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20 The PCB Design Magazine • February 2015 Kirk Fabbri is owner of KsPT engineering Consulting, llC and electrical engineer for l-3 Communications – Avionics systems. he can be reached at kirk.fabbri@l-3com.com. the characteristic impedance of the connecting nets (75 ohms)? To see these effects, the via is altered to have a 75 ohm impedance and a 13ps delay (5.4 mil pad, 8 mil drill). Running the wizard again on this scenario shows the effec- tive characteristic impedance is now 67.2 ohms, indicating that the vias are no longer loading the driver as much as before. The drill/pad size chosen for the via aren't entirely realistic. This is merely for purpose of demonstration. Important Points to Remember • The simulator calculates the driver imped- ance based on the transistor characteristics in the IBIS model and recommends termination based on the topology and characteristic im- pedance of the system. • The receiver capacitance parameters in- cluding C_comp, pin capacitance, and package capacitance all impact the loading on the driv- er, thus changing the effective Z0 parameter. • The wizard does not appear to consider the system distributive until the interconnec- tion length becomes approximately equal to or longer than 1/6 th of the driver's transition time. • When not considered to be a distributive system, changes to the capacitive properties of the IC are no longer included in the wizard's calculation of the effective Z0 parameter. • Vias have an effect on the loading of the driver, but their affects can be minimal depend- ing on the driver transition rates. As the driver speeds increase, their effects become more pro- nounced. PCBDESIgn References 1. Signal Integrity: Simplified, ISBN 0-13- 066946-6. 2. Mentor Graphics LineSim User Guide, July 2013. 3. Right the First Time, A Practical Hand- book on High-Speed PCB and System Design, Volume 2, ISBN 0-9741936-1-5. 4. IBIS (I/O Buffer Information Specifica- tion) Version 6.0, September 20, 2013. Figure 10: The transmission line model is calculated from the individual lC sections of the via transitioning through the layers. feature EFFECTIvE CHARACTERISTIC IMPEDAnCE continues