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I think I understand why, if you're using those resistor values you mentioned in an R-2R sort of circuit, using the internal 20 kohm (or so) pullup resistor for the pin you're going to use.
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The pullup is mainly to provide source voltage to the R-2R. The example i was working from had an external 5v source (which i intended to take from the LCD), plus a ground, plus the input wire. The resistor values were mainly just the most convenient ones from my spare parts box. I originally tried to use some SMT 4-resistor arrays, but the soldering was just too hard to stick.
Just a few minutes ago, I noticed my ADC scheduler was wrong, as I was using a stack instead of a queue. That should be fixed now.