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88 I-CONNECT007 MAGAZINE I MAY 2026 Figure 3: Matching the Spartan 6 driver to the transmission line. (Source: iCD Termination Planner) above. Reducing the signal trace to reference plane dielectric thickness can also reduce cross- talk while not requiring additional space. Also, dif- ferent technologies should not be mixed as higher voltages create higher crosstalk, and long parallel trace segments should be avoided. Crosstalk also depends on the load, which may vary considerably when driving banks of memory modules, for example. Keep in mind that the total crosstalk on a victim trace is the sum of the cross- talk from each of several nearby aggressors. Small discontinuities, such as vias and non-uni- form return paths on a bus, are also becoming an important factor for the signal integrity and timing of high-speed systems. They produce impedance discontinuities due to the local return inductance and capacitive changes. Impedance discontinuities create reflected noise, contribute to differential channel-to-channel noise, and may promote mode conversion. In the case of differential pairs, the transformation from differential-mode to common- mode typically occurs on bends and non-symmet- rical routing, near via and pin obstructions, but can also be caused by small changes in impedance due to return path issues. One must also understand the importance of referencing and how to control the return displace- ment current flow of a signal. Each signal layer should be adjacent to and closely coupled to a ref- erence plane, creating a clear, uninterrupted return path and eliminating broadside crosstalk. As the layer count increases, this concept becomes easi- er to implement, but decisions regarding returning current paths become more challenging. The return current of a high-speed, fast-rise time digital signal will always follow the path of least inductance, which is directly beneath the signal path, as in Figure 4. However, discontinuities tend to divert the return current, increasing the loop area, inductance, and delay. The best way to iden- tify the discontinuities is to follow the signal path and imagine the return path closely coupled on the nearest plane. If multiple planes are present in the layer stack, the displacement current will still take the path of least inductance and closely follow the signal trace. If a discontinuity (e.g., split plane) in- terrupts this return flow, then the return current will be forced into a distant plane where it has a clear run, creating increased loop area and hence more inductance. A via that connects signal traces referenced to different planes also creates discontinuities. In other words, the return current has to jump be- tween the planes to close the current loop, which B E YO N D D ES I G N