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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Tue, 4 Jun 2024 22:58:54 -0000 (UTC)
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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 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics