AD7792 readback is wrong

i use ad7792 with pt100。

the voltage of pt100 is 0.107V,

7792 setting is  REF=3.3v   1/16, ( 0.20625v ) IOUT=1mA,

But it“s readback data is always  half of 65536,

It's wrong.I wonder know how.

Best regards.

Aaron