The HMC984 VCOp/n inputs have a common mode limit of VCCPD-1.4.
 does this limit specifies that no more than 1.4Volt can be applied across the internal 50 Ohm pull-up resistors ? otherwise the internal 50ohm could be burn out due to a constant dc power 1.4V^2/50ohm=40mW power through such resistor ?
 does the limit specifies that no more than 3.6V (=5V-1.4V) common mode should be applied for such resistors ?
in the case where 2.5V (which is smaller than 3.6V) common mode is applied for such 50 ohm, there would be a constant DC power 2.5V^2/50ohm=0.125Watt=50mA^2*50ohm through such 50 ohm pull-up resistor; a signal current 15mApp or 750mVpp (=15mApp*50Ohm) single ended should be typically applied on such common mode.
Which definition is specified in the absolute maximum table of HMC984 ?
 is correct. We don't specify this limit as 5V-1.4V since VCCPD can vary from 4.5V to 5.5V.
thanks for the feedback. Is there a maximum power rating for the internal 50 ohm resistor so that the cross-resistor-terminal dc voltage has to be constrained too ?
See page 2 of the datasheet. The maximum single ended signal on VCOp or VCOn is 875 mVpp (2.8 dBm).
is the spec of 875mVpp (single ended) that you suggested the AC signal which is injected onto the common mode ? I was referring in my previous inquiry for the DC power due to the common mode where we can tentatively assume the added ac signal power is comparatively smaller than the DC power defined by the (VCCPD-common_level)^2/50 watt.