Hi Robert,
The input span on the AD9642 is differential across the Vin+ and Vin- terminals of the ADC. Each input has a common mode of 0.9V which is set with the Vcm pin. The means each pin will have a signal swing of 0.9V +/-0.875Vpp such that the total differential signal swing is 1.75Vpp.
One other thing to consider...if you are AC coupled then there should be no worries on the common mode compatibility of the driver and the AD9642, but if you desire to be DC coupled you should ensure the common mode levels are compatible.
Regards,
Jonathan