Issue link: https://iconnect007.uberflip.com/i/1529118
28 DESIGN007 MAGAZINE I NOVEMBER 2024 Return Current Paths 1. Return current paths follow the path of least inductance rather than coupling to a plane further away. 2. For offset striplines, whichever plane is closest to the trace has the most influence on impedance. It's advisable to have a deliberate offset to eliminate this possibil- ity and to be certain which plane will be used for the return path. 3. e impedance of broadside coupled traces is affected by the mechanical regis- tration of layers of the substrate during the fabrication process and should be avoided. 4. Current always flows in a loop. e out- bound pulse charges the local parasitic capacitance as it propagates down the transmission line and returns to the driver. 5. e ground plane is not a dumping ground for unwanted signals. 6. In a DC circuit, the return current takes the path of least resistance. 7. At high speed, the return current takes the path of least inductance, which just happens to be the reference plane (either ground or power) directly above or below the trace. 8. Return current tends to couple to the sig- nal conductor, falling off in intensity with the square of increased distance. 9. Because of skin effect, the high-frequency fields cannot penetrate the plane. 10. e current density will be the same regardless of the frequency. 11. Placing the trace closer to the reference plane will reduce crosstalk even if the trace spacing remains the same. 12. It is important to consider how the return current propagates in the planes. When you plan your stackup, be aware of which plane(s)—either power or ground—will be the return path for your critical signals, and make sure there is an unobstructed return path. 13. Stitching vias should be used to connect ground planes or decaps between power and ground planes to provide return cur- rent paths. 14. If there are multiple ground planes, then place a stitching via as close as possible to each layer transition (signal via). 15. If power planes are also used as the reference plane, then place decoupling capacitors as close as possible to each layer transition. Crosstalk 1. Crosstalk is the unintentional electromag- netic coupling between traces on a PCB. But crosstalk can also be induced in the return path, which oen gets overlooked. 2. Tightly couple traces to reference planes to reduce crosstalk. 3. In the multilayer digital environment, Figure 3: Proximity of return current to signal trace. Figure 4: Differential signal return current.