![]() Together, these quantities determine the impedance seen by the signal. ![]() The location where the return path is introduced depends on the capacitance between the signal trace and adjacent conductors, as well as the self-inductance for the circuit formed by the signal trace and the conductor in question. Let’s address the second question first as it helps explain the answer to the first question. If you’ve read what I’ve written above and you’re still wondering what happens to the return current in a complex PCB, you’re probably asking yourself: what happens when the current is coupled to a ground plane or other grounded conductor? Why would this happen in the first place? Both are valid questions. Note that the parasitic impedance is not localized to adjacent conductors thanks to inductance, which can create a complicated ground return path in a multilayer board.Ĭan you track the ground return path for these traces? Getting Back to a Solid Ground Return Path The primary quantities that determine the initial reference plane are the parasitic capacitance between the signal trace and nearby conductor and the circuit’s inductance. The situation becomes more complicated when we are routing in a multilayer stackup with multiple plane layers as the reference conductor can change along the signal path. The exact ground return path followed by common-mode noise depends on its frequency content as this determines the reactance seen by the signal. Common-mode noise, once induced in a given trace, will try to follow the same path as your signal back to ground. Read this guide to learn more about designing a mixed signal return path for a single plane layer.Ĭommon-mode noise path. Because reactance is a function of frequency content in your signal, the signal return path becomes more difficult to predict at moderate frequencies. The parasitic capacitance between a signal-carrying conductor and its nearest reference conductor, as well as the loop created by the circuit, determine the reactance seen by a switching signal. This is one reason why high speed signal traces should be routed close to a reference plane on an adjacent layer. The loop inductance is lower when the loop is tighter. A circuit with a large current loop will have larger parasitic inductance, making it more susceptible to radiated EMI. The loop inductance created by a return path determines a circuit’s susceptibility to EMI. Whether the return signal is induced in your chassis, power plane, or other conductor, it will be drawn back to ground due to the potential difference between your ground conductor and a conductor held at higher potential.Īside from being a characteristic of ringing as a signal propagates, a signal’s return path determines the following behavior:ĮMI susceptibility. No matter where the return path runs in your board, it will always try to get back to the low potential point on the board, i.e., the ground return point back to the power supply. The ground return path for a signal may not actually flow through ground it could pass through the chassis, a power plane, or some other grounded conductor. Whether they are intentionally placed in your board, such as a ground plane for signal traces, or an unintentional reference plane that lies close to the signal traces, can be difficult to determine if you do not carefully track the location of signal traces throughout your board. Reference planes are an inherent part of the signal transmission path. This is where it helps to distinguish between ground planes and reference planes as both can form part of the return path in your PCB. Multiple plane layers and conductors can form the ground return path, even if the conductor is not grounded. The situation becomes more complicated when you’re working with higher layer counts. When your PCB has a small layer count (e.g., a 4-layer board with two plane layers), it becomes rather easy to determine the return path and deliberately design it to prevent EMI. Make sure none of these traces crosses a ground plane gapįollowing the path back to ground can quickly become complex in a complicated multilayer PCB.
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