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Design007-Oct2018

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46 DESIGN007 MAGAZINE I OCTOBER 2018 10 Fundamental Rules of High-speed PCB Design, Part 2 Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA In last month's column, I introduced the 10 fundamental rules of high-speed PCB design (Figure 1). The first rule was to establish design constraints before commencing the design. This prime strategy sets constraints upfront based on pre-layout analyses or rec- ommendations and guidelines and is integral to the design flow to maintain the established requirements. This month, I will elaborate on the importance of controlling the impedance and floor planning the placement based on connectivity. II. Control the Impedance: Match the transmis- sion line impedance to the driver and load. Create the stackup and define terminations to match the impedance. For perfect energy transfer, the impedance of the driver must match the transmission line— assuming there is a high-impedance load. A good transmission line is one that has constant impedance along the entire length of the line, so no mismatches result in reflections. Digital design typically uses a characteris- tic impedance of 50–60 ohms. However, this value becomes more critical as the edge rates increase. Different technologies also have spe- cific impedance requirements. For example, Ethernet is 100 ohms, USB is 90 ohms differ- ential, DDR2 memory is 50/100, and DDR3/4 is 40/80 single-ended/differential impedance. Thus, controlling impedance simultaneously on each signal layer with many different tech- nologies can become a challenge. Further, as operating voltages decrease, the associated Figure 1: The 10 fundamental rules of high-speed PCB design.

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