Re: AD5791

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
 

Never mind. The 1 us is the settling time, so the BW is wider than 350 kHz.


Cheers

Phil Hobbs

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