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System design considerations: board terminations

Posted: 02 Jul 2010     Print Version  Bookmark and Share

Keywords:system design  board terminations  design considerations 

As systems increase in speed, the PCB connections, which allow these components to communicate with one another, begin to behave like transmission lines. These effects were always present, but as edge rates and transmission rates increase, the effective line lengths become longer and transmission line effects become especially important—important to the extent that if they are not addressed, the system will not work. This application note starts from these points and discusses the practical aspects of terminations, and the various types of terminations. The following pages give a detailed overview of different terminations.

A signal propagating along a bus will "reflect" from any part of the bus that is different from the rest of it. These reflections add and subtract in odd combinations and cause the original pulse to be distorted and corrupted, thereby causing data loss. In order to prevent or minimise the reflections from the ends of the bus, terminators are added to "absorb" the energy from the pulses.

Except for a couple of instances, this discussion covers the what and why of terminations and will avoid making long derivations for the formulas. Although a few simple formula derivations will be given, most of the more complex ones have been done before in several excellent textbooks, one of which is referenced at the end of this paper. These references offer excellent further reading.

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