Electronic – How does a digital potentiometer work

digital potentiometerintegrated-circuit

What the title says. I'm curious how exactly a digital potentiometer works. Is it something like a pair of pass transistors like below?


simulate this circuit – Schematic created using CircuitLab

Or is it more like a series of switched resistors like below?


simulate this circuit

Or is it something else entirely, that I can't think of?

I looked around, and found exactly one source suggesting that it is the latter case, but I couldn't find anything else talking about the internals of a digital pot at all. Maybe there's more out there, though, and I'm just really bad at finding things.

Best Answer

Second option is a good representation. You can fabricate well matched resistors in CMOS processes, and also well matched analog switches. As ever, the absolute value tolerance for passives in CMOS is high, but the matching is excellent. Have a look at the datasheet for the PGA2311, which is a audio potentiometer.

The first option would be a bit grim, as you've alluded to. The MOSFETs are non-linear, and you have issues with the bulk connection with changing \$V_\mathrm{DS}\$.