why cannot measure voltage on VREF of AD5666 when I set up "Standalone mode,reference on"?

Hello,

Recently I have a problem about AD5666. That is my AD5666 is controlled by FPGA via SPI. I send command according to 

.My command is .I can measure all signal about sync,clr,sclk,din ,thai is same to .But I cannot measure 2.5V on VREF pin.It is always 0 V. Why ? where is the problem?how to make AD5666 work normally? thanks