doubts using ADC in AVR

I have tried and tested al features of AVR and are working satisfactorily except the ADC part.
Actually for my applications, the ADC resolution required is atleast 12 bits. So i used the Oversampling method suggested by atmel application notes to enhance resolution upto 16 bits. It works well and readings are quite stable. But the problem occurs when i reduce the analog input to less than 10% of the full scale. here the linearity drastically differs. I have also removed all digital outputs from portC to eliminate cross interference of digital noise but the problem persists. suggest any solution if possible.

did you see the ADC response graphs on application notes?.. what they say about oversampling?

try it in normal mode with 12bit resolution and see the response. if its still the same then must be some problem with interfering signal or something.

can i see your circuit?

actually the circuit is the same as in the avr application note on atmels site. the only modification i have done is to connect a multiplexed display using port B and Port D.
the application note no is AVR121 on atmel application notes.

the normal mode is 10 bit resolution and works fine.##SANITISED##addtext(':mad',true)

do this.. can you remove or stop using multiplexed display..
try sending the ADC o/p of 16-bit to UART (high 8-bit and low 8-bit).. and see if there is any change in that..

Also make this change in hardware.. connect a 100nF cap close to VCC pin of controller. can i see your circuit?