AD9517-4 Single-Ended Mode

I have started using AD9517-4. Could anyone give me any advice as below?

On page4 in datasheet Rev.E, there is information about REFIN. I use only REF1 and I put 0.1uF at REF2.

So I use this device as single ended mode. I put 10MHz via 0.1uF DC cut into REF1. and VCP=VS=3.3V.

1. In this case,  the mode is AC coupled, but the data sheet says the input frequency is 20M~250MHz.

Why does 10MHz REFIN work? 

2. I have changed the input level from 0.6Vp-p to 3Vp-p. The device works properly in this all range.

It seems the REF1 port outputs DC. Though I put AC 10MHz the level is centered at +1.6V.

So it seems to meet all specification for DC and AC coupled for input sensitivity(except 10MHz frequency), which means that

10MHz swings 0.1V to 3V for DC coupled and 0.8Vp-p typ. for AC coupled.

Again let me ask why 10MHz with 0.6Vp-p work properly?  If it is AC mode, it should work from 20MHz. I am cofused.

Best Regards