Measuring Input or Output Capacitance of a buffer

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ashourism

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Hi

i am trying to measure the input and output capacitance of a buffer stage (for example) as i need to measure several other stages

i set up the following circuit



and i set the input voltage to 1.2 with AC Magnitude and Amplitude to 1.2 , also vdd is set to 1.2

then i calculated Zin = I/1.2 * f

however i get extremely variable outputs as in files and also no imaginary parts get in the output csv file from Cadence how is possible?

also how am i supposed to measure the output capacitance

Thanks

- - - Updated - - -

So i made a mistake by dividing by 1.2 i divided by 1 and now i have a linear line with frequency but this is the total input Zin how i get the Input Capacitance (Imaginary)?
 

so i used Cadence calculator to get the imaginary part of (I/V) then averaged it to get the slope this will be the capacitance?

What about the output i tried to the same technique but i got a non-linear curve
 

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