Thursday, December 04, 2008     Register | Login | Search | Contact Us
     
   
     
     

Many of you already received communications about the move of the Cadence user community into cadence.com. And many of you have already joined, with over 4000 registrations in the first two weeks.

The new Cadence Community enhances the ability of Cadence users to connect and collaborate. In addition to moving the community into cadence.com -- enabling single sign-on for community, Sourcelink and Cadence events -- the new site is organized around nine technology segments, giving you easy access to product information, training, forums and blogs. Some of the new features include:
  • Ability to respond to posts via e-mail
  • Technology-specific blogs
  • Latest Web 2.0 social networking capabilities
  • Public profile options
  • Private messaging
  • Friends lists
Visit the new Cadence Community today at www.cadence.com/community and join the discussions!

Registration note: Due to the scope of the enhancements and the new SSO registration system, we were not able to migrate existing cdnusers.org member accounts. So new registrations are required, but this enables a broader set of functionality we think you'll enjoy.

Forum note: Under the guidance of forum moderators, we have taken the 20+ cdnusers.org forums and consolidated them into 11 forums on the new site. Posts have been brought over so you can leverage that posting history. CDNusers forums will be set to read only starting 7/30, and cdnusers.org will be redirected to the new community on 8/4.

Best regards,
Mike and Tom

Michael A. Catrambone - Steering Committee Chairman
Distinguished Engineer
PCB/Mechanical
UTStarcom, Inc.

Tom Diederich
Cadence Community Manager
Home
Forums
Forums > Custom IC > Custom IC Electrical Design

Subject: Tip of the Week: Proper use (and misuse) of cmin for transient sims
Posting to forums is available to community members only.
Login or Register
Rate this topic:
   
Author Messages
Hugh
Posts: 25
Online: User is Offline
7/15/2008 11:26 AM  
As many of you know, the "cmin" parameter (under transient options) can be a really useful tool for getting difficult transient simulations to converge and/or run faster.  Essentially, "cmin" attaches a virtual capacitor from each node in the circuit to ground, with whatever value cap you specify.  (Typically anywhere from a tenth of a femptofarad to several femptofarads, depending on the circuit.)  This smoothes out sharp transitions and allows the simulator to converge more easily.  The improvements in speed can be dramatic.
 
However, I have noted several ways that you can abuse cmin and have it cause serious simulation problems or erroneous results.  Following is a list of abuses, which I'm sure is by no means exhaustive:
  • cmin should never be used with extracted parasitic views.  The additional nodes in such views magnify to the nth degree the effects of cmin, because now instead of a few dozen virtual caps you have thousands of them.  Serious circuit mischief can result.  If you are using cmin for your schematic sims, be sure to remember to turn it off when you go to parasitic views.
  • cmin should not be used with digital schematic views.  These views have a large number of often-tiny transistors, and very small values of cmin can seriously affect the performance of digital circuits.  
  • cmin should be used with extreme caution in any circuits that have a free-running frequency (oscillators and vco's), because the cmin combined with the high impedance nodes in such circuits can tune your oscillating frequency in unexpected ways.
  • Similarly, in RF circuits in general, including filters, cmin should be used sparingly.
- Hugh
Posting to forums is available to community members only.
Login or Register
Forums > Custom IC > Custom IC Electrical Design > Tip of the Week: Proper use (and misuse) of cmin for transient sims


ActiveForums 3.6
     
Copyright 2006 Cadence Design Systems, Inc.