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Solving IBIS Model AC/DC Mismatch Error

Understanding AC/DC Mismatch IBIS Models

By Timothy Coyle

Introduction

The use of IBIS models in simulations for high-speed design is pretty common now among PCB designers. While IBIS models are easy to simulate with they can sometimes be difficult to debug if you have an issue with the model. One of the most common issues is trying to resolve the AC/DC Mismatch error that you can get from running the IBIS golden parser. This article will show how these mismatches are calculated and what they mean so you can fix them.

The Golden Parser and the AC/DC Mismatch Error

The IBIS Golden Parser is a free utility made available from the standard group that maintains the IBIS specification that parses an IBIS model file and reports warning and error messages about the IBIS model. The parser mostly checks for syntax issues but also does some basic data validation. The most important data validation check it does is check that the DC endpoints from the I/V curves match the AC endpoints from the V/T curves. If the mismatch between the DC and AC endpoints are less than ten percent it reports a warning, and a mismatch over ten percent is an error.

Load Line Analysis

The IBIS Golden Parser uses load line analysis on the I/V curves to calculate the DC endpoints (Voh and Vol) and then checks to see if those match the AC endpoints from the V/T curves. Using a load line is often used in circuit design to find the operating point of an amplifier. Below is an example of applying a load line to the IBIS pulldown curve to calculate the DC endpoints and comparing them to the AC endpoints of the IBIS V/T data.

A load line can be drawn on the IBIS pullup I/V curve as well to validate the DC and AC endpoints. Using load line analysis is a valid way to check the IBIS data.

Conclusion

This article described how the IBIS Golden Parser uses load line analysis from traditional circuit design to provide an easy but useful data check on the IBIS I/V and V/T data. Once you understand how the IBIS Golden Parser works, you can then easily see if you have data integrity issues between your I/V and V/T curve data in an IBIS model.



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