Re: AD5791

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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