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pipeline ADC missing codes

Started by Unknown September 16, 2021
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?





-- 

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.



  
On Thursday, September 16, 2021 at 10:23:46 AM UTC-4, jla...@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?
Not sure what you mean by "high frequency" noise. If you are talking about out of band noise, that is never a good idea. There is no need for dithering "noise" to be out of band. If the signal and noise are in band the ADC should work as indicated in the data sheet. If the part does not work as advertised, discuss your results with the maker. It's that simple. -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
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...
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. -- 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.
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? 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. The RC on the input covers a multitude of sins, for sure. ;) 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
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.
>The RC on the input covers a multitude of sins, for sure. ;) > >Cheers > >Phil Hobbs
-- 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.
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? Are they at the MSB carries? Do they go away if you put caps on the inputs? Is the part self-calibrating? 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
On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened
jlarkin@highlandsniptechnology.com wrote in
<i0o6kgpiu14mo0g7e4dpodrgoqaun3rrt8@4ax.com>:

>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.
How can you have RF noise if you use a Nyquist filter?
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.
> >Cheers > >Phil Hobbs
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
On Thu, 16 Sep 2021 17:15:50 GMT, Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:

>On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened >jlarkin@highlandsniptechnology.com wrote in ><i0o6kgpiu14mo0g7e4dpodrgoqaun3rrt8@4ax.com>: > >>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. > >How can you have RF noise if you use a Nyquist filter?
Nothing's perfect. 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