If you (or your company) can spend some money on it you could consider
buying a ultra-stable voltage/current source such as
Analogic Model 8200 (http://www.analogic.com/Level4/TM4/8200.pdf).
An alternative route : in the early days, when anything above 12 bit DAC's
were hard to find and very expensive, I built a pretty good DAC using a
stable voltage source and PWM-ing that. After good filtering of the output
signal I got 15 bit accuracy. Accuracy was mainly determined by the drift
of the opamps, but todays state-of-the-art opamps should have much less
drift than those of 20 years ago.
Or : try something with one of these 24 bit audio DAC's such as AD1955,
they are cheap (< 10 Euro) and if you ask very nice they even send samples.
At 18:57 12/05/2004 +0200, you wrote:
I wonder if someone on this list has already had a problem similar
to this, and has found a simple solution.
I have a super-filtered and stable DC voltage of about 8V.
I need to make it variable (with negligible current requirements)
from abt 0 to abt 8V, with a minimum step size of abt 10 uV.
I can use part of the processing power of an AVR microcontroller
for this task. One solution that comes immediately to the mind is
to use a 20-bit DAC, fed somehow with that stable voltage, but I am
in search of something that doesn't need hard-to-find components.
I played with the idea of a charge pump, where you control with
a couple of output bits of a port of the micro the charge and discharge
of a capacitor for variable lenghts of time, so to achieve the wanted
result, but I am unsure of the implementation details. The response time
requirements are very relaxed, there will be an adjustment of that
voltage let's say once every two minutes or more.
Has anybody suggestions or links ? Many thanks
73 Alberto I2PHD