AD8318 Maximum Input Voltage
Becasue there isn't a max voltage, only a max power. You can put in whatever voltage you want, as long as the power referenced to 50Ω is kept below 12dBm. The requirement forces you to look at the frequency content of your signal and ensure that the total power of that signal does not exceed the rating. So measure or calculate the total power of whatever is upstream and keep it below that requirement.  

At DC you should limit the voltage to this requirement.