the Altera FFT IP function well?

The following is IP Config

https://www.alteraforum.com/forum/attachment.php?attachmentid=7157 https://www.alteraforum.com/forum/attachment.php?attachmentid=7158 https://www.alteraforum.com/forum/attachment.php?attachmentid=7159

When I force the DC to the sink_real,and sink_imag to 0 ;The FFT IP work as following;who can tell me why and how to solve.

https://www.alteraforum.com/forum/attachment.php?attachmentid=7156