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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Wed, 5 Jun 2024 14:06:07 -0400
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On 2024-06-04 21:11, Phil Hobbs wrote:
> 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 

LA22

> Lab Amplifier.  Works great, and is cheap like
> borscht.)

https://hobbs-eo.com/products/la-22-lab-amplifier

> 
> Cheers
> 
> Phil Hobbs
> 


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com