AVR-Chat

You can get away with much higher impedance than 10K if your application can wait for a settling time after setting the A/D input MUX before you do the conversion. You have to charge up the capacitance of the A/D input, so if your impedance is very high, you need to wait for that to happen. I've done it with a 500K divider. If you aren't changing the MUX (ie you only have one a/d input) then you don't need to wait.

Also be aware that the internal reference is not very accurate - data sheet says it can be from 1.0V to 1.2V, and my experience is that you do get that wide variation across different devices. We ended up having to use an external reference.

-- 
Tim Mitchell