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Controlled Impedance Design

Controlled impedance—it’s all about transmission lines. For perfect transfer of energy, the impedance of the driver must match the transmission line. A good transmission line is one that has constant impedance along the entire length of the line, so that there are no mismatches resulting in reflections. But unfortunately, drivers do not have the exact impedance to match the line (typically 10–35 ohms) so terminations are used to balance the impedance, match the line and minimize reflections.

Reflections occur whenever the impedance of the transmission line changes along its length. This can be caused by unmatched drivers/loads, layer transitions, different dielectric materials, stubs, vias, connectors and IC packages. By understanding the causes of these reflections and eliminating the source of the mismatch, a design can be engineered with reliable performance.

Impedance matching slows down the rise and fall times, reduces the ringing (over/undershoot) of clock drivers and enhances the signal quality of a high-speed design. The ringing is dramatically reduced by adding a series terminator.

To read this article, which appeared in the May 2015 issue of The PCB Design Magazine, click here.