AD8145 doesn't work exactly.

Dear Sir/Madam,

I need your help.

My schematic:

VREF is provided directly by AD5765. I set the common mode voltage of Vin to 0.5 V and the differential mode voltage to 1.0 V.

AD5765's output voltage is set to 0 V.  Than I measure the voltage of AD8145's REF pin. However, it's not 0 V(Of course, AD5765 doesn't work exactly now). The voltage on REF pin has following features：

Its frequency is the same as Vin;

The following pictures show how its Vp-p  varies with frequency:

My question is that why REF pin of AD8145 is not 0 V ?