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12 The PCB Design Magazine • April 2014 by Barry Olney in-CirCuiT Design PTY lTD One of the greatest myths in PCB design is that we only have to route signal traces from pin-to-pin to make a complete connection. And, that ensuring these traces have matched delay is the only timing issue we need to con- sider. However, current is not a one way trip—it must complete the circuit back to the source in order to provide the round-trip current loop. This misconception comes from the fact that we only draw the pin-to-pin connections on the schematic and ground the chips at one point. Current always flows in a loop. However, it does not go down to the end of the trace, to the load, and then begin to make its way back to the source. But rather, the outbound pulse charges the local parasitic capacitance as it propagates down the transmission line and returns to the driver. As the pulse progresses down the line, current returns to the source as the wave front moves until it finally reaches the load. If the re- turn path is disrupted and does not flow directly beneath the trace, the loop area and hence delay are extended. This generally results in increased emissions of radiation. In a previous column, The Dumping Ground, I discussed why the ground plane is not a dump- ing ground for unwanted signals. Most PCB designers think that the ground only serves to make the routing easier, allowing the designer to ground anything, anywhere without having to run multiple tracks. Generally, a component requiring a ground connect is just grounded at feature There are no One-Way Trips!

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