How to check the load capacitance of a node in cadence virtuoso?

Dear zuiying,

zuiying said:

How to check the load capacitance of a node in cadence virtuoso?For example，I want to get the total load capacitance of the "sample_convert" node in the image below.

You indicated in your post that you are trying to "get the total load capacitance of the sample_convert node". However, I don't see an explicit load on the output node "sample_convert". Is there any circuitry attached to node "sample_convert" that is not shown in your posted screenshot?

What specific "load capacitance" are you trying to assess - in other words - what specific circuit elements form the "load capacitance"? Or, perhaps, are you trying to determine the effective capacitance at a given frequency of node "Sample_convert"?

There are multiple methods to determine the capacitance of a node or the load capacitance at a given frequency using spectre, but I am not sure which method will provide the answer you are hoping to find.

Spectre can provide a tabular listing of node capacitances following a transient or DC analysis using its captab capability. The instructions for using this can be found in the spectre reference manual. Figures 1 and 2 show the location in the DC and Transient anaylses selection GUI where this option can be enabled respectively.

It can also find the various capacitances using a post-layout based netlist. There is an instructional video describing this at URL:

https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1O3w000009y1JhEAI

However, this information will not capture the effective capacitance as a function of frequency. Other methods, both large and small-signal based can be used to determine the effective capacitance of a node at one or more frequencies by determining the real and imaginary impedances of a node. For example, for a small-signal analysis of the nodal impedance, the following Troubleshooting article at URL:

https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1Od0000000nVZZEA2&pageName=ArticleContent&oMenu=People%20who%20viewed%20this%20also%20viewed

has some information on how to compute the impedances versus frequency and, ultimately, capacitance versus frequency:

support.cadence.com/.../ArticleAttachmentPortal

Sorry I cannot be more specific in my response suiying, but I am not exactly sure what you are hoping to compute.

Shawn

Figure 1

Figure 1

Figure 2

Hello! There are many circuits attached to node "sample_convert". I didn't show them because they are too many. I am sorry it bothered you. In fact, I want to get the load capacitance of "sample_convert" so that I can design inverter chains to optimize the time delay of "sample_convert". I tryed the captab option and got this.

What do "fixed" and "variable" mean? Fixed capacitance and variable capacitance?

By the way, there are three options in detail, "node", "nodetoground" and "nodetonode". For optimizing time delay, which option should I choose to calculate capacitance?

I am a cadence novice, not very good at using it. Thank you for your help!

Dear zuiying,

zuiying said:

There are many circuits attached to node "sample_convert". I didn't show them because they are too many. I am sorry it bothered you. In fact, I want to get the load capacitance of "sample_convert"

Thank you for this clarification - it helps me better understand your need!

zuiying said:

I tryed the captab option and got this.

What do "fixed" and "variable" mean? Fixed capacitance and variable capacitance?

Exactly. Fixed capacitors are voltage invariant and variable capacitors will change as the voltage across them changes. Hence, if you are interested in the load capacitance from a large-signal perspective, using the small-signal value of a variable capacitor in a large-signal simulation may not provide a good estimate of the load since the actual variable capacitance value is changing as the voltage changes over time. A large signal impedance analysis can provide a more accurate assessment of the capacitance. However, if you just want to determine a "worse-case" capacitance, perhaps using the captab option in a transient simulation and examining the capacitance as the voltage varies with time will allow you to choose a "worse-case" value.

zuiying said:

By the way, there are three options in detail, "node", "nodetoground" and "nodetonode". For optimizing time delay, which option should I choose to calculate capacitance?

The capacitances can be listed as capacitances from a nodes to all nodes or from a node to ground (only). The Troubleshooting article at URL:

support.cadence.com/.../ArticleAttachmentPortal

details this and provides an example. As to which you should use, neither is totally accurate as the node to ground option only provides the capacitance from a node to ground and nodetonode provides the sum of all capacitances at a node (not all of which may connect to ground). This is why I mentioned the use of a large or small-signal impedance analysis to determine the capacitance. However, as mentioned, if you want a "worse-case" value, clearly the nodetonode option might be best.

Shawn

Hello！Can I ask you one more question？When using captab function, how can I set to print only the nodes I specify? By default all nodes are printed. Thank you！

Hello！Can I ask you one more question？When using captab function, how can I set to print only the nodes I specify? By default all nodes are printed. Thank you！

zuiying said:

When using captab function, how can I set to print only the nodes I specify? By default all nodes are printed.

If setting this through the dc or tran option form, you can't specify the list of nodes. The spectre info analysis supports this via the node parameter; see "spectre -h info" for details. The form doesn't provide this option.

However, there is another way of doing it, which is to set up the captab via the Outputs->Save All form instead:

I used the Click_To_Add, picked captab in the What column, called it capInfo and then entered node=[inp inn outdiff] in the Others column to just output the captab for those three nodes. The Detail, Sort and Threshold at the bottom are the same as the choices you get on the dc options form when selecting captab.

You can also use this in a transient analysis by using the infotimes and infonames fields on the Outputs tab of the tran analysis; you'd set the first to the times that you want it to run at during the transient, and the second to whatever you called the captab analysis in the form above (capInfo in my case). There seems to be a little buglet that in the spectre log it reports the capacitances for all nodes at the second time (the first is OK), but the result database correctly only saves the desired capacitances and so the Results->Print->Capacitance Table works correctly with the subset of nodes you asked for.

Regards,

Andrew