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10 The PCB Design Magazine • February 2015 feature In this investigation, it is the capacitive con- tribution of the different components that are of interest, and how they affect the characteris- tic impedance the driver sees. For the following examples, the HyperLynx Termination Wizard is used for investigating these effects. It works on a variety of termination schemes including series, parallel, Thevinin, AC, and combinations of those topologies. Figure 1 shows a simple point-to-point sys- tem in HyperLynx 9.0 LineSim. Included is a driver (flash signal I/O pin) and a simple LVC- MOS input connected by a one-inch, 50-ohm transmission line. Running the termination wizard on this net reveals the following results shown in Figure 2. The wizard shows that the driver impedance is 52.7 ohms (based on the IBIS model pull-up/ pull-down curves and the 50-ohm load line), the driver transition time is 855ps, the charac- teristic impedance of the transmission line is 50 ohms, and the effective impedance is 46.5 In a typical interconnect, there lie multiple places where capacitance plays a factor in the signal integrity. This includes the driver and re- ceiver output/input capacitance, as well as the packages, vias, and the transmission lines. Fail- ing to optimize these parameters can often lead to unwanted reflections, excessive radiated and or conducted emissions, and sometimes failure of components and systems. 1 Reflections can occur anytime there is an im- pedance mismatch on the line. Sources of mis- matches are plentiful and include trace width changes, vias, stubs, reference plane changes, and even the so-called fiber weave effect. In this case, a trace can encounter a different dielec- tric constant depending on whether it is routed over glass or the epoxy resin in the dielectric material. 3 by Kirk Fabbri KsPT eNGINeerING CONsulTING