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[SOLVED] Converting Fan Out of 4 Inverter to Capacitive Load

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r_usso

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Hello Everyone,

I am designing a circuit in which I am supposed to add 4 cmos inverters as load. I can either implement the inverters as the output or can calculate their capacitive load and just place a capacitor of the same value in the output.

The problem is I do not know how to calculate the capacitive load of the inverters. Any suggestions where can I find how to calculate it?

Thanks in advance.
 

If these are inverters from a library, you should find their input capacitance in their description.

Otherwise use the dimensions of the poly gates together with the gate oxide thickness and its ε , + the dimensions of the gate connections together with the field oxide thickness and (the same) ε to calculate Cin . This results in a (rather rough and lower limit) input capacitance estimation.

Best chance would be to run a DC analysis at both 0/1 output operating points with the operating conditions selected by your requirements, incl. realistic load. This would give you the best results for its input capacitance - as good as its model is characterized.
 
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