Ioannis, Sam,
Valid concern that MAY need attention!

If the voltage drop over the output resistor does present a problem it can easily be adjusted for by changing the gain of the amplifier/attenuator with P1.

BUT.....

The datasheet for the 16F877 (which is the PIC I used and I suspect others are similar), table 15-12 says that the recommended impedance of the voltage *source* is max 10k. So the 1k shouldn't make that large impact on the result. I couldn't find any info on the actual input impedance of the ADC. Where did you find that?

Further on, I belive that if the input impedance was 10k the ratio would be ~9/10 with a 1k source impedance.

Thanks!
/Henrik Olsson.