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Beyond Design: Impedance Matching: Terminations

The impedance of the trace is extremely important, as any mismatch along the transmission path will result in a reduction in signal quality and possibly the radiation of noise. Mismatched impedance causes signals to reflect back and forth along the lines, which causes ringing at the load. The ringing reduces the dynamic range of the receiver, eats into the noise budget and can cause false triggering.

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. For perfect transfer of energy and to eliminate reflections, the impedance of the source must equal the impedance of the trace, as well as the impedance of the load. When the signal path has an impedance discontinuity, place a terminator at the point of line discontinuity to equalize the impedance.

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Editor's Note: This column originally appeared in the October 2013 issue of The PCB Design Magazine.