use an Oscillator instad of a crystal for the REFCLK?
if the XTAL mode is not used, you should ac couple both REF CLK inputs. The
capacitor values of 0.1uf or 0.01uf should suffice. In a single-ended
configuration, tie the unused REF CLK ac capacitor to ground or AVDD for
bypassing. A voltage swing of 200mVp-p to 1Vp-p should suffice.