# How do I measure the input capacitance of an Inverter?

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#### samiran_dam

##### Full Member level 2 Dear all,

I want measure the input capacitance of an inverter for the vinput range of 0 to 3.3 V

I am following the approach mentoined below:

1. at the input of the inverter I connect a dc voltage in series with a ac source.
2. then I am doing a parametric analysis (sweeping the dc value from 0 to 3.3 V) under which I am doing an ac analysis
3. Then I plot the imaganiray part of the V(F)/I(F) and multiply it with a certain frequency value to get the inverse of the capacitance value

the expression I am using in cadence spectre is: value(imag((VF("/net028") / IF("/VG/PLUS"))) 1)

is the method correct?

#### saurabhr8here

##### Member level 3 I remember doing something similar once, but there is a simpler way to get the input capacitance of an inverter if you'd like to do back-of-the-hand gate delay calculations and stuff. Create a chain of inverters as shown in the figure attached.

Now, you can use the optimization routine in HSPICE or vary Cdelay till the delay from nets c to g (charging/discharging the capacitor) is equal to the delay between the nets c and d. When both delays are equal, the capacitance Cdelay is equal to the input capacitance of the inverter X4. Hope this helps, let me know if you have any questions.

Thanks!

I would like to mention that this method is from the excellent textbook "CMOS VLSI Design: A Circuits and Systems Perspective, 4th Edition by Neil Weste and David Harris". There are quite a few other HSPICE routines provided that prove to be helpful in solving digital circuit simulation problems.

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