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Design >> Mixed-Signal Design >> capacitance calculation in spectre
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Message started by jfyan on May 1^st, 2005, 7:24am

Title: capacitance calculation in spectre
Post by jfyan on May 1^st, 2005, 7:24am

hi all:

in spectre, if i run a .tran analysis, i can get node capacitances,but there are 3 types(such as node,nodetonode,nodetoground) and fixed value and varying value. so if i want to know the capacitance of a oscillating node during the simulating a ring oscillator, what can i do ?

thanks in advance.

Title: Re: capacitance calculation in spectre
Post by Geoffrey_Coram on Jun 6^th, 2005, 9:01am

If the node is oscillating, the capacitance is also oscillating. You can probably get the capacitance at a collection of timepoints using the reported node capacitance, or figure out the appropriate device capacitances from the connected devices and add them up.

I'm not sure how accurate the reported capacitances are, though: if you have a small resistance between the node of interest and a huge capacitance, does that capacitance show up? You may want a driving point impedance, computed by capturing the transient operating point and using that as the op pt (skipdc?) and then injecting a current into the node and measuring the imaginary voltage.