AD7685 - 5V Ref and 5V Input, not full scale output?

I am currently using the AD7685 in my design with the following circuit layout (apologies for sloppy presentation!).

The problem i am having is that when a voltage of 5V is applied to the input i am getting an output of 61497, rather than the expected 65535 (or thereabouts). So a full scale input is not producing a full scale output. After this i noticed that all other inputs between 0-5V contained an "offset" error. The smaller the input voltage, the smaller the error.

Has anyone else had this problem or possibly know what my problem is or what stupid mistake i have made?


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