Srikanth,
ADC's are used for sampling of the anlog signal....for proper digitilizating , one need to have faily smoth analog signal.....but many cases analog signal get affected because of noise mostly high frequecy bcz of interference, gound unbalances, unstable impediances matching etc... such high frequency signls leads to variation in the digitalized output....so one need to filer and get smothing of analog inputs.....there is another method is followed many times is averaging of digitialized output of adc ...to avoid even effect of random noise....
I hope it answer your question....
Good Luck