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Input circuitry and input capacitance of SA/NA

Question asked by r2c on Jan 23, 2014
I am asking about the input circuitry of Spectrum Analyzer, but this question also applies to Network Analyzer in my case.

The thing is, I am trying to simulate some IC and take into account not only its pads but also transmission line *AND* the input circuitry with its parasitic capacitance of the measurement equipment (in this case SA and VNA). 

I was skimming through the manuals and some other docs about SA and NA, and I saw the block diagram of the whole thing, but I couldnt find anything telling how much capacitance in pF is represented at the input of SA/VNA for the case of 50 Ohm direct coupling?

Also, I want to know what would be proper way of putting input circuitry of the measurement equipment in my simulation testbench? I have attached the picture file with couple variants, does it seem to be relatively legit? 

If not I'd like to see your opinion, thanks.