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One of the most common questions we see from many designers is how to properly ground their PCBs. This includes ground plane design, placement of grounded vias, and other important techniques to reduce noise in a PCB. The fact is, grounding is the foundation on which we build our systems, and a thorough understanding of PCB grounding techniques is essential.
Ground loop formed by two trace connections
We normally talk about signal integrity in terms of high speed and high frequency signals, but any PCB needs to have a stable ground to ensure signals are clean and noise-free. Proper grounding comprises routing return signals to a ground point, and properly designing ground planes. Let’s take a look at some PCB grounding techniques and ways to ensure proper grounding throughout a board.
What is a Ground?
This might sound like a simple question, but the distinction between different types of grounds is fundamental. An electrical ground is a conducting body that acts as a common return path for the current from various devices. Usually it is referred as the zero-potential node and all the other voltages in the system are referred with respect to this node. Following are the different types of nodes that are referred to as grounds-
- Floating grounds: A floating ground is simply a large reference conductor in an isolated system. A floating ground is not physically connected to earth.
- Earth ground:This is literally a physical connection to the earth. This acts as a safe return point to deplete surplus current.
- Chassis ground: The electronics in a PCB cannot connect directly to an earth ground (this normally happens through the power supply), but the metal chassis can act as a good ground. This is usually used to ground the body of an enclosure as a safety measure.
- AC ground: These are low impedance ground paths that block DC return current. This is normally created by connecting to a ground plane through a capacitor.
- Virtual ground: These grounds can be found in negative feedback circuits at the inverting end of operational amplifiers. Connecting 0 V to the non-inverting input will pull the inverting input 0 V, and this value is usually held constant through feedback. This is an unstable node and cannot be used as a return path for other circuits.
PCB Grounding Techniques and Your Layout
Any large piece of copper on the PCB connected to the ground is called ground plane. In a two-layer board, this is usually spread across the bottom layer, while traces and components are arranged on the top layer. In a multilayer PCB, one of the interior copper planes is normally dedicated to a ground plane.
If the ground plane doesn’t entirely cover a complete layer, then you should always make sure you do not create any closed rings in your ground plane as this makes your ground plane susceptible to electromagnetic interference (EMI). Note that EMI can be induced in the ground plane from other components on the board or from external sources. The conductive rings act as an inductor, and any external magnetic field can induce a voltage/current in the ground loop.
Similarly, placing unnecessary traces between the ground pins of two components creates a ground loop. This is an especially potent source of noise between digital circuits that mimics the behavior of ground bounce. It also creates an effective inductor that increases susceptibility to EMI. Each component must be connected to a solid ground plane individually to avoid ground loops.
Ground loop formed by two traces connecting to a ground plane
When using a chassis ground, you can avoid ground loops by placing a void in the ground section that connects to the chassis, as shown below. The use of a capacitor provides an AC ground point. This is an ideal situation for electrical equipment that will run off of wall power and needs to have a return directly back to earth.
Elimination of ground loop antenna
In a multilayer board, ground planes on different layers are connected through vias. These connections help you access the ground plane anywhere throughout the PCB. Vias also help reduce the ground loops in the system. They provide a shorter return path for the current through a low impedance ground point.
Sometimes, pieces of copper may resonate at 1/4 the frequency of the current flowing through it. This is one reason that you should try to route the shortest possible connections between components using controlled impedance techniques. Placing grounded stitching vias at appropriate distances can help eliminate these oscillations as they provide a capacitive path back to ground. As a rule of thumb, these ground vias must be placed at 1/8th of a wavelength or less from the relevant conductor.
Ground Planes in Your Stackup
In a multilayer PCB, the arrangement of power, signal, and ground layers in the stack has major effects on signal integrity and will influence your routing strategy. It is important to keep a ground plane near signal planes in order to minimize the current’s return path. In a 4-layer board, the power and ground planes are normally kept on the inner layers, while signal traces and components are placed on the outer two layers.
Analog and Digital Component Arrangement
Components should be arranged on the signal layer close to the ground so that the return paths are short and traces coupled to ground. If the PCB contains analog and digital components, then the ground connections must be placed very carefully. The analog and digital sections of the board should be physically separated, but they still need to connect to the power supply return path.
Mixed-signal ground connections
Some might suggest completely splitting the digital and analog ground and then connecting them using a ferrite bead, but this can create more EMI and noise problems than it solves, especially if you are working at very high frequencies. A good way to connect these sections is to place the power supply return path between the two planes so that return currents from either section will not enter the other plane. It is important to note that no traces should not be routed over a gap between two ground planes as this creates long current return path that is highly susceptible to EMI. The space between the ground planes can be used to place mixed-signal components like ADCs.
Designing a high-performance PCB requires attention to detail, and grounding is just one of many design aspects that require your attention. One rule of thumb to follow here is “grounding before routing,” meaning you should consider the location of ground connections in your PCB before routing signal traces. It is important that you do not leave any floating planes on your PCB and connect them to ground instead. Layout editors have design rule check (DRC) features that will inform you of any floating net.
No matter which PCB grounding techniques you need to implement in your PCB, Upverter® provides a high quality PCB editor with excellent routing tools to design boards from start to finish in a browser-based interface. You’ll also have access to real-time DRC tools to help avoid any rework in your layout. Upverter’s browser-based platform gives you access to your work from anywhere.
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