Sujet : Re: AD5791
De : bill.sloman (at) *nospam* ieee.org (Bill Sloman)
Groupes : sci.electronics.designDate : 05. Jun 2024, 06:37:29
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <v3oq2e$qmhh$1@dont-email.me>
References : 1 2 3
User-Agent : Mozilla Thunderbird
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.pdfThey were still working on the Josephson junction array 10V reference back then
https://en.wikipedia.org/wiki/Josephson_voltage_standardand it back commercially available in 1989.
-- Bill Sloman, Sydney-- This email has been checked for viruses by Norton antivirus software.www.norton.com
AD5791
Re: AD5791