Could You tell me please what is the input voltage range? I can't find this
value in the datasheet!
With dual supply operation +-5V there is a valid input voltage range for AD8638
-4.9V to +3V. It is a bit difficult to identify it in the datasheet:
For single supply 5V it is −0.1V to + 3V (datasheet page 3)
For single supply 16V it is −0.1V to +14V (datasheet page 4)
Out of this results a common input voltage range of -Vs-0.1V to +Vs-2V. In your
case with +-5V: -4.9V to +3V.