Double counting C

Home

Modeling & Simulation
What is Modeling?
Our Book

Our Methods
What is Unique?
Who We Work With
Team Effort

Services We Provide
Services & Support
Classes & Seminars

Tools, Resources &
Publications
Resources
Websites
References
Glossary
Technical Papers

About Us

Contact Us

Site Map

Error: the Double counting of C_comp in a Simulator

The IBIS Spec has always made it clear that driver model [Ramp] data (alternatively, V-T curve data) should include the loading effects of C_comp. The voltage-time behavior of the switching pulse emanating from the driver (IBIS Output or I/O model) should not be affected by the C_comp element appearing across the driver output.

How can this be and how are errors avoided? After all, the driver should see its C_comp element as a load across the output. Computation of the launched waveform could then result in an erroneous double counting of C_comp.

Why is C_comp even included in the IBIS model? Simply because a reflected pulse waveform returning to the driver from the receiver, load, or mismatch, will be loaded by the effects of C_comp. Accurate computation of multiple reflection behavior requires that the simulator include C_comp effects.

One popular way to avoid this potential error in a simulator program is:

"To sum this problem up, I usually say that C_comp needs to be visible from the outside looking in, but invisible from the inside looking out. . . .

. . . I connected a duplicate C_comp on the output of the PWL [piecewise linear] source(s), which had the V-t curve(s). Then I just simply included an additional term in my math, which combined the I-V and V-t curves so that the current of this duplicate capacitance was subtracted from the total result. I know this will have some error as the actual waveform deviates from the V-t curve, but this error is smaller than double counting C_comp (zeroing it out).

Of course, when you write the code of your simulator you have much more control over writing an algorithm that does a better job than this.

Arpad Muranyi

Intel Corporation"

How does a user tell if their software has implemented this simulation correctly? The answer is to start with an IBIS driver model where the value of C_comp can be modified. Do two simulations. Do one simulation with the value of C_comp very low, perhaps .01pf to 0.1 pf. Do the second simulation with the value of C_comp very high, perhaps 10 pf to 100 pf. Check the output ramp rates / V-T curves at the driver in both simulations. They should not be affected by the value of C_comp. Note that driving a matched transmission line simplifies the investigation.

Modeling & Simulation | Methods | Services | About Us