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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Wed, 5 Jun 2024 01:11:33 -0000 (UTC)
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john larkin <jl@650pot.com> wrote:
> 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?
> 
> 

;)

I believe you may have got one in your stocking in January. 

(For others: we sell a nice 20-MHz AC-coupled preamp for noise
measurements, the LA-20 Lab Amplifier.  Works great, and is cheap like
borscht.)

Cheers 

Phil Hobbs 

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