how to remove the dc from data of AD6688

hello,when I use AD6688 to catch noise or signal.I found there is  dc frequence when I pwelch the spectrum .It makes the stray when mixed the dc and local oscillator from AD6688.

I found the phenomenon always existed. However,when I configure the AD6688 resigster through the SPI. I didn't find the resigster about how to remove the dc.

Please help me whether there are resigsters about it .Thank you!