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Subject: Re: input capacitance in cmos gates

Date: 03/31/00 at 5:58 AM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: input capacitance in cmos gates posted by mark on 03/31/00 at 3:51 AM:

: Does anybody know a way of calculate the input capacitance of any cmos gate? Can it be calculated using spice? Can we aproximate it as the sum of all the gate to bulk capacitances of the transistors connected to the input(in pararell)

: Thank you

Hi, Mark.

If we say that such-and-so-many CMOS gate inputs can be driven by one output port of another gate of the same family, then a first and admittedly simplistic estimate of the input capacitance would be the device's capacitance driving capability at the output divided by the fan-out.

For instance, if a CMOS output is specified into a 50 pF load and you know you can successfully drive ten inputs from a single output, the maximum CMOS input capacitance could be estimated as 5 pF.

Good luck.

John Dunn - President
Ambertec, Inc.

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