Hi,
The input (basically) only sinks current when it's configured as an output and set low. When being used as an input for the ADC it obviously needs to be set to input - just as Mark has pointed out. Then it "sinks" current only when the input is connected to the S/H circuit which leads us to the next issue: Your 100k pot viloates the maximum recommended input impedance by an order of magnitude.

/Henrik.