PCB grounding

Guide to PCB Grounding to Ensure Low Noise and EMC

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PCB grounding is perhaps the most important system level consideration a designer can address in complex systems. Simple designs have a very simple strategy that almost always works well as long as the PCB layout is constructed correctly. More complex systems involving an earth connection, multiple boards, or multiple electrical units will have more complex grounding strategies. This is an area that can be quite confusing and will determine the noise immunity of your system.

The grounding strategy and the physical layout of ground in a PCB will also be a major influencer of passing EMC. Most of the EMC failures that are addressed in PCB design are solved by looking at the grounding strategy, followed by the routing strategy. In this article, we’ll give a comprehensive view of grounding in complex PCBs, including in systems with mixed-signal PCB layout and routing, as well as systems involving a chassis/earth connection and power systems.

Getting Started With PCB Grounding

Ground in a PCB and in the surrounding physical construction of a system plays multiple roles, including noise immunity, references for signal measurements, and EMC. In general, in a PCB, there are three types of ground regions to be considered in a complex system:

  • Earth ground: This is the ultimate ground reference as it is used in utility wiring. It provides the best form of safety for any system when it is available. This involves a literal connection to the earth through a utility cabinet.

  • Chassis ground: This is sometimes called a frame ground and it literally refers to the system frame, assuming there is a conductive frame in the system. The frame ground is normally connected to earth when an earth connection is present.

  • Signal ground: What most guidelines won’t tell you is that this is somewhat arbitrary; it could be a ground plane in a PCB, but it could also be any other large conductor that can carry return currents back to the system power supply. In general, chassis ground should not be the signal ground if the user will ever interact with the chassis directly.

How these three regions (when all are present) are connected, and if they need to be connected, can be complex when multiple devices are present in an electrical system. The connections between these grounding regions are specified in your PCB design software (in the circuit board schematics) and in an electrical diagram for the entire system.

 

PCB grounding symbols

Typical symbols used for ground in schematics.

 

Grounding on a PCB

How you apply ground on a PCB really depends on what the PCB needs to do. The guidelines for an isolated power supply will be different than in a small device that will be powered with a battery. PCB Grounding with an earth connection and an isolated power section will be different than in a system with an earth connection and a simpler non-isolated power supply. At some point, you may find that you don’t even need the earth connection as you’ll be fine with a floating ground, even if you are pulling AC power from the grid or a generator to power the device.

Grounded structures may also appear on the circuit board to provide shielding, additional paths for return currents, or convenient connections to ground for certain components. This is normally placed as copper pour, which provides some additional benefits beyond providing convenient ground connections.

To see how this all works, we have to actually think about how power is brought into the PCB and whether multiple devices are present, as well as whether they have to share ground. We’ll try to cover most of the basics here, although some of these topics are rather complex and require their own articles.

AC Power With Earth Connection

This is a standard method for bringing power into a board that requires grid power and will run at moderately high current. In the diagram shown below, we’re bringing 3-wire single-phase AC into the system and stepping it down to the required level on the board with a transformer. The separation between the primary ground (PGND) and secondary ground (SGND) in the PCB stackup means we now essentially have an isolated power supply on the board with the rest of our important components, which could be high speed digital or precision analog components.

 

PCB grounding with isolation

Grounding with isolation on the input power stage.

 

The separation between PGND and SGND is used to ensure galvanic isolation between the input and output. This is typical in a system that runs at enough current to create safety concerns, and the two sides are bridged with a Y-type capacitor with a capacitance that is much larger than the TX winding capacitance. The result is that high frequency noise on the SGND side can easily flow back into the PGND side and back to earth through the mounting hole connection. This also provides a path for ESD events at an I/O, particularly if the I/O is exposed to a user. In this case, when you have such an I/O passing to a connector, the connector should be connected to SGND so that any ESD event at that connector is diverted to earth through the PGND mounting hole rather than into nearby components.

AC Power Without Earth

This is common when safety is not so much of a concern. Consider a simple AC/DC power adapter that plugs into the wall; this device may use isolation between a PGND and SGND side of the adapter, but these devices will not have any earth connection. This is perfectly fine, even in a high current device like a laptop; you still get isolation between the input and output power, but you need some other way to get a safety ground. The end device might have its own safety ground method built into its chassis, or it will just take advantage of its own ground plane for safety.

Direct 2-Wire DC Connection With Earth

This would be used if you’re powering a device from a DC power supply that is a separate unit, and it has its own earth connection. The earth connection is generally not connected to the board, instead being connected to the chassis. If you are using a DC power supply with its own earth/chassis connection, you can then connect the chassis together with this type of power arrangement. The idea is that the chassis provides some safety against ESD, as well as eliminating any large floating metal bits that could provide EMI coupling around the design. What you should not do is connect the system ground to the chassis as you don’t want to create a ground loop or cause high currents to flow through the chassis as this might create a safety hazard.

 

PCB grounding with earth DC

Grounding in a DC system with an earth connection.

 

Direct 2-Wire DC Connection, No Earth

In the case of a 2-wire DC connection without earth, the design basically has floating grounds with respect to any other ground. The strategy here is, if there is a metal chassis, to ground it near the DC input or battery terminals. This way, any return currents in the device won’t flow through the metal chassis, which is not meant to be used as a signal return. This usually only becomes problematic when there is some high frequency RF source present in the design, which might induce capacitively-coupled common-mode noise around the design through the chassis. In such an environment, an earth connection would be better as there will be a low impedance path that diverts noise.

 

PCB grounding without earth

Grounding in a DC system with a chassis ground but without an earth connection.

 

Grounding With Multiple Earths

This situation generally arises when routing a cable between two pieces of equipment, and the cable might have some shielding that connects to the grounds on each end. You have to be careful here as the two earth connections could have a DC potential difference between them, which could be around 10 V as measured in the lab. The result is that, if the earths are bridged (such as using shielding along a shielded cable), the ground offset may cause a large DC current flow that fries the cable. One option to create a high impedance connection for DC but allow high frequency AC noise to pass is to use a capacitor-based connection on each end of the shielding, which can tolerate the ground offset. Otherwise, you should probably not bridge the earths.

 

PCB grounding with ground offset

 

What is interesting about this is that this was the original motivation for using differential pairs to transfer data across long links. Differential signals can tolerate very large ground offsets, which might arise when equipment is connected to different circuits. Quantifying signals based on a difference value rather than a sum or individual levels is what creates the immunity to ground offset, as well as the immunity to common-mode noise.

What About Precision Analog?

This is more difficult to generalize and is layout-specific. Typically, if you’re working with low-frequency analog and digital, you can use separate ground if the device can’t be laid out such that return paths do not interfere with each other. Most systems will just have a single PCB ground plane rather than using something like a star ground. I always recommend new designers use a single ground plane because star grounds or using different ground regions improperly encourages bad routing practices, and it encourages a designer to think about how return paths propagate in the board.

In the case of low frequency analog measurements, such as you might do with time-averaged optical measurements, using a dedicated analog ground or differential analog channel may be your only option to prevent noise from interfering with your measurements. This is the approach we take in aerospace sensor measurements, even running on 4-20 mA measurements, or with specialty sensors that do not use a standard interface.

 

PCB grounding PIR sensor

Sensors like this PIR sensor may experience noise from other circuits on the PCB due to interfering return paths. Sometimes, you might not have any choice but to use disconnected grounds.

 

Whenever ESD is a real concern, such as at an I/O or connector, the standard method is to make a connection to GND or PE (protective earth) via the chassis. This would involve using an ESD protection circuit with a TVS diode, a pair of Zener diodes, or some other method. Depending on how this is placed, it will allow the design to withstand high voltage ESD events that easily reverse bias a diode-based ESD circuit, but it will not allow the chassis to earth to be used as the return path for regular signals in the design. This element of ESD protection is vital in high reliability systems, such as in commercial space or medical devices.

 

When you’re ready to start your next high-reliability design, you should work with a design firm that understands PCB grounding and how it affects EMC, noise, and power distribution in your system. NWES helps aerospace OEMs, defense primes, commercial space companies, and industrial equipment companies in multiple industries design modern PCBs and create cutting-edge embedded technology. We've also partnered directly with EDA companies and ITAR-compliant PCB manufacturers, and we'll make sure your next design is fully manufacturable at scale. Contact NWES for a consultation.

 



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