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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Wed, 05 Jun 2024 14:32:15 -0400
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On Wed, 05 Jun 2024 10:56:18 -0700, john larkin <jl@650pot.com> wrote:

>On Wed, 05 Jun 2024 12:50:59 -0400, Joe Gwinn <joegwinn@comcast.net>
>wrote:
>
>>On Tue, 04 Jun 2024 13:56:36 -0700, 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.
>>
>>Aren't 6.5-digit DMMs exactly 1PPM?
>>
>>Joe Gwinn
>
>I think the best I've seen is something like 4 PPM.  For north of
>$14K.

It might be easier to buy a 1ppm voltage reference and calibrate
against that.

..<https://en.wikipedia.org/wiki/Voltage_reference>

For a product instance:

..<https://voltagestandard.com/001%25-10v-reference>  costs $140.

Joe Gwinn