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 Hobbs

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.



-- 

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 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.

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.

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

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.

-- 

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

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

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.

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.

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.