> But: if I do have to design this beast, Does anyone have any favorite
> A/D and D/A convertors that are 14 bit accurate or better?  I don't
> normally use parts that accurate and would prefer to have people whom
> I trust make suggestions rather than just starting to pour over
> data-books and find out that the parts that at first seem to make
> sense are now unobtainium.  SPI preferred, I think, but willing to
> look at anything.

Here is a very lightly edited comment from my friend Ken.
(I took out his description of a product of his).

Note his very last paragraph re required DAC resolution.

Russell: Need for a super DAC may be able to be relaxed if you have a
lower resolution DAC and can "nudge it up or down a bit" independently
and use the ADC to control what is happening.



              Russell

____________

Linear Tech have pretty much all the IC's Dwayne should need  - and a good =
SPICE
simulator so he can be pretty sure his design will work (and meet spec) bef=
ore
he buys or builds anything.

We've just done a very high precision programmable voltage source with
programmable output resistance for an ATE system and pretty much used LT de=
vices
exclusively  - and the SPICE simulations we did were right on the money.

We opted to use I2C-bus devices throughout for the programmable aspects.  T=
hat
meant the unit needed no uC and no firmware  - just a USB-I2C bridge (easil=
y
done using FTDI's devices) with just a Windows DLL and app to do all the
control.  That saved a lot of time, and also neatly solved the problem that=
 the
unit (in fact each of the five sub-units it comprises) could be readily
galvanically isolated from ground and from each other (using I2C-bus isolat=
ors
from Analog Devices)  - an essential requirement for the application.

Achieving an accuracy of 0.01% would be a challenge, and would definitely
require that Dwayne know what he is doing  - especially when it comes to PC=
B
layout (1A flowing in the wrong place in a PCB can cause havoc for mV-level
control and feedback signals).  How much of a challenge depends on the exte=
rnal
environment (temperature range especially) as much as anything else, but pr=
oper
analog design techniques (to take account of opamp input bias currents etc.=
),
and proper component selection (with adequately low temperature coefficient=
s)
will also play an important part.  The required closed-loop bandwidth will =
also
dictate the types of active components he will need to use (and the detaile=
d
circuit topology), and he will have to watch out for nasty analog "gotchas"=
 that
simulators don't ever pick up  - things like thermal tails.

Trim-on-test will also be essential if 0.01% precision is to be achieved  -=
 just
picking a 1-lsb accurate 14-bit DAC will definitely not get you there for t=
he
overall system (or anywhere near it).

We have done analog designs to higher levels of precision than that  - but =
you
have to get creative  - and you definitely have to know what you are doing =
 -
and have the time to re-spin prototype PCB's as it's very unlikely you'll g=
et
the layout exactly right the 1st time (it took three spins for our
...... thing ....  - and that is working to an accuracy of 0.025%).

I can't really comment too much on the circuit topology without knowing the=
 full
spec but if he really needs bipolar compliance he should probably be lookin=
g at
a buffered Howland topology or derivations thereof.

Note also that 10uA to 1A range mentioned implies a 10uA programming resolu=
tion
over 5 decades (a resolution of 1 part in 100k)  - which would require a 17=
-bit
resolution DAC.  That would significantly constrain the choice of DAC devic=
e.


             Ken
--=20
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