Dear sir,

the AD936X ADC maximum input is 0.625Vpk, when calculated ADC input power, will use the below equal:

P=Vrms^2/50, which Vrms=0.625Vpk/sqrt(2);

the ADC input resistor really is 50 ohm? actually, the ADC input resistor usually equal to a few Kohm resistor, such as 1Kohm;

why not using the really ADC input resistor to calculate ADC input power?

Internally ADC is terminated with 50 ohm characteristics impedance which forces the effective input impedance of ADC close to 50 ohms as long as input impedance into the ADC itself is much larger than 50 ohms.