Ok, I checked into it and there is a 51 ohm termination resistor connected to the external clock test point. If you are able to remove resistor R11 from the back of the EVM you should be fine.
Your clock source seems to have a 150ohm output resistance, so the load resistance needs to be high. The ADS1293 is getting only a fraction of amplitude of your external clock because of the 51 ohm resistor.
Please let me know if removing R11 solves your problem.
Thanks,
Sergio