I have an evaluation board EVAL-AD9649-80, with a driver AD8138. They are DC connected. Now I found a very strange phenomenon that the digital output data value is about 60% of the analog input voltage. It looks like there is a scaling during A to D conversion. I changed the reference voltage which is 1V from internal to external, but the results are the same. I'm wondering which are the possible reasons that cause this phenomenon? Thank you!