RCs to fast ADC inputs. I assumed that was to offset some > >>>charge injection thing, but maybe not. Those might be lowpass filters. > >>> > >>>Has anybody run into this effect? > >> > >>Well you sample at 2f and have a nyquist filter at 1f? > >> https://en.wikipedia.org/wiki/Anti-aliasing_filter > >> > >>Scope ADCs do not use a filter ADFAIK... > > > >The issue isn't aliasing, it's missing codes. > > > >In fact we're not volating Nyquist with our main signal, but we think > >a little very-HF noise is making the ADC miss codes. > How can you have RF noise if you use a Nyquist filter?sometimes signals are naughty and sneak in through the backdoor
Reply by ●September 16, 20212021-09-16
On Thu, 16 Sep 2021 12:21:43 -0700 (PDT), John Walliker <jrwalliker@gmail.com> wrote:>On Thursday, 16 September 2021 at 18:39:10 UTC+1, John Larkin wrote: >> On Thu, 16 Sep 2021 17:15:50 GMT, Jan Panteltje >> <pNaonSt...@yahoo.com> wrote: >> >> >On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened >> >jla...@highlandsniptechnology.com wrote in >> ><i0o6kgpiu14mo0g7e...@4ax.com>: >> > >> >>On Thu, 16 Sep 2021 14:48:40 GMT, Jan Panteltje >> >><pNaonSt...@yahoo.com> wrote: >> >> >> >>>On a sunny day (Thu, 16 Sep 2021 07:23:38 -0700) it happened >> >>>jla...@highlandsniptechnology.com wrote in >> >>><iuj6kgd5abi888ppa...@4ax.com>: >> >>> >> >>>>I've been researching this and see a lot of papers and appnotes that >> >>>>are mostly the same. >> >>>> >> >>>>We conjecture that some capacitive-DAC pipeline ADCs do not act as if >> >>>>they have a single sample-and-hold in the front end, but are more >> >>>>complex and have, essentially, multiple s/h elements. One consequence >> >>>>is that the presence of very high frequency components of the signal >> >>>>can cause missing codes if the sample aperatures are not absolutely >> >>>>identical. I think we may be seeing this happen. >> >>>> >> >>>>Sometimes we deliberately add dither noise to improve ADC histograms, >> >>>>but it could be that very high frequency noise has the opposite >> >>>>effect. >> >>>> >> >>>>I've seen data sheets and appnotes that suggest adding series >> >>>>resistors or RCs to fast ADC inputs. I assumed that was to offset some >> >>>>charge injection thing, but maybe not. Those might be lowpass filters. >> >>>> >> >>>>Has anybody run into this effect? >> >>> >> >>>Well you sample at 2f and have a nyquist filter at 1f? >> >>> https://en.wikipedia.org/wiki/Anti-aliasing_filter >> >>> >> >>>Scope ADCs do not use a filter ADFAIK... >> >> >> >>The issue isn't aliasing, it's missing codes. >> >> >> >>In fact we're not volating Nyquist with our main signal, but we think >> >>a little very-HF noise is making the ADC miss codes. >> > >> >How can you have RF noise if you use a Nyquist filter? >> Nothing's perfect. > >However, the datasheet is very specific in its claim that there are no missing codes. >So either the devices are broken and don't meet their specification or something >about the design is provoking the problem. I don't see how the DNL plots in the >data sheet could have been obtained with a device behaving as you describe. > >John >No, I can't imagine that TI would sell parts this bad. It's the same on multiple boards. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●September 16, 20212021-09-16
On Thu, 16 Sep 2021 13:46:10 -0700, John Larkin <jlarkin@highland_atwork_technology.com> wrote:>On Thu, 16 Sep 2021 12:21:43 -0700 (PDT), John Walliker ><jrwalliker@gmail.com> wrote: > >>On Thursday, 16 September 2021 at 18:39:10 UTC+1, John Larkin wrote: >>> On Thu, 16 Sep 2021 17:15:50 GMT, Jan Panteltje<A lot of snippage>>> >>John >> > >No, I can't imagine that TI would sell parts this bad. It's the same >on multiple boards.Agreed. They're not dumb. Is there a TI evaluation board that would allow you to compare the performance to your own design? It probably wouldn't be practical to compare the same chip because things are so tiny and difficult to rework nowadays, but you could at least see if the chip works as advertised in (presumably) the best possible assembly. Board layouts for many high-speed converters are critically layout-sensitive. Also, some bypass caps are better than others. But you do a lot of high-speed stuff and probably know this, if you didn't you'd be out of business by now.
Reply by ●September 21, 20212021-09-21
On 17/09/2021 00:23, jlarkin@highlandsniptechnology.com wrote:> I've been researching this and see a lot of papers and appnotes that > are mostly the same. > > We conjecture that some capacitive-DAC pipeline ADCs do not act as if > they have a single sample-and-hold in the front end, but are more > complex and have, essentially, multiple s/h elements. One consequence > is that the presence of very high frequency components of the signal > can cause missing codes if the sample aperatures are not absolutely > identical. I think we may be seeing this happen. > > Sometimes we deliberately add dither noise to improve ADC histograms, > but it could be that very high frequency noise has the opposite > effect. > > I've seen data sheets and appnotes that suggest adding series > resistors or RCs to fast ADC inputs. I assumed that was to offset some > charge injection thing, but maybe not. Those might be lowpass filters. > > Has anybody run into this effect? >I never did a pipeline converter so take what I say with a grain of salt... I think they have to do a bunch of tricky stuff, like there are sometimes extra duplicate stages added part-way along the pipeline so that the first stage comparator doesn't need to be accurate to as good as 1 lsb, only the 1st stage DAC must. So when the first stage gets its decision wrong, some subsequent stage will have extra range, or be duplicated, and then error-correcting logic fixes the msb based on the later bits. Without that feature, missing codes could result. I think they need to pay a lot of attention to metastability too, i.e. the bit that gets passed to the logic that generates the output code had better be the same as the bit that gets passed to the DAC for that stage of the converter, which means they need to use the same latch, not a different identical one, to drive both, and probably a lot of other measures. Metastability would cause big errors though, not what you're seeing. You could look in JSSC or patents to find some of the tricks they use in old parts like that one. If you can identify whether there is a pattern to the codes that are missing (are they adjacent to a major bit transition for example) that might help someone to figure it out.
Reply by ●September 21, 20212021-09-21
On Tue, 21 Sep 2021 17:01:03 +1000, Chris Jones <lugnut808@spam.yahoo.com> wrote:>On 17/09/2021 00:23, jlarkin@highlandsniptechnology.com wrote: >> I've been researching this and see a lot of papers and appnotes that >> are mostly the same. >> >> We conjecture that some capacitive-DAC pipeline ADCs do not act as if >> they have a single sample-and-hold in the front end, but are more >> complex and have, essentially, multiple s/h elements. One consequence >> is that the presence of very high frequency components of the signal >> can cause missing codes if the sample aperatures are not absolutely >> identical. I think we may be seeing this happen. >> >> Sometimes we deliberately add dither noise to improve ADC histograms, >> but it could be that very high frequency noise has the opposite >> effect. >> >> I've seen data sheets and appnotes that suggest adding series >> resistors or RCs to fast ADC inputs. I assumed that was to offset some >> charge injection thing, but maybe not. Those might be lowpass filters. >> >> Has anybody run into this effect? >> > >I never did a pipeline converter so take what I say with a grain of >salt... I think they have to do a bunch of tricky stuff, like there are >sometimes extra duplicate stages added part-way along the pipeline so >that the first stage comparator doesn't need to be accurate to as good >as 1 lsb, only the 1st stage DAC must. So when the first stage gets its >decision wrong, some subsequent stage will have extra range, or be >duplicated, and then error-correcting logic fixes the msb based on the >later bits. Without that feature, missing codes could result. > >I think they need to pay a lot of attention to metastability too, i.e. >the bit that gets passed to the logic that generates the output code had >better be the same as the bit that gets passed to the DAC for that stage >of the converter, which means they need to use the same latch, not a >different identical one, to drive both, and probably a lot of other >measures. Metastability would cause big errors though, not what you're >seeing. > >You could look in JSSC or patents to find some of the tricks they use in >old parts like that one. > >If you can identify whether there is a pattern to the codes that are >missing (are they adjacent to a major bit transition for example) that >might help someone to figure it out. > > >Out of 1024 codes, something like half of them are unreasonable. There are random clusters of one or two empty or nearly empty histogram bins with a big neighbor getting their hits. It seems to be related to the input signal we are applying, possibly some too-high-to-see oscillation. TI support is clueless. We plan some mindless experiments, change parts here and there and see what happens to the statistics. -- Father Brown's figure remained quite dark and still; but in that instant he had lost his head. His head was always most valuable when he had lost it.
Reply by ●October 5, 20212021-10-05
John Larkin wrote:> On Thu, 16 Sep 2021 12:21:43 -0700 (PDT), John Walliker > <jrwalliker@gmail.com> wrote: > >> On Thursday, 16 September 2021 at 18:39:10 UTC+1, John Larkin wrote: >>> On Thu, 16 Sep 2021 17:15:50 GMT, Jan Panteltje >>> <pNaonSt...@yahoo.com> wrote: >>> >>>> On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened >>>> jla...@highlandsniptechnology.com wrote in >>>> <i0o6kgpiu14mo0g7e...@4ax.com>: >>>> >>>>> On Thu, 16 Sep 2021 14:48:40 GMT, Jan Panteltje >>>>> <pNaonSt...@yahoo.com> wrote: >>>>> >>>>>> On a sunny day (Thu, 16 Sep 2021 07:23:38 -0700) it happened >>>>>> jla...@highlandsniptechnology.com wrote in >>>>>> <iuj6kgd5abi888ppa...@4ax.com>: >>>>>> >>>>>>> I've been researching this and see a lot of papers and appnotes that >>>>>>> are mostly the same. >>>>>>> >>>>>>> We conjecture that some capacitive-DAC pipeline ADCs do not act as if >>>>>>> they have a single sample-and-hold in the front end, but are more >>>>>>> complex and have, essentially, multiple s/h elements. One consequence >>>>>>> is that the presence of very high frequency components of the signal >>>>>>> can cause missing codes if the sample aperatures are not absolutely >>>>>>> identical. I think we may be seeing this happen. >>>>>>> >>>>>>> Sometimes we deliberately add dither noise to improve ADC histograms, >>>>>>> but it could be that very high frequency noise has the opposite >>>>>>> effect. >>>>>>> >>>>>>> I've seen data sheets and appnotes that suggest adding series >>>>>>> resistors or RCs to fast ADC inputs. I assumed that was to offset some >>>>>>> charge injection thing, but maybe not. Those might be lowpass filters. >>>>>>> >>>>>>> Has anybody run into this effect? >>>>>> >>>>>> Well you sample at 2f and have a nyquist filter at 1f? >>>>>> https://en.wikipedia.org/wiki/Anti-aliasing_filter >>>>>> >>>>>> Scope ADCs do not use a filter ADFAIK... >>>>> >>>>> The issue isn't aliasing, it's missing codes. >>>>> >>>>> In fact we're not volating Nyquist with our main signal, but we think >>>>> a little very-HF noise is making the ADC miss codes. >>>> >>>> How can you have RF noise if you use a Nyquist filter? >>> Nothing's perfect. >> >> However, the datasheet is very specific in its claim that there are no missing codes. >> So either the devices are broken and don't meet their specification or something >> about the design is provoking the problem. I don't see how the DNL plots in the >> data sheet could have been obtained with a device behaving as you describe. >> >> John >> > > No, I can't imagine that TI would sell parts this bad. It's the same > on multiple boards. >John, did you ever get to the bottom of this mystery? Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
