So I built a poor-man's 4-bit DAC by using some resistors to form an R2R ladder. Worked well - I could create something that looked like a stair-step sine wave which, after being buffered with an op-amp and fed through a low-pass filter, gave me a decent sine wave.
I kind of arbitrarily chose 12k and 24k resistors (1%) for the ladder, and it worked well. Whole thing was built up on a solderless breadboard, but since the speeds were low (under 3kHz) the breadboard capacitances and resistances, and the lead inductances, didn't really matter.
I then got to thinking - the actual resistor values don't matter, and higher resistor values would mean lower current draw, right? So I swapped out for some 1.8M and 910k resistors. The resultant waveform went all over the place. Jagged, not resembling any reasonable sinewave, etc.
I'm wondering why. I can see two possibilities: 1) with resistances that high, the poor connections of the solderless breadboard are enough to throw the tolerances all over the place or 2) the very low currents through these resistors (microamps) aren't enough to drive the op amp in a stable fashion (or even the scope probe when I probe upstream of the amp).
I'm tending towards #1, in which case a properly built up PCB should resolve the issue, but before I start soldering I was hoping for a sanity check. Thoughts?