AD7949 SDO error


I have one problem with AD7949.

Please refer the attached.

Regarding SDO_NG sheet, after Power up, I have control in accordance with a sequence that is specified on the datasheet, but SDO does not go out. It works the same even after the 4th cycle. I believe that he period of tCYC is longer than the specification, and tCONV is more than 2.2uS. 

On the other hand, regarding SDO_OK sheet, I changed only tCONV from 3.3msec to 1sec. So It works correctly.

Can I ask you to give me the hint to solve this problem?

Are there any constraints such as settling time?

If you have any question, please let me know.

Best Regards,