Voltage swing level of AC-coupled CML
CML is used both DC signals and AC protocols that have no DC.

So consider the caps for  AC, Xc(f)= 0 Ohms, thus the current swing is determined by Rt.


  which means there's 4mA flowing through RT1 and RT2. 


Since the differential switch supplies 16mA shared by the internal equal 50 Ohm loads, the current peak swing is 8 mA not 4.