AD9957's  External REF_CLK

Hi, everyone!

Here I have a problem with the Evaluation Board of AD9957. I want to drive the AD9957 using an external signal source instead of the 25MHz Crystal on the board. The problem is that the Evaluation Board of AD9957 requires the REF_CLK input level be 50-1000 mV(singled-ended). But most of the oscillator's Vcc does not meet this requirement. Do you have any solution?  Thanks very much!

Best Regards!