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?
Did you follow the AD7685 layout guidelines for your circuit (placing reference decoupling capacitor close to the REF and GND pins, etc)? Please also note that as per the REF02 datasheet, "While the REF01/REF02/REF03 series of references are designed to function stably without any external components, connecting a 0.1 μF ceramic capacitor to the output is highly recommended to improve stability and filter out low level voltage noise."
What are you using for your input signal? What are the tolerances of your resistors? It may also introduce error to your reading.