connecting Xilinx XUP expansion headers

Hello,
I'm having a problem connecting a Xilinx XUP board with an ADC board.
The scheme is very simple, from XUP board I send a clock (20MHz) to the 
ADC board that sends back 16bit parallel samples and a clock (20MHz). 
These signals (1.8V) enter to the low speed connector of the Xup board. 
The cable I chosed for the connection between boards is an ATA-133 cable.
Actually, that cable is causing the problem because signas at the output 
of ADC pins are good but, if I look at them at the end of the cable, 
they are corrupted (clock is very bad). I think that cable has too high 
capacitance to be driven from the ADC board buffers.
Could you suggest me another kind of cable that I can use with the XUP 
low speed connector? Using single cables would be better (I think no 
but...)? Could I do something to make things better using the ATA-133 cable?
Thank you in advance.
Bye
Giovanni