On the other hand if all you care about is DC precision you can use a very simple, though not necessarily easy, circuit. You need a stable voltage derived from

On the other hand if all you care about is DC precision you can use a very simple, though not necessarily easy, circuit. You need a stable voltage derived from a stable voltage reference, and a stable clock to PWM that stable voltage. Then you "just" low-pass filter the heck out of the PWM to get an adjustable DC value, and calibrate out any offsets. I've seen this used in adjustable voltage standards (forget which) and the venerable Fluke 5700A Multifunction Calibrator[0]. [0]: https://www.youtube.com/watch?v=qhr6h7wuqo0 https://www.youtube.com/watch?v=qhr6h7wuqo0