Common Mode Differential Analog Input
It's a bit more complicated than that. Have you seen the example driver filter circuit?

It's designed to be driven by a differential signal, centered on 0.5V. The datasheet talks about a swing of 0V to 1V. That should be your swing. So a 40% amplitude wave should have a peak at (0.5*0.4) + 0.5 on the positive rail and -(0.5*0.4) + 0.5 on the negative rail.

Additionally you need to provide a suitably terminated driver. The "negative input impedance" is a bit weird, but what it means is that the inputs will run away and self-destruct unless their bias current goes somewhere. The datasheet says "it is reccomended to have termination to ground or 0.5V through a resistor less than 1k", and indeed their example driver filter terminates to ground through a 100 ohm resistor.

(Why is the filter there? It's there to round off the edges of your signal coming from the DAC in order that it modulates cleanly).