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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Wed, 5 Jun 2024 14:37:29 +1000
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On 5/06/2024 6:56 am, john larkin 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?
> 
> 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.

In fact it needs a stable thin-film array. Provided the thin-film 
resistors are on a common substrate, the divide ratio can be quite a bit 
more stable than the individual resistances which are at the same 
temperature and made of metal despoited at the same time.

> 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.

Very stable four terminal references can be bought - they aren't cheap 
but there's nothing crazy about the prices or availability. You do have 
to be careful of voltage drops in the relevant printed circuit traces - 
I once had to fix a circuit where the voltage reference was grounded at 
the wrong end of trace carrying the return current from a big EPROM.

The quick fix was soldering a chunk of copper wire onto the track, but 
changing the layout to something closer to star grounding was the long 
term solution.

> I wonder how ADI tests these parts. I can't buy a 1 PPM accurate DVM.

Not a enough money? No access to liquid helium? NIST seems to have 
managed it in 1984.

https://www.nist.gov/system/files/documents/calibrations/im-34-2a.pdf

They were still working on the Josephson junction array 10V reference 
back then

https://en.wikipedia.org/wiki/Josephson_voltage_standard

and it back commercially available in 1989.

-- 
Bill Sloman, Sydney


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