Questions about AD7682

   I am using AD7682 to make a data aquisition system.
   I connected the COM(pin10) to GND and used the internal reference 2.5V.
   But when the amplitude of the input sinewave exceed about 1.2V, half of the reference voltage, it seemed that the waveform ploted according to the converted result not exactly the right.
   Why it performs like this?
   By the way, the input signal composed of about 600mV DC offset and a sinewave about 600-700mV peak to peak?
   When I used the internal 4.096V reference, should the input voltage below about 2V? Or it could be 4.096V?
   Thank you!