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Everyday App Note: The Top 5 Techniques for Terminating Transmission Lines

Sam Sattel

everyday-app-note-series

Everyday App Note: The Top 5 Techniques for Terminating Transmission Lines

Today’s Everyday App Note puts an end to transmission line troubles five termination techniques. Inside you’ll find wisdom from Doug Brooks, founder of UltraCAD. He’ll be talking about proven strategies to reduce the risk of signal reflections with adequately terminated transmission lines.

Doug Brooks, founder, and president of UltraCAD. (Image source)

Who Is This App Note For?

This app note was written at a time when circuit engineers and layout designers were two different jobs. Times have changed of course, but signal reflection problems remain just as prevalent today.

impulse_noise

The effect of noise caused by signal reflection. (Image source)

For those of you just designing board layouts, you’ll walk away with an understanding of how termination strategies work. If you’re designing circuits, then you’ll be using these strategies on your schematic designs. And if you’re wearing both hats, as many of us do, then you can use this knowledge at the start of your design process!

Why Read This App Note?

Noise can wreak havoc on data, producing a variety of problems like overshoots, undershoots, or reflections. All of these problems ultimately lead to misreading logic states and poor signal timing.

The key takeaway from this app note is to design traces that look like transmission lines. It’s not just about limiting traces to a certain length to get that signal from the source to sink. Even long traces can properly control reflections with the strategies discussed inside.

Some of the termination techniques discussed in this app note includes:

  • Parallel Termination. This technique uses a single resistor from trace to ground and performs well with distributed loads. However, it also provides a continuous DC path to ground.
  • Thevenin Termination. This technique uses a pair of resistors, one for ground, the other for power. However, the ideal resistor value can be hard to determine and is only suited for two-state devices.
  • AC Termination. This technique adds a capacitor in series with a parallel terminating resistor to block DC. However, depending on the capacitor used there can be problems with power dissipation or overshoots.
termination-technique

Each termination technique has its own pros and cons to consider. (Image source)

As you can see, there’s not one termination technique out there. Each strategy has its own tradeoffs that need to be considered as part of your design process. The end goal is still the same though, producing clean signals that get to their destination on time!

Download this application note now from UltraCAD to learn the top 5 techniques for effective transmission line termination!

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