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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Fri, 7 Jun 2024 14:22:25 +1000
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On 7/06/2024 3:57 am, 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.

"Accuracy" is an interesting word. The waveforms your are working 
aren't actually square waves - with zero edge transition - but have more 
or less exponential rising and falling edges.

You don't cares if the rising an falling edges have much the same stable 
shape, and if each exponential has effectively got to the rail before 
the next one starts.

Different rise and fall times complicate the error analysis, which is a 
tedious exercise (and one that I've done from time to time). One of 
engineers I worked with - and interviewed when he was first hired - did 
a six-digit DVM based on delta-sigma principles and he talked me through 
some of the problems during the job interview. I'd met them before on 
less accurate systems, which meant that he impressed the hell out of me.

He did some very nice work after we hired him - some of it on one of my 
projects.

-- 
Bill Sloman, Sydney
> 


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