How do I determine the 50ohm buffer current?
The current can be huge different for ac couple case and without ac couple case.
For the case of 2VDD and 500mV ac swing:

AC couple case: 500mV/50ohm=10mA      (AC signal only)
without AC couple case: 2V/50ohm=40mA!! (DC + AC signal)

Just wanna know which case should be normally used for high-speed digital signal in gigahertz range? I think the without AC couple case, the current is too large and ac couple case should be used. Pls commend.