All About Grounding in Electronics Design and PCB Layout
The concepts of grounding techniques, earthing, making ground connections, and chassis ground are all very convoluted in electronics, despite international standards that have tried to separate concepts and terminology. Grounding is important in every aspect of electronics design, electrical work, and of course, in PCB design. All circuits will need a reference connection, which is something that we call ground, but the exact reference is defined in different ways for various systems.
If you're unsure of how grounds work in different types of electronics and how to use ground connections, there is no simple answer that applies to every system. Different types of electronics will have different ways of defining their potential reference, and all ground are not always at the same potential, contrary to what you might have learned in an introductory electronics class. In this article, we'll take a systems level approach to defining and integrated digital grounds, analog grounds, chassis grounds, and eventually, an earth ground connection. Keep reading to learn how ground eventually gets connected to your PCB and ultimately to every component in your system.
What is Ground in a Circuit and What Does It Do?
There are a few ways to define ground, depending on who you ask. Physicists define it in a particular way (mostly theoretical), while electricians and electrical engineers might be literally referring to the ground beneath your feet (earth ground). In electronics, we sometimes refer to ground as performing various functions interchangeably. Here are some of the primary functions of ground in electronics:
- Ground provides a reference point that is used to measure voltages. All voltages are defined in terms of the electric field (and potential energy) between two points. One of these points can be defined as "0 V", and it just so happens that we call this 0 V reference a "ground". This is one of the reasons we say that a ground plane in a PCB is a "reference plane".
- Ground can be used to provide a path for return current to power source, thus completing a circuit.
- Conceptually, ground acts as a big reservoir of charge that also defines the direction of current flow. Since we take ground to be our 0 V reference, voltages above or below this value (positive or negative) will drive current flow in different directions with respect to the location of ground.
- Ground provides a point where electric fields terminate. This is really a variation on the first point. If you ever had to solve method of images problems in an electromagnetics class, you should remember that ground is defined as an equipotential surface held specifically at 0 V. Note that this definition also applies to any conductor held at a specific voltage (e.g., a power plane in a PCB).
- The voltage drop across a perfect ground conductor is 0 V. In other words, if you measure the voltage between any two points in a ground reference, you should always measure 0 V. This is a restatement of point 2 above.
In PCB design, we often talk about ground in terms of points 1 and 3 because it defines how power is supplied to components, and how digital/analog signals are measured in a design. EMI/EMC folks will sometimes talk about ground in terms of point 4 as this basically describes the function of shielding materials. Everyone accepts point 5 as gospel, although point 5 does not happen in reality.
Now that we have these points covered, there are some things to realize about grounding and various types of grounds in electronics.
All Grounds Are Imperfect
Although all ground regions are intended to have the above characteristics, the real nature of conductors means they function differently when used as a ground reference. In addition, the geometry of a ground region determines how it interacts with electric and magnetic fields, which then influences how current moves into and within a ground region. This is why different signals will have a particular return path that depends on their frequency content. In addition, all grounds have non-zero resistance, which leads to the next point concerning real grounds.
Not All Grounds Are at 0 V
Conductors that are left floating, or conductors in a system that are reference to different power sources, may not have the same 0 V potential. In other words, you could have two ground references for two different pieces of equipment, both being connected to the same reference, but if you measure the potential between them, you would measure a non-zero voltage.
This can even happen when two devices reference the same conductor as a ground connection. If you were to measure the potential difference across a long conductor (e.g., with a multimeter), it could be non-zero, meaning some current is being driven along the conductor. This difference in potential along a large ground or between two ground connections is called "ground offset". In larger multiboard systems, or in areas like industrial and networking equipment, ground offset is one of the drivers for using differential signaling (e.g., CAN bus, Ethernet, etc.). Because differential protocols use the voltage difference across two wires, their respective ground references are irrelevant, and signals can still be interpreted.
Types of Grounds in Electronics
In electronics, it's easy for a new designer to get confused by the various terminology used for grounds involved in PCB design: digital, analog, system, signal, chassis, and earth ground. Add to that the fact that symbols for representing ground are mixed and often misused, something which I and certainly guilty of doing purely for convenience. Regardless, there are some standard ground symbols that are used in electrical and electronics engineering, including in your electronics schematics.
Different types of ground connections are called out in schematics using symbols defined in the IEC 60417 standards. The common symbols normally used in PCB design are shown below:
The signal ground symbol can be used for digital or analog ground, just make sure to apply the right net name (I sometimes use AGND for analog ground, and DGND for digital ground). The chassis ground is sometimes connected back to earth ground, depending on how the system is constructed and how it receives power. Finally, the safety ground can sometimes be connected directly to earth via a neutral wire, or to chassis, or possibly to earth via a low inductance chassis connection.
The term "earth ground", or simply "earth" in electronics, refers to a literal connection to the earth. In other words, the potential of the earth is being used as our 0 V reference. If you've ever looked at a utility pole that carries power lines, you can sometimes see a wire running down the side of the pole and into the dirt. This is an earth ground connection is imperfect as the resistance in the soil along the cable can be high. However, using earth provides the large reservoir of charge that is characteristic of a desirable ground connection. This connection is not intended for carrying current when loads draw power, it only carries current when dissipating spurious currents (e.g., noise or ESD events).
One important point to note in electronics is that not all systems will have chassis ground. Normally, this term refers to a metal chassis that is in an enclosure, and a connection is made to the chassis. In 3-wire AC systems (hot, neutral, and ground wires), or in 3-wire DC systems (DC+, DC common, and ground wires) the chassis ground is normally connected to earth ground at the point where power comes into the plug in the system. A part of the system may also be connected to chassis ground to sink noise or for safety reasons (e.g., ESD protection), such as the example shown below. This arrangement provides common-mode noise filtration for an AC or DC input on a 3-wire connection.
This type of earth ground connection from provides three functions:
- Because the chassis is now set to a global 0 V reference potential, the chassis now acts as a Faraday cage and provides broadband shielding.
- It provides a safety function that dissipates spurious currents (ESD, shorts, or noises) back to earth. This is one reason we sometimes call the chassis ground a "safety ground".
- It can provide a low-impedance sink for common-mode noise on this input EMI filter without placing a ferrite or large choke on the board.
In a battery-powered system, or in a system with a simple 2-wire DC power connection, the PCB ground plane can be tied back to the chassis via mounting holes. The idea here is to ensure there is no floating conductor as an ungrounded conductor can act as a radiator due to capacitive coupling of current into the chassis. An ungrounded chassis or other floating conductors in the board can be sources of radiated EMI that can be easily eliminated by connecting to a ground.
Analog and Digital Ground
Analog and digital grounds are two different issues from earth and chassis ground. Typically on the PCB, you can have a chassis ground connection as described above and the connection to the earth ground for safety. Meanwhile, you should have a ground plane on the PCB that supports both analog and digital return paths; you should not have physically separated ground nets. These physically separated ground can create strong radiated emissions when overlapped in the stackup, particularly at parallel plate waveguide frequencies. Instead, do everything over a single ground reference in your PCB
To learn more about these points around analog and digital ground, read this article on star grounding as it explains the major reasons you shouldn't use physically separated ground planes.
Should You Connect Signal Ground to Earth?
It's not very often you will do this directly This might be appropriate in the case of high-voltage DC batteries/PSUs or similar systems that are being tested. Generally, you might connect the chassis ground to earth, which then connects to a circuit that references a PCB ground plane on the input side (e.g., the input EMI filter before a rectifier). In an un-isolated 3-wire AC system, or in a 3-wire AC system that gets rectified to DC, if you connect the signal reference ground in a circuit to earth, you're just shorting out the negative wire on the AC or DC line. Don't do this because now the chassis can be a big current-carrying conductor! There is now a risk of shock (in high voltage/current systems) or intense EMI (in high frequency systems). When this is done, current will move back to the earth connection as long as that is the path of least reactance back to ground, and that path might be through someone who touches the device while it carries high current.
In a 2-wire system (no earth ground connection), there are varying guidelines on how or if to connect the signal ground back to the chassis. Some guidelines say multipoint grounding is okay, others say to use a single point near the I/O, and still others say to use a single point near the power connector for safety. If RF noise is a problem throughout the system, you can tie back to chassis at multiple points to dissipate noise, but you probably have a bigger problem in your layout because you did not build the stackup correctly, and the device is just receiving too much radio energy. Focus on building the stackup correctly and you might not need to stitch mounting hole connections all over the board, you'll just need it at a few points. You should not make a connection back to earth in this case.
Summary on PCB Grounding Techniques
With a PCB being the vessel that holds your electronics, it's important to get grounding correct. As we discussed above, your grounding strategy is relevant for safety, EMI/EMC, and systems design, so it's important to get it right. While the presence of multiple grounds in your design might seem confusing, the best schematic editor tools and PCB layout software will help you keep track of ground nets throughout the design as you create your physical layout.
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