The R-2R network can provide digital-to-analogue conversion of arbitrary precision using resistances of only two values. Is there a corresponding circuit architecture that switches resistance values to provide a variable resistance?
I want to implement a digitally programmable resistance. The dream solution would be something that could produce all of the E24 series over 5 decades, i.e. 100, 110, 120, ... 82M, 91M, 100M, but I would settle for a proof of concept - a design (scalable) that could accept say, a 4-bit digital input and give 16 exponentially or linearly increasing resistance values. The unit needs to be a two-terminal fully isolated unit and would probably use reed relay switches to provide isolation and minimize switch resistance losses.
A trivial solution to this would use a resistor of every required value, a corresponding reed relay switch, and a whole heap of logic to turn on the appropriate switch. I'm hoping there is a circuit that uses one switch per bit and fewer resistors.