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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Tue, 04 Jun 2024 17:15:03 -0700
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On Tue, 4 Jun 2024 22:58:54 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>john larkin <jl@650pot.com> wrote:
>> On Tue, 4 Jun 2024 21:53:13 +0200, Jeroen Belleman
>> <jeroen@nospam.please> wrote:
>> 
>>> On 6/4/24 19:48, john larkin 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.
>>>> 
>>> 
>>> But you can power the chip from +/-16V and the LSB can be in
>>> the 25uV ballpark. The Johnson noise of 7.5nV/rtHz doesn't
>>> seem so bad then, does it?
>>> 
>>> Jeroen Belleman
>> 
>> That helps some. +-14v is about the limit on the references. We'd have
>> to divide down to get our +-10v range back, and that would need some
>> crazy stable resistors.
>> 
>> Looks like the other way to get the noise down would be to parallel a
>> number of DACs. Times 8 channels! Ballpark $100 per DAC, which is
>> actually feasible.
>> 
>> It will of course need crazy-low-noise hyper-stable references.
>> 
>> I wonder how ADI tests these parts. I can't buy a 1 PPM accurate DVM.
>> 
>> 
>
>Its quoted rise time is 1us, corresponding to a 3 dB bandwidth of about 350
>kHz, or 550  kHz noise bandwidth. 
>
>With 7.5 nV 1-Hz noise, the total RMS noise should be about 5.6 uV, just
>about half a LSB at 10V FS. 
>
>Not that shabby. 
>
>Cheers 
>
>Phil Hobbs 

Three DACs in parallel with +-16 refs, divided down to +-10, pencils
out around 3.2 nv/rthz.

I'm going to need a very good preamp to measure the noise, something
below 1 nv/rthz. Any ideas?