Re: AD5791

On Fri, 7 Jun 2024 14:28:18 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2024-06-06 22:38, john larkin wrote:

On Thu, 6 Jun 2024 22:19:22 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

On 2024-06-06 13:57, john larkin wrote:

On Thu, 06 Jun 2024 10:15:45 -0700, boB <boB@K7IQ.com> wrote:
>

On Tue, 04 Jun 2024 10:48:00 -0700, john larkin <jl@650pot.com> wrote:
>

https://www.analog.com/en/products/ad5791.html
>
That's an amazing part. 20 bit DAC with 1 PPM accuracy and 0.05 PPM
per degree C tempco.
>
My main gripe is its 3.4K output impedance, which makes a lot of
Johnson noise. I suppose I could run a bunch in parallel.

>
>
Nice part but costs way too much for any products we make.
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boB

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What do you make?
>
We live on the lunatic fringe of electronics, things that are really
hard to do, things with extreme exponents. It makes money because it
has little competition, but the money is a side effect. I do it
because it's fun.
>
There must be something cool that we can do with a 1 PPM accurate DAC.
>
TI has a 20-bit delta-sigma DAC that's about $12, but it's only linear
to 15 PPM. I don't understand how a d-s DAC or ADC can even be that
good. It would seem to need femtosecond edge accuracies inside.
>

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I expect that the deterministic part of the jitter gets pushed out to
high frequency by the noise shaping.
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Random jitter you'd have to deal with by averaging.
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Cheers
>
Phil Hobbs

 
I was thinking about rise/fall time asymmetry, changing average values
as duty cycles squirm all over the place.
 

>
Yeah, part of which is deterministic and part random.  DAC noise shaping
AIUI makes the the DS sum run in a limit cycle even for a fixed code, so
that most of the switching junk is up at high frequency where it's
easier to filter out.  However, I'm not a delta-sigma expert.
>
(They call them sigma-deltas for some reason--possibly related to gang
insignia.) ;)
>
Cheers
>
Phil Hobbs

Looking at the classic d-s architecture, a midscale code will be
almost a square wave from the feedback quantizer, but near full scale
the duty cycle will be very small. Edge density will change with input
signal.

Every blip has a rise and a fall and they have to be match exactly to
get the right average.

Maybe real chips do something else, charge balancing maybe.