Subject: another C_comp confusion question

From: "Wilco Hamhuis" <[email-masked]>
Date: Wed, 16 May 2001 15:39:36 +0200

Hi all,

Betty Luk initiated this topic a few months ago. Recently I received a
differential IBIS model from a vendor. I noticed that the C_comp value was
very small, 162fF. Apparently the C_comp value is also included in the VT
tables. In my case I don't have the VT tables. The driver model only
consists out of the pullup and down curves. Gnd, power, rise and fall aren't
present (ramp data is present). When I use this model in a XTK simulation I
see very large oscillation, straight through the logic threshold levels.
Increasing the C_comp value a bit and the results become significant better.
I find it VERY disturbing that C_comp has such a large impact on the
simulation results. My questions:

 - is 162fF a realistic C_comp value according to your experience?
 - does anyone have any experience with questionable simulation results due
to C_comp value?
 - what is the relationship between the VT tables and C_comp?

Tanks in advance,

Wilco
Signal Integrity Expert

---

From: Bob Khederian <[email-masked]>
Date: Wed, 16 May 2001 10:50:46 -0400

Hi All,

I have also had a problem with Ccomp, but the other extreme.  I have seen
Ccomp values of 5pF.  Recall according to the IBIS spec, "Ccomp defines the
silicon die capacitance".  I find it difficult to beleive that the
capacitance of a die (we deal with memory here) is 5pF.  Has anyone had
similar concerns?  Has anyone addressed this with any of the silicon
vendors?  

Bob.

-----Original Message-----
From: Wilco Hamhuis [mailto:[email-masked]]
Sent: Wednesday, May 16, 2001 9:40 AM
To: Ibis
Subject: another C_comp confusion question

Hi all,

Tanks in advance,

Wilco
Signal Integrity Expert

---

From: "Milt Schwartz x3261" <[email-masked]>
Date: Wed, 16 May 2001 11:48:37 -0700 (PDT)

Our C_comp measurements at National Semiconductor Interface Group.
Based on bench TDR, subtracting out the package parameter which we get in a matrix from our packaging group.

LVDS differential driver outputs; range is 3.15pF to 5.3pF depending on how "strong" the driver output is and the type of ESD protection circuitry.

Receiver outputs, are full CMOS swing: Range from 5pF to 7pF, again depending on how "strong" the output is and the type of ESD protection.

These are all CMOS process, typically 0.35 micrometer.

regards, milt schwartz
Interface Applications

> From [email-masked] Wed May 16 08:36:48 2001
> From: Bob Khederian <[email-masked]>
> To: "'Wilco Hamhuis'" <[email-masked]>, Ibis <[email-masked]>
> Subject: RE: another C_comp confusion question
> Date: Wed, 16 May 2001 10:50:46 -0400
> MIME-Version: 1.0
> X-Mailer: Internet Mail Service (5.5.2653.19)
> Sender: [email-masked]
> 
> Hi All,
>

---

From: "Dagostino, Tom" <[email-masked]>
Date: Wed, 16 May 2001 12:03:48 -0700

The smallest specified C_comp I've seen is around 700fF.  The largest
spec'ed beleive it or not was around 35 pF.  Most of what I have seen falls
in the range Milt points out.  Remember, the data from a datasheet usually
has some pad in it.

-----Original Message-----
From: Milt Schwartz x3261 [mailto:[email-masked]]
Sent: Wednesday, May 16, 2001 11:49 AM
To: [email-masked]
Subject: RE: another C_comp confusion question

Our C_comp measurements at National Semiconductor Interface Group.
Based on bench TDR, subtracting out the package parameter which we get in a
matrix from our packaging group.

LVDS differential driver outputs; range is 3.15pF to 5.3pF depending on how
"strong" the driver output is and the type of ESD protection circuitry.

Receiver outputs, are full CMOS swing: Range from 5pF to 7pF, again
depending on how "strong" the output is and the type of ESD protection.

regards, milt schwartz
Interface Applications