On Tue, 5 Oct 2021 16:42:21 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>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 HobbsNo. I tried some experiments with the driving circuit, looking for oscillations or whatever, and only managed to make the missing codes a little worse. We'll live with the box's specified jitter for now, but I want it to be much better. The fix may be to spin the board and use a different ADC. -- 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
pipeline ADC missing codes
Started by ●September 16, 2021
Reply by ●October 5, 20212021-10-05
Reply by ●October 5, 20212021-10-05
John Larkin wrote:> On Tue, 5 Oct 2021 16:42:21 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> 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 > > No. I tried some experiments with the driving circuit, looking for > oscillations or whatever, and only managed to make the missing codes a > little worse. > > We'll live with the box's specified jitter for now, but I want it to > be much better. The fix may be to spin the board and use a different > ADC. >Yikes. "Here there be dragons." 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 ●October 5, 20212021-10-05
John Larkin <jlarkin@highland_atwork_technology.com> wrote:> The fix may be to spin the board and use a different > ADC.What does "spin the board" mean? Thanks.
Reply by ●October 5, 20212021-10-05
On Wed, 6 Oct 2021 00:40:10 -0000 (UTC), John Doe <always.look@message.header> wrote:>John Larkin <jlarkin@highland_atwork_technology.com> wrote: > >> The fix may be to spin the board and use a different >> ADC. > >What does "spin the board" mean? > >Thanks.Revise the schematic and layout. Roll the rev letter. -- 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
On Tue, 5 Oct 2021 20:34:03 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> On Tue, 5 Oct 2021 16:42:21 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> 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 >> >> No. I tried some experiments with the driving circuit, looking for >> oscillations or whatever, and only managed to make the missing codes a >> little worse. >> >> We'll live with the box's specified jitter for now, but I want it to >> be much better. The fix may be to spin the board and use a different >> ADC. >> >Yikes. "Here there be dragons." > >Cheers > >Phil HobbsWe have done this same dpll many times, but not with this ADC. Maybe I can hack a quickie adapter board, to try a different ADC. That's a lot easier than revving a 10-layer board with about 1000 parts. -- 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 6, 20212021-10-06
On Tue, 05 Oct 2021 18:22:19 -0700, jlarkin wrote: [mega snip]> > We have done this same dpll many times, but not with this ADC. > > Maybe I can hack a quickie adapter board, to try a different ADC. > That's a lot easier than revving a 10-layer board with about 1000 parts.Following your OP I did have a look at the TI data sheet, and couldn't see anything obvious that would cause it. We use a similar part from AD, the AD9649, running at 57.6 MHz. No problems so far, the sort of missing codes you describe would completely break our measurement. Whether you can buy any at the moment is a different matter :)
