Subject: Re: [SI-LIST] : meaning and value of C_comp

From: [email-masked]
Date: Tue, 21 Mar 2000 16:13:11 -0600

Here's another thing to consider:  what if you have an on-chip series
resistor?  Physically speaking, some of the C_comp is distributed on the
FET side of the resistor, and some of it is on the pad side.  Behavioral
simulators assume that all C_comp is in once place and all of the impedance
is in one place (from a network topology point of view).  When the device
is tri-stated you still have a distributed network in real life, but you
have a very high impedance plus a lumped capacitance in your behavioral
model.  This almost begs for a distributed model.

I like to use a TDR to measure pin capacitance in the lab, but I'm not even
sure that a TDR will give you the right answer in this case.

Greg Edlund
Advisory Engineer, Critical Net Analysis
IBM
3650 Hwy. 52 N, Dept. HDC
Rochester, MN 55901
[email-masked]

---------------------- Forwarded by Gregory R Edlund/Rochester/IBM on
03/21/2000 04:06 PM ---------------------------

"D. C. Sessions" <[email-masked]> on 03/21/2000 02:53:40 PM

To:   [email-masked]
cc:   [email-masked]
Subject:  Re: [SI-LIST] : meaning and value of C_comp

Weston Beal wrote:
>
> Dear IC experts,
>
> I've been working a problem that might involve the C_comp parameter in an
> IBIS file.  In discussion with others, I find some difference of opinion.
> the IBIS spec says that C_comp represents the capacitance on the die.  I
> understand this to be the bond pad, clamp diode reverse bias capacitance,
> and final stage transistor channel capacitance.  What other effects are
> important contributors to C_comp?  I suspect that the bond pad is the
> dominant factor in todays technology.  Is that correct?  What are typical
> values for the components I've listed?

C_Comp is whatever value most closely approximates the high-frequency
response that an incoming wave 'sees' hitting the IC.  Personally, I
prefer deriving it from doing a best-fit optimization matching a cap
to a fully extracted SPICE netlist of the physical part.  For most
I/O cells this runs just a bit over 1.0 pf, mostly in the gate/drain
capacitance of the output devices.  The other contributors (diffusion
to bulk, pad to substrate) have such high ESR as to be negligible.

Warning: most test groups want to measure input capacitance at 1 MHz,
and this gives dramatically higher values.  There's been blood on the
floor in JEDEC meetings over this very difference.

--
D. C. Sessions
[email-masked]

---

From: "D. C. Sessions" <[email-masked]>
Date: Tue, 21 Mar 2000 15:32:35 -0700

> sure that a TDR will give you the right answer in this case.

The good news is that it doesn't matter.
Even the dominant effect of C_comp is down in the ~50 ps ballpark; the
second- and third-order contributors are truly in the noise.

-- 
D. C. Sessions
[email-masked]