Re: AD5791

On Tue, 04 Jun 2024 21:16:54 -0400, Steve Goldstein
<sgoldHAM@alum.mit.edu> wrote:

On Tue, 04 Jun 2024 17:15:03 -0700, 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?

>
You can't use +-16 references, there's a 2.5V minimum headroom
requirement (datasheet page 4).  The part is tested and guaranteed
with +-10V references; it's _possible_ the nonlinearity will be a
little worse if you increase to, say, +-13.5V refs.  This is a
consequence of the design internals. I don't know if this was ever
characterized, you'd probably have to check it yourself.

Sorry, right, the refs could be 14, to reduce the Johnson noise by
40%.  Then dividing down to +-10 becomes a problem.

>
>
You asked about testing.  I don't know how this specific part is
tested, but in general there are (at least) a couple of ways.
>
Many automatic testers have a super DVM available as a system
resource, often an HP3458A.  This works well but is slow, hence is an
expensive solution, i.e. it adds a lot of test time (cost).
>
Testers also often have a super-precision system DAC against which you
can make differential measurements.  With an in-amp gaining up the
difference between the system DAC and the DUT (Device Under Test) by
something like x100 you could use the system's fast ADC - 12 bits
might even be enough. There might need to be some averaging involved.
Even with waiting for the in-amp to settle this may still be easier
and faster than the system DVM.
>
Either way, testing to 20 bits takes time, and time costs money.
>
Thermocouple effects can become an issue in testing something like
this.  One of the first parts I designed at ADI was a very linear
custom VFC with very low offset and offset drift specs.  I also
designed and built the trim and test fixtures and needed to use
high-purity copper wire and Cd-Sn solder as the part dissipated a lot
of power (it was a chip-and-wire hybrid full of bipolar stuff - this
was the 1980s, before there was precision analog CMOS). It's probably
a lesser issue for AD5791 as the power dissipation is much lower than
my part had.  Probably just the Cd-Sn solder would have sufficed as it
had 1/10 the thermocouple effect against copper compared with Pd-Sn
solder, but the fixtures were one-offs so I went al -in.
>
I think I still have that roll of solder and the special flux. Now
it's hazmat.

The super DVMs don't usually offer a scanner option, so we'd probably
build a test box with an 8 or 9-channel relay scanner for the DVM and
a super-low noise AC-coupled amp for the noise measurements.

We use some cute little DPDT telecom relays for stuff like this,
latching relays to reduce coil-heating thermoelectrics.  We could even
gap-pad the tester PCB to a big aluminum plate to make it isothermal,
although a few layers of 2oz copper would do that pretty well.

Test speed wouldn't be a big deal. Overnight test and cal would be OK.
We're not making ICs!

That DAC is astounding, which makes it hard to test.