Tips and Tricks for Separating Analog and Digital Ground Planes

March 12, 2021 Cadence PCB Solutions

Key Takeaways

  • Learn the difference between analog and digital ground planes.

  • Find out why it’s important to separate analog and digital ground planes.

  • Discover how to keep analog and digital ground planes apart.

If you’ve ever had Siamese fighting fish as pets, you’ll know that their beauty lies in their ferociousness. These tiny fish are aggressive fighters and will battle to their death when placed in a tank with other fish. Even if they’re kept apart in separate tanks alongside each other, the mere sight of other fish gets them agitated. 

Just as keeping fighting fish apart is essential, separating analog and digital ground planes is necessary, otherwise, you risk having all sorts of currents ‘battling’ each other. In this article, we will focus on what analog and digital ground planes are and why it is so important for them to be kept apart.

What Are Analog and Digital Ground Planes?

Separating analog and digital ground planes is important

Analog and digital ground planes serve as return paths for their respective signals

You might be asking yourself, “Wait, is there more than one ground plane?” If you have the habit of pouring a huge copper ground plane on a PCB, the question may take you by surprise. While the ground plane serves as the return path of signals over a large surface, it can be categorized as either an analog or digital ground plane.

Ground planes are usually placed on the inner layer of the PCB. An analog ground plane would be one that’s placed under analog components and traces. Meanwhile, a digital ground plane refers to the ground plane that serves as the return path for digital signals. Both grounds are marked differently on a schematic.

Analog and digital ground symbols

Analog ground and digital ground symbols 

Technically, both analog and digital ground planes share the same voltage potential of 0V when measured. However, the types of signals that are returned in both planes differ, hence the specific label of each plane. In a mixed-signal design, you’re bound to have both types of ground planes on a PCB.

Why Separating Analog and Digital Ground Planes Is Important

Separating analog and digital ground planes graphic

Keeping analog and digital ground planes apart prevents noise interference

Digital signals, particularly high-frequency ones, are known for their potential to couple noise to the others like analog and ground traces. On the flip side, analog signals are highly susceptible to interference. Put them together and, well, you can see where this relationship is going—unfortunately, just like the Siamese fighting fish, separating both types of signals is not enough to prevent noise interference. 

Whatever happens on the trace will behave similarly on the ground plane. All signals will require a return path. If you have a single ground plane for both analog and digital signals, there’s a high risk of noise coupling from digital to analog signals via the ground return path. 

While interference from a non-separated ground plane isn’t a life or death battle, a few “agitations” may happen. A clock signal may be coupled into an audio channel, where you’ll get a periodic ‘tick’ on top of the audio. What’s worse, really, is that it’s incredibly difficult to pinpoint the exact location on a ground plane that causes the interference. Therefore, the wisest thing to do to mitigate noise interference is to separate analog and digital ground planes in the design. 

How To Separate Analog and Digital Ground Planes

Single point connection for separating analog and digital ground planes

Keep analog and digital ground planes separated with a single point connection

Analog and digital ground plane separation can be tricky because it’s not a “separation” in the truest sense of the word. If you split the planes completely, they won’t be at the same differential potential. But, if you connect both planes at multiple points, you’ll have ground loops that will become a source of electromagnetic interference. So, what’s the solution?

The best practice for “separating” analog and digital ground planes is to connect both with a single connection. A single connection prevents ground loops while keeping both planes on the same differential potential. Ideally, you’ll want to have a mixed-signal component placed between the joining point of both planes.

Separating the ground planes may not be enough to prevent noise coupling if the return paths are far from ideal. You’ll also need to ensure that signals crossing between the ground planes have the shortest return path. This means routing the traces connecting both analog and digital components across the single connection point. If you’re routing the trace over a split plane, the return path will cover a large area and is susceptible to interference

Assuming that you’re using an intuitive PCB design software, separating analog and digital ground planes shouldn’t be an issue. OrCAD PCB designer has the necessary tools for component placement and ground plane pour to achieve the ideal analog and digital ground separation. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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