Understanding Ground Planes in Your Two-Layer PCB

April 5, 2018 Altium Designer

Ground planes in a two-layer PCB

 

 

Back when I was a young and hungry grad student, my first PCB ground plane design was used to collect analog signals from several sensors. When looking at a voltage graph of my measurements, the noise level was horrible and completely masked the signal I was trying to measure. I soon realized that I had completely bungled my ground plane connections, and ground loops were ruining my signals.

 

Placing your ground plane and routing your ground connections is one of the most important design guidelines in PCBs with two or more layers. Doing this correctly can help suppress EMI, crosstalk, and ground loops. These noise sources degrade signal integrity, and proper circuit board ground plane design techniques will ensure that your device shows maximum performance.

So Where Should I Put My Ground Plane Anyways?

If you are just beginning to venture into PCB design, you may hear terms like “ground plane”, “EMI”, and “traces” thrown about. Your first PCB design is also likely to be a two-layer board. Sure, all of these terms are easy to define but how do you put this all together to design a high-quality PCB?

 

In a two-layer PCB, the ground pins plane generally appears on the bottom layer of the board, and the signal traces and components appear on the top layer. Rather than place return traces on the board and send each tread to the ground lead of your power supply, it is better to route your signals to the ground plane.

 

There are several reasons this should be done. First, if your return signals are running in the ground plane, the return signals will follow their corresponding signal traces as closely as possible. This minimizes the circuit board loop area that contains your electrical signals. When the distance between a signal trace and its return is minimized the circuit loop is less susceptible to EMI and crosstalk.

Prevent EMI with proper routing techniques

Prevent EMI with proper routing techniques

 

Designing Your Ground Plane for Maximum Performance

When placing a ground plane in your PCB layout, you should be mindful of the location of your traces and your electronic components. Some circuit designers have a tendency to place the ground plane over the entire bottom layer, and then remove the portions that contain electronic components. This creates a problem as you could create a ground plane that forms a conductive ring around your electronic components.

 

So what’s the big deal, you ask? If the ground plane forms a ring around a component, the ground plane itself now becomes susceptible to interference from EMI. A closed conductive ring in the ground plane acts as an inductor and an external magnetic field can induce an electric current within the ground plane. This current is called a ground loop, and it can cause excess noise in the rest of your PCB.

 

This is important to keep in mind during the circuit layout phase. You will need to be creative with how you place your components and route your traces. It is a good idea to place components so that the traces between them be kept as short as possible. Once you have mapped out your traces, place your ground plane so that it runs completely beneath them.

 

As you start to place your ground plane beneath your traces, you may find that some of your traces cannot be covered without forming a ring in your ground plane. You may need to move traces and components around on the board until you can get the layout just right. This takes some extra time, but the investment is worth the extra effort.

 

Circuit layout software with proper routing techniques

Prevent EMI with proper routing techniques

 

 

The final step requires that the traces be routed back to the ground plane using a through-hole via. It is tempting to make many quick connections directly from the top layer to the ground plane. But making multiple connections to the ground plane will create a voltage differential between each via and will form ground loops. The better solution is to run all traces to a single via connection to the ground plane.

Two Layer PCB? More Than One Ground Plane

Because of the trace routing and ground ring issues mentioned above, it may be better to use multiple ground planes in your PCB layout. This can make routing and component placement easier. This is common practice in most multilayer PCB designs.

 

If you must use multiple ground planes, they should not be daisy chained to each other. Daisy chaining your ground planes and connecting return places to each plane is equivalent to placing multiple returns traces to different points on a single ground plane. A better method is to connect each ground plane to the power supply individually in a star topology. This isolates the ground planes from each other and prevents ground loops.

 

A ground plane can be effectively created in a two-layer PCB using a technique called gridding. Power and ground traces are routed in a differential manner, mimicking a pair of power lines. Every ground trace can be expanded to fill up as much of the empty PCB space as possible, and all the remaining empty space can be filled with ground planes. This technique will give your two-layer board the same level of noise reduction as a four-layer board.


Altium Designer contains multiple tools that can help you implement all of these design suggestions. The ActiveRoute and PDN Analyzer tools make routing and power distribution analysis a breeze. Talk to an Altium expert or start a free trial today to learn more.

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Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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