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Design007-Mar2020

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36 DESIGN007 MAGAZINE I MARCH 2020 In a previous column "Beyond Design: In- terconnect Impedance," we saw that the im- pedance of the interconnect is the most crit- ical factor in high-speed PCB design. Unfor- tunately, the source impedance of a digital IC driver is typically lower than the impedance of the transmission line (10–35 ohms). This is far from the ideal situation for the perfect transfer of energy, and, in most cases, results in reflec- tions and electromagnetic radiation if not ad- dressed. Whenever a signal meets an impedance vari- ation along a transmission line, there will be a reflection, which can seriously impact signal integrity. By understanding the causes of these reflections and eliminating the source of the mismatch, a design can be engineered with re- liable performance. In this month's column, I will look at how to effectively terminate trans- mission lines. In an ideal world, the energy emanating from an IC output driver would travel through the PCB transmission line and be totally ab- sorbed by the load. However, if energy is not completely absorbed, then the residual will be reflected back along the interconnect, reach- ing the original source of energy at the out- put driver. Reflected energy acts like a standing wave and adds or subtracts to the original sig- nal, causing ringing. Resonance can develop at a signal's fundamental frequency or harmonics resulting in multiple bounces and emission of radiation. This situation only occurs when the round-trip delay of the interconnect exceeds the signal rise time. Even if the multilayer PCB has been de- signed with controlled impedance in mind, im- pedance discontinuities can still occur due to input gate capacitance, branches, stubs, or test pads; variations in dielectric materials; a neck- Transmission Line Termination Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA

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