Let's continue to explain the next note on PCB routing:
In PCB design, the power plane is often used as a reference plane for signals. When signals need to return through the power plane, they reference the power and ground. To switch the reference from the signal layer to the power layer, stitching capacitors should be added at the points of return current and source current. These capacitors help stabilize the potential difference between the power plane and the signal layer, reducing noise and interference.
If the receiver and source are powered by the same power rail, bypass capacitors can be used as stitching capacitors. These capacitors should be placed as close as possible to the start or exit points of the signal to ensure signal stability and integrity. The ideal value for stitching capacitors is typically between 10nF and 100nF, which helps provide sufficient charge storage to stabilize the power voltage while having a low equivalent series resistance (ESR) and equivalent series inductance (ESL) to reduce power supply noise.
When differential signals switch from one layer to another, the reference ground plane also switches. Therefore, it is necessary to add stitching via near the via where the layer changes. These via allow the return current to switch between different ground layers, thus maintaining signal integrity. When dealing with differential signals, stitching via should be placed symmetrically to ensure that both halves of the signal can return current evenly, maintaining the balance of the differential signal. As show in the figure below:
