Running those pots at 20mA when their absolute max rating is 11mA is a bad idea. Are you sure that you can't get a low enough resistance using 10mA?
Actually, in general I need the full current range of the Vactrols, 0 to 40mA... I hadn't realized that the digital pots were so limited. I just copied the circuit from the arduino examples page.
If I go the DAC route, then I guess the current to the LED will again be pulsed by the DAC? Or would the OP-AMP flatten then into a constant DC voltage?
I was thinking, could I not use the op-amp with the digital potentiometer voltage divider to keep the current output by the pot wiper very low, while controlling the output current through the LED as the voltage output of the pot divided by R?
Most op amps won't provide 40mA output current, especially quad ones. For 40mA output I would boost the op amp output using a transistor, as in the attached schematic.
Thank you for the circuit! I've looked at it but of course have more questions, sigh. I tried to understand the circuit but I'm really only guessing... Will the DAC + op-amp+transistor allow for the full current range through the LED, i.e. 0 to 40mA? yes, in steps of 0.156mAWhat type and value of capacitor would be needed at the DAC ref2 connection? 0.1uF ceramic will be fineThe DAC produces a variable voltage from 0 to 1 Volt? yes, if you apply 1V to the REF pins and don't enable the x2 amplifier when you send commands to the DAC, then it will produce 0 to 1V outputThen the op-amp maintains that 0 to 1V on it's output, then the transistor amplifies the current? the op amp maintains that 0 to 1V across the 25 ohm resistor, and nearly all the current in that resistor passes through the LEDThe LED sees 5V input always? the transistor acts as a constant current source, so the LED drops whatever voltage it needs when passing that current. With that circuit and a 5V supply, up to about 3.5V max is available for the LED. I am assuming this is enough for your Vactorols.But how is the current at the LED controlled? see aboveThank you again,Bob