pipeline ADC missing codes

[jla...@highlandsniptechnology.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?





--

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.

[Rick C]

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

[Jan Panteltje]

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...

[jla...@highlandsniptechnology.com]

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.

[Phil Hobbs]

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

[jla...@highlandsniptechnology.com]

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


--

[Phil Hobbs]

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?

[Jan Panteltje]

On a sunny day (Thu, 16 Sep 2021 08:20:49 -0700) it happened
jla...@highlandsniptechnology.com wrote in
<i0o6kgpiu14mo0g7e...@4ax.com>:

How can you have RF noise if you use a Nyquist filter?

[John Larkin]

On Thu, 16 Sep 2021 12:57:00 -0400, Phil Hobbs

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

[John Larkin]

On Thu, 16 Sep 2021 17:15:50 GMT, Jan Panteltje

Nothing's perfect.

[John Walliker]

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

[Jan Panteltje]

On a sunny day (Thu, 16 Sep 2021 10:38:09 -0700) it happened John Larkin
<jlarkin@highland_atwork_technology.com> wrote in
<vov6kghibf6iklg5v...@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

[Phil Hobbs]

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.

[Rick C]

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

[Lasse Langwadt Christensen]

sometimes signals are naughty and sneak in through the backdoor

[John Larkin]

No, I can't imagine that TI would sell parts this bad. It's the same
on multiple boards.

--

[Steve Goldstein]

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
><jrwal...@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.

[Chris Jones]

On 17/09/2021 00:23, 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?
>

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.

[jla...@highlandsniptechnology.com]

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.

[Phil Hobbs]

John, did you ever get to the bottom of this mystery?

[John Larkin]

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.

[Phil Hobbs]

Yikes. "Here there be dragons."

[John Doe]

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.

[jla...@highlandsniptechnology.com]

Revise the schematic and layout. Roll the rev letter.

[jla...@highlandsniptechnology.com]

On Tue, 5 Oct 2021 20:34:03 -0400, 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.

[Rhydian]

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 :)