So ideally don't you want the S parameters of the ADC front end to match the balun optimially? asked by Steven N.
Yes, s-parameters are a great start but this opens up another subject as well for the match. Since you know the load (R||C combo) what you can do is simply do the math instead. After taking several s-parameters and verfying this ADC measurement technique I have found that over the years that the math gives you just as good of a starting point. See the following link.. http://electronicdesign.com/analog/high-speed-adc-input-impedance-measured-versus-mathematical-approach