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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Fri, 7 Jun 2024 14:28:18 -0400
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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.
>>>>
>>>> boB
>>>
>>> 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.
>>>
>>
>> I expect that the deterministic part of the jitter gets pushed out to
>> high frequency by the noise shaping.
>>
>> Random jitter you'd have to deal with by averaging.
>>
>> 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

-- 
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