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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: AD5791
Date: Tue, 04 Jun 2024 20:50:03 -0700
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On Wed, 5 Jun 2024 01:11:33 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> 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. 

Yes, I forgot somehow. Thanks.

We'll likely need another, if we go ahead with this project, and I'll
have the customer buy some too.



>
>(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