I wonder if someone on this list has already had a problem similar
to this, and has found a simple solution.

Problem:
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