18F4585 adc-in Pull-up ????

[sougata]

Hi,

Please note that the ADC results are dependent on the supply accuracy. Since your Vref is derived from VDD (I hope). If you require accurate results try using a temperature compensated Vref. May be a TL434.
Also since you are using a low pass assure you have enough sampling time.

[Bruce]

Use smaller resistor values on your input filter. Recommended max is 2.5K.