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Simulators >> Circuit Simulators >> How to avoid Parasatic resistance deletion
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Message started by Faisal on Apr 10^th, 2007, 8:49am

Title: How to avoid Parasatic resistance deletion
Post by Faisal on Apr 10^th, 2007, 8:49am

Parasatic resistor "rs" has been deleted becuase its value of 2.4375 mOhm (R/MFactor) was smaller than minr.

I want this parasatic ressistance to be included in my simulation. How can I set MFactor or R or minR with Spectre Simulator.

Title: Re: How to avoid Parasatic resistance deletion
Post by Geoffrey_Coram on Apr 10^th, 2007, 12:09pm

spectre -help options

see the part about "minr" (it's the last parameter in my version of spectre, but then there are some notes).

Title: Re: How to avoid Parasatic resistance deletion
Post by Andrew Beckett on Apr 11^th, 2007, 7:07am

Do be careful about setting minr very low, because it can lead to convergence difficulties. See Ken's book http://www.designers-guide.org/Books/#Kundert-1995 page 24.

Regards,

Andrew.

Title: Re: How to avoid Parasatic resistance deletion
Post by Faisal on Apr 12^th, 2007, 2:46am

Thank for the responses. I have a Cadence version 5.10.41.500.3.58. I am using the Virtuso Analog Design Environment, the simulator is set to Spectre.

In Simulation->Options->Analog, I can only find the gmin option but not the minr, in the resistance section.

Do I need to look some where else ?

Title: Re: How to avoid Parasatic resistance deletion
Post by Geoffrey_Coram on Apr 12^th, 2007, 4:28am

I don't use ADE; I hope Andrew can help you with that.

You can also set minr on the model card for the device(s) you're concerned about -- eg, if you want to set minr for some very wide output devices but not necessarily for minimum-width devices.

Title: Re: How to avoid Parasatic resistance deletion
Post by Faisal on Apr 12^th, 2007, 4:48am

I am getting this warning for a very large size device i.e. pass transistor in the LDO. In addition I also get this warning for a few normal size transistors.

A related question is how important is to model these parasatic resistances, when they are being deleted ?

Title: Re: How to avoid Parasatic resistance deletion
Post by krishnap on Apr 12^th, 2007, 5:55am

Is the parasitic resistance of the device model or being obtained from the
Parasitic extraction from the layout?

Title: Re: How to avoid Parasatic resistance deletion
Post by Faisal on Apr 13^th, 2007, 1:10am

This is coming from the device model and not from the layout.

Title: Re: How to avoid Parasatic resistance deletion
Post by Andrew Beckett on Apr 13^th, 2007, 6:31am

I believe minr only shows up in the "Resistance Options" section of the Setup->Options->Analog form if you're using an MMSIM version of spectre. However, it does seem to be supported in some IC5141 versions of spectre. You could always go to the bottom of the Analog options form and type in "minr=nnn" in the Additional Options field.

HOWEVER, the chances are that this won't work. Since local (model) minr is by default 0.1, setting it globally to something lower won't help - since it will already have been removed by virtue of the local minr.

Is it really going to make that much of a difference?

Regards,

Andrew.

Title: Re: How to avoid Parasatic resistance deletion
Post by Geoffrey_Coram on Apr 13^th, 2007, 6:42am

Andrew Beckett wrote on Apr 13^th, 2007, 6:31am:

Is it really going to make that much of a difference?

I've heard that resistance is quite important to model correctly for those LDO output devices ...