What is the default setting for the control register (described on p. 12 of the data sheet)? Specifically what is the default of the range bit? I don't believe it is defined in the data sheet.
As mentioned on the datasheet, the AD7927 upon power up may be in any operating mode and to ensure that the part is placed into a required operating mode, the user must perform a dummy operation. Kindly refer to the figure 24 on p.21 of the AD7927 datasheet. 3 dummy conversion places the AD7927 into the auto shutdown mode and in this dummy operation, DIN is tied high. On the third dummy cycle, the user should write the desired control register configuration of the AD7927 to place the part in auto shutdown mode.