The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> node capacitance
https://designers-guide.org/forum/YaBB.pl?num=1077279932

Message started by boa on Feb 20^th, 2004, 4:25am

Title: node capacitance
Post by boa on Feb 20^th, 2004, 4:25am

I want to estimate non-dominant poles in folded-cascode OTA (circuit design, no layout parasitics yet).

How can I estimate the capacitance of a node from Spectre simulation?

Operational point simulation in Spectre gives 18 capacitances: 16 transcapacitances Cij = dQi/dVi, where i, j = D, G, S, B and cjd, cjs capacitances (source- and drain-bulk capacitances) (ASFAIK, these capacitances come from BSIM3v3 model).

I would like to deal with terminal capacitances, i.e. Cgd, Cgs, Cdb, Csb.

How can I get them (or maybe an overall node capacitance??)

Title: Re: node capacitance
Post by Andrew Beckett on Mar 4^th, 2004, 9:24am

Spectre can output a capacitance table with the captab
option on the info analysis (spectre -h info). This can be
done from the analog design environment too (if I remember
rightly, it's an option on the DC analysis options form).

As for representing the partial derivatives in a more useful
form, here's a note I wrote a few years ago on this:

Quote:

An earlier spectre implementation only gave the values of three
capacitors which were closer to the intuitive value of what the
capacitance should be, but many customers asked for the complete set of
partial derivatives, so that is what spectre currently provides.

Here is a summary of the differences in the capacitance operating point
figures between 4.4 and 4.4.1 BSIM3V3 models in spectre.
---------------------------------------------------------------

In the following section Cxx refers to the parameter calculated by
the bsim evaluator inside the code. The lower case cxx refers to the
value reported by the operating point.

In 4.4:

cgs = -Csg + pModel->OverlapCgs * pInst->Width * pInst->MFactor
cgd = -Cdg + pModel->OverlapCgd * pInst->Width * pInst->MFactor
cgb = -Cbg + pModel->OverlapCgb * pInst->Length * pInst->MFactor
cbd = Cjd
cbs = Cjs

In 4.4.1:

Cgsovl = pInst->pSDModel->OverlapCgs * pInst->MFactor;
Cgdovl = pInst->pSDModel->OverlapCgd * pInst->MFactor;
Cgbovl = pInst->pSDModel->OverlapCgb * pInst->MFactor;

cgs = Cgs - Cgsovl
csg = Csg - Cgsovl
cgd = Cgd - Cgdovl
cdg = Cdg - Cgdovl
cbd = Cbd
cbs = Cbs
cjd = Cjd
cjs = Cjs

Here's a mapping between the parameter name in 4.4, and what it
corresponds to in 4.4.1:

4.4 4.4.1
------------
cgs = -csg
cgd = -cdg
cgb = -cbd
cbd = cjd
cbs = cjs

The parameter names are those reported by the operating point in the two
versions.