Preventing Ground Loops in Your PCB Design

I think we’ve all been there. You buy that awesome stereo system only to hear that familiar humming sound in the background. When you take it back to the store the clerk blames the manufacturer. What components are to be considered a problem? The decoupling capacitor or ground wire? How about the bypass capacitor or PCB ground loop design interference? The stereo manufacturer will blame the component manufacturer and the component manufacturer can’t blame anyone. In reality, PCB ground loops that form due to shoddy design are the source of the problem.

What is a Ground Loop in PCB design?

A ground loop in PCB design refers to an unintended current path formed between different ground points on a circuit board. This can occur when there are multiple ground connections at different potentials or when the ground plane is not properly designed.

Ground loop PCBs generate noise in electric circuits, sometimes called ground rings. Large currents can exist in ground planes, and a voltage differential between ground connections causes the formation of a ground loop. Ringing or humming sounds in some audio systems are just one manifestation of the PCB ground ring. Read on to find out about preventing PCB ground loop design.

Why Does Ground Routing Matter Anyways?

In basic electronics, it is understood that electric currents circulate within closed loops. In a PCB layout, signals are routed using signal traces and adjacent return paths. As a signal reaches full strength and travels through the board, it creates a current loop with the signal and return traces. The magnitude of the induced return current is influenced by various factors. Considering a trace and its ground plane in isolation, the return current is induced in the ground plane of the PCB through parasitic capacitance between the trace and the ground plane.

So why is this important? If the trace is closer to the ground plane, the capacitive impedance seen by a signal on a trace will be lower, which forces the return path to follow closer to the area beneath the trace. This means that if you want to ensure a reliable return signal to the ground, your signal and return should be placed as close together as possible. Placing a signal trace closer to its ground plane will ensure lower loop inductance, which helps reduce susceptibility to EMI. By placing a ground plane below signal traces, the return signal will naturally form below the signal trace and your circuit will be complete.

Ground Plane Connections

When the ground plane is placed directly below the plane containing your signal traces, all of your signal traces will induce their return path directly in the ground plane. This should illustrate the convenience of using a large ground plane to route return signals rather than routing return traces individually.

No ground plane is a perfect conductor; it has some resistance and reactance. If two signal traces connect to the ground plane at different points, a small voltage differential can exist between these two connections. This is a major source of PCB ground loops in the ground plane. PCB ground loop and return path potentials tend to be on the order of microvolts, but this is still enough to cause signal integrity problems, especially in low-current devices.

Proper planning can mitigate several potential PCB ground loop problems

While the PCB ground ring that arises due to a ground loop can never be eliminated, it can be significantly reduced such that its effects on signal integrity are minimized. Rather than connecting ground connections at different points, it is better to route traces to a ground connection with the ground plane. This minimizes any potential differential between PCB traces and ground plane connections by simply reducing the distance between them.

The ground return to the power supply should also be connected to a ground plane at a single point. When a ground plane is connected to the power supply only at a single point, the entire ground plane will be held at nearly the same potential. If the ground plane is connected to the power supply return at multiple points, PCB ground loops can form due to the voltage differential between these connections. Using a single and proper grounding connection point can eliminate PCB ground rings and loops.

The Right Topology

Unfortunately, only simpler designs with low component interconnectivity will allow the placement of a ground plane that spans below every signal trace. Spanning a ground plane below signal traces is generally a good idea in lower-frequency devices. Keeping the area enclosed by your signal traces and the ground plane small also reduces susceptibility to external EMI.

Spanning the large ground plane under every component may not even be desirable in high-frequency applications. For example, in high-frequency mixed-signal circuits driven by crystal oscillators, placing a ground plane directly below the signal clock creates a center-fed patch antenna. This will exacerbate EMI issues, and signal integrity is likely to be degraded without significant shielding.

If you elect to use multiple ground planes, a PCB ground loop can be prevented between ground planes by using the proper topology. Rather than connecting ground planes in a ring-like or daisy-chain topology, ground planes can be connected to the power supply ground in a star topology. Daisy chaining your ground planes can cause PCB ground loops between ground planes. A star topology connects each plane directly to the power supply and eliminates loops between ground planes.

Use a star topology to connect multiple ground planes

When multiple ground planes are used in your design, take care to avoid routing traces over multiple ground planes. Traces should only be routed over their ground plane. This is especially important in mixed-signal design. For example, if a digital signal is routed over an analog ground plane, noise coupling can occur between the digital and analog signals. This defeats the entire purpose of the star topology.

The PDN Analyzer^™ tool in Altium Designer^® allows you to optimize your design so that signal integrity and PCB ground ring issues are minimized. Furthermore, the 3D PCB design interface can certainly help visualize your designs. To find out more, talk to an Altium expert today.