Misconception About Differential Signals in PCB Design

One common misunderstanding in PCB design is the belief that the differential signal does not require a ground plane as a return path, or that the differential traces serve as return paths for each other. This misconception arises from a lack of understanding of the high-speed signal transmission mechanism.

Understanding the Role of Ground Plane

It is essential to recognize that the differential circuit still utilizes the reference plane as the signal return path. The emitter currents of transistors Q3 and Q4 in the receiving end are equal and opposite, canceling each other out at the ground (I1=0). This design feature makes the differential circuit less susceptible to ground bounces and noise signals on power and ground planes.

Coupling and Return Path

In differential traces, the coupling between them is typically minimal, with most coupling directed towards the ground. Therefore, the primary return path for the differential trace remains on the ground plane. In cases where there is a discontinuity in the ground plane, the coupling between the traces can provide an alternate return path.

Importance of Impedance Control

It is crucial to maintain impedance control in differential traces to avoid EMI radiation. Removing the ground impedance loop for common mode signals can lead to increased EMI, impacting signal quality. While differential traces can be routed on different signal layers, this practice is generally discouraged due to impedance variations and potential introduction of common mode noise.

EMI Considerations

At lower frequencies, EMI may not be a significant concern. Studies have shown that the attenuation of radiated energy from a differential trace is sufficient to meet electromagnetic radiation standards. Designers should focus on maintaining proper trace spacing to minimize crosstalk and noise interference.

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