ADC12D1800RFRB external clock frequency < 300MHz

Hi all
I'm using the ADC12D1800RFRB reference board with the xc4vlx25 FPGA on it. I'm trying to use an external clock that, for certain tests, needs to be fixed at 200MHz.

The problem is that the board doesn't recognize the clock until it reaches almost 400MHz.
How can i solve this problem? On the adc datasheets it's written that it can works with a clock not lower than 150MHz setting the LFS bit in the config register.

I did it but it still doesn't work.

I hope someone can help me.

Thanks in advance.