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Grounding is a critical concept for any electronic circuit and any system dealing with an electric current. Everything from the electric grid to a home to a printed circuit board (PCB) has a ground. PCBs are critical to the functioning of nearly all electronics, and each PCB needs proper grounding to function correctly.
People use the term ground to describe various concepts. In this article, we’ll discuss these concepts, the importance of ground in a PCB and the different methods one can use for grounding in a PCB.
A ground is a conducting body that acts as an arbitrary node of potential voltage and a common return for electric current. It is a point of zero reference or zero volts. The ground is the reference against which you base the signal.
In electronics, the ground is the name given to a certain point in the circuit. In a circuit with one battery with a positive and negative terminal, the negative terminal is usually called the ground.
Some circuits have connections called positive, negative and ground. In these cases, the ground is the middle point between the negative and positive terminals measured in voltage. If the voltage is nine, the ground would be 4.5 volts. You would, however, call the ground zero, the positive terminal 4.5 volts and the negative terminal -4.5 volts. You can do this because the voltage is a measurement between two points, and there is still a difference of nine between 4.5 and -4.5.
Improper use of grounding techniques can dramatically reduce the performance of a system. You must manage various aspects of grounding including controlling spurious ground and signal return voltages, which can worsen performance. External signal coupling, common currents and other issues can cause these voltages. Properly routing and sizing conductors, employing differential signal handling and using ground isolation techniques help to control these unwanted voltages.
There are also special considerations when working in a mixed-signal analog and digital environment. Grounding can help to minimize noise when working with signals that have a wide dynamic range.
There are various types of nodes that get called grounds including floating grounds, virtual grounds and earth grounds.
- Floating grounds: These nodes are reference points in an isolated system and are not physically connected to the earth.
- Virtual grounds: These nodes can be found in a negative feedback circuit at the inverting terminal of an operational amplifier. When the non-inverting input is at zero volts, the feedback will make the inverting terminal match it in a stable circuit. The value is not a stable return for other circuits and is only held by feedback.
- AC grounds: These nodes have low-impedance DC values. This DC voltage is stable even when exposed to small disturbances. Because of its DC value, this node can’t be used as a proper ground, but because it is stable, it can be used as a reference point.
- Earth grounds: In a large electrical system, the earth’s ground is literally a connection to the ground. Every house, for example, has a copper pole that is stuck in the earth to deplete surplus currents.
- Chassis grounds: The electronics in a PCB cannot connect to the physical ground, but a chassis ground serves the same purpose. This ground is the connection of a safety wire from the AC mains to the product’s case or chassis.
Because an earth ground and chassis ground serve the same function, these terms are often used interchangeably along with the term safety ground.
When it comes to grounding a PCB, there is no one-size-fits-all approach. To determine the best way to ground a system, you need to understand the way the currents within it flow. There are, however, various methods to choose from and some tips for best grounding practices that apply across the majority of systems. To determine the approach that works for your board, you’ll need to ensure you understand the board’s design and may need to try several techniques.
There are various techniques that one can use for grounding a PCB. The following are some of the most common approaches used today.
1. Ground Plane
One common technique is to use a ground plane, which is a large piece of copper on a PCB. Typically, PCB manufacturers will cover all of the areas that don’t have a component or trace on them with the copper ground plane.
In a two-layer board, the standard PCB ground plane rules indicate that the ground plane should be placed on the board’s bottom layer, while the components and signal traces are on the top layer.
It is best to avoid creating a ring of conductive material formed by the ground plane, as this makes the ground plane more susceptible to electromagnetic interference (EMI). This conductive ring acts as an inductor, and an external magnetic field may cause an electric current called a ground loop. You may end up with a conductive ring if placing the ground plane over the whole bottom layer and then removing the parts that have electronic components. To avoid this issue, make traces as short as possible, and after mapping them, put your ground plane so that it runs entirely underneath them. You may need to adjust the layout of traces and components to avoid having to create conductive rings.
The ground plane is also often on both sides of the board. In some cases, the plane on the component side is kept at the supply voltage, and the plane on the other side of the board is grounded. The ground plane is connected to the ground pins of the components and connectors to keep the ground voltage at the same level through the whole PCB.
On a two-layer PCB, you may also use more than one ground plane. Each plane should connect to the power supply individually to keep the planes separated and prevent ground loops from occurring.
2. Ground Plane Vias
If there are ground planes on both sides of the PCB, they will be connected through vias at many different places on the board. These vias are holes that go through the board and connect the two sides to each other. They allow you to access the ground plane from anywhere you can fit in a via.
Using vias can help you to avoid ground loops. They connect the components directly to the ground points, which connect through low impedance to all of the circuit’s other ground points. They also help to keep the length of return loops short.
Pieces of copper, such as ground planes, may resonate at one-quarter of the wavelength of the frequency of the current which is flowing into it. Putting stitching vias around the ground plane at specific intervals can help to control this. A practical rule of thumb is to place ground vias at one-eighth of a wavelength or less. This works because a stub on a trace only starts to become an issue at one-eighth of a wavelength.
To create vias, you drill small holes through the board and pass thin copper wires through them before soldering them on each side to form the necessary connections.
3. Connector Grounds
All of the connectors in a PCB should be connected to the ground. In connectors, all signal conducts must run in parallel. Because of this, you must separate connectors using ground pins.
Each board will likely need more than one connector pin leading to the ground. Having just one pin may cause issues with impedance mismatch, which can cause oscillations. If the impedance of two connected conductors does not match, the current flowing between them may bounce back and forth. These oscillations can alter the performance of the system and cause it not to work as intended. The contact resistance of each pin of a connector is low but may rise over time. For this reason, it is ideal to use multiple ground pins. Approximately 30 to 40 percent of the pins in a PCB connector should be ground pins.
Connectors come in various pitches and can have different numbers of rows of pins. The pins of a connector may also be parallel to the PCB surface or at a right angle to it.
PCBs contain one or more integrated circuit chips, which require power to operate. These chips have supply pins to connect them to an external power source. They also have ground pins, which connect them to the ground plane of the PCB. Between the supply and ground pins, there is a decoupling capacitor, which serves to smooth out oscillations in the voltage being supplied to the chip. The opposite end of the decoupling capacitor connects to the ground plane.
One of the main reasons for the use of decoupling capacitors is related to functionality. A decoupling capacitor can act as a charge storage device. When the integrated circuit (IC) requires additional current, the decoupling capacitor can provide it through a low inductance path. Because of this, it is best to place decoupling capacitors close to the IC power pins.
Another primary purpose is to reduce the noise put into the power and ground plane pairs and reduce EMI. Two main issues can cause this noise. One is a decoupling capacitor that does not provide adequate current resulting in the lowering of the voltage at the IC power pin temporarily. The other is an intentional current sent between the power and ground planes using a via with a fast-switching signal.
You should choose the placement and number of decoupling capacitors for a design based on their two functionalities. Often, distributing the capacitors across the entire board is the best approach — try placing some near the IC ground and power pins to use. Using the highest value of capacitance is also recommended, and it is best to keep all of the capacitors at the same value. You may also want to use a combination of high equivalent series resistance (ESR) and normal capacitors.
Grounding is an essential part of any PCB design. All PCB designs must follow certain grounding practices. Here are several tips to remember when grounding.
Make sure nothing in your PCB layout in unattached. It is advisable to fill any open space with copper and vias that connect to your ground plate. By doing so, you ensure that there is a structured path that enables all of your signals to get to the ground efficiently.
If you have a dedicated ground layer, as many four-layer boards do, ensure that there aren’t any route traces on it. Dividing up your ground layer by adding route traces creates a ground current loop. Instead, ensure that the ground layer stays whole.
Every PCB should have a single point where all grounds come together. Often, this is the metal frame or chassis of the product. It could also be a dedicated layer of the board. This single point is often referred to as a star ground because the various conductors extend from this location in a pattern that somewhat resembles a star. In mixed-signal applications, there may be separate analog and digital power supplies that have separate analog and digital ground that meet at the star point.
It is best to minimize the number of vias along your ground paths and to send component grounds as directly to the ground plane as possible. Adding additional vias to a board creates further impedance. This consideration is especially crucial for fast transient currents that can cause an impedance path to becoming a voltage differential.
5. Design Grounding Before Routing
The ground should be designed before any routing. The ground is the foundation for the routing process, so it’s crucial to design the ground correctly. If a ground is designed poorly, the entire device is at risk, while this is not the case if one signal does not work as expected.
6. Understand How Your Currents Are Flowing
Understanding where currents are going on a board can help ensure proper grounding. It’s essential to consider where the signal is going to as well as the return path it will take. The sending and return path of a signal have the same current, and this can impact ground bounce and power stability.
7. Prepare for Dynamic Variance Between Grounds
In a multi-board system, when sending ground connections between boards, it is important to plan for a dynamic variance. It is especially critical when dealing with applications that require long-distance cables. Optical isolators, low voltage differential signals and common-mode chokes can help to keep variance under control.
8. Keep Mixed-Signal Considerations in Mind
When dealing with analog and digital signals together, you need to be careful in your planning. The analog parts of the board should be kept isolated, including analog-to-digital converters (ADCs) and digital-to-analog converters. You can tie an ADC’s ground back to a common ground point where you can pass digital signals to other sections of the PCB.
Proper grounding is a crucial consideration for all PCBs. There is often confusion surrounding this concept, and implementation can be difficult. Ensuring that you understand the flow of current in your design, and employing some of the practices and techniques described in this article can help.
Partnering with an experienced PCB supplier like Millenium Circuits can help as well. We can help to ensure that you receive PCBs that use the proper grounding techniques for your applications. Contact us with any questions or for help finding the perfect PCBs for your next project. Request a quick quote to get started today.