2. If you feed the ADC from a source resistance of more than 10K, then you need to take precautions to prevent the increased source resistance affecting the reading. There are two simple ways of doing this. One is to connect a capacitor of 0.01uF or greater between the analog pin and ground. This has the side effect of slowing down the rate of change of the signal, but this is acceptable in some applications. The other is to introduce a delay in the code for analogRead() between selecting the input channel and starting the conversion. The delay needed is about 1us per additional 10K source resistance.