MAX999 draws current through negative input
IIN+ = 100μA 2.1V means you may limit  current  the source voltage with a series R   2.1V/100 μA =21k is the threshold of conductance of the clamp.  

Current into Input Pins .....................................................±20mA

Then consider driver Vol/Iol limits and (Vol(=0 @ 50 Ohms)) Thus divider divider is 4V (=Veq) / 1.8k (=Req) = 2.22 mA  So you are OK. But with cap you need to add 330R ~1k in series = Rs on input.

Your driver Spec = TBD