In most electronic systems, noise reduction is a critical design issue. Similar to challenges like power constraints, environmental temperature changes, size restrictions, and requirements for speed and accuracy, managing ubiquitous noise factors is essential for successful final designs.
Focus on Grounding for High-Frequency Applications
We do not consider techniques to reduce “external noise” (noise that arrives with the signal into the system) because it is generally beyond the direct control of design engineers. In contrast, preventing “internal noise” (noise generated or coupled within the circuit or system) from disrupting the signal is the direct responsibility of design engineers. Today, we will discuss “grounding,” specifically targeting high-frequency operations.
The Role of Grounding
Grounding typically involves connecting a circuit, device, or system to a good conductor that acts as a reference potential point or surface, creating a low impedance path between the circuit/system and “ground.”
Grounding Interference Types
Ground wires serve as a potential reference for circuits or systems. However, any conductor has impedance. When current flows through ground wires, voltage exists across them due to Ohm’s law, indicating that these are not equipotential bodies. This results in ground interference, which can significantly impact digital circuit performance by introducing voltage differences across grounding points.
As mentioned above, ground wires, being conductors, possess certain impedances. Impedance, by definition, is composed of two parts: resistance and reactance.
Impedance and Interference in Ground Wires for High-Speed Digital Circuits
The impedance of conductors is a function of frequency; as the frequency increases, the impedance rises quickly. For high-speed digital circuits, the clock frequency is very high, and the pulse signals contain a rich high-frequency component, thus producing significant voltage across ground wires. This makes the impedance of the ground wires a considerable source of interference in digital circuits.
In the PCB design of electronic products, suppressing or preventing ground wire interference is one of the main issues to consider. Interference typically occurs between different circuit units, components, or systems, and ground wire interference refers to signal disturbances caused by using a common ground wire. The signals mentioned here usually refer to alternating signals or transition signals. There are many forms of ground wire interference, which are generally categorized into three types: ground loop interference, common impedance interference, and electromagnetic coupling in ground loops. The diagram below clearly illustrates the causes of these three types of ground wire interference.
Types of Ground Wire Interference in PCBs
1. Ground Loop Interference: In this type, different currents flow through each conductor, creating differential mode voltages that impact the circuit. Specifically, the ground current from “other circuit unit B” in the ground loop formed by points J, N, L, and M affects amplifiers A1 and A2. This interference is caused by the loop current formed by the cable and ground wire, hence, it is termed ground loop interference.
2. Electromagnetic Coupling in Ground Loops: On the actual PCB, the ground loop formed by J, N, L, and M encircles a certain area. According to the law of electromagnetic induction, if there is a changing magnetic field within the area enclosed by this loop, it will induce currents within the loop, creating interference. The larger the area enclosed, the more severe the interference due to changes in the surrounding magnetic field.
3. Common Impedance Interference: Upon careful examination of the circuit structure shown in the diagram, it’s observed that one of the connections among J, N, L, and M is redundant. Removing any one of these can still satisfy the connectivity requirements of the grounding points and eliminate the ground loop. The decision on which connection to remove leads to considering another type of interference—common impedance interference.
Evaluating Options for Reducing Ground Loop Interference
1. Remove J: This is the worst option. After removing J, the ground loop appears to disappear, but a more problematic loop forms (I, N, L, M), with I being a signal line, thus making the interference worse than when J was included.
2. Remove M: The loop disappears, but we find that the ground current for amplifier A2 now needs to flow through J and N to reach the ground zero point. Note that segment N is the common ground line for A1 and A2, so the voltage drop caused by A2’s ground current on N adds interference to A1. This type of interference, caused by sharing a segment of ground wire, is known as “common impedance interference.”
3. Remove L: This does not solve the common impedance interference between A2 and A1 and instead introduces a common impedance interference issue between “unit B circuit” and A1, A2.
4. Remove N: This appears to be the last option. Doing so would make M the “common impedance” for A1 and A2, also creating interference. However, there are still problems! However, it’s noted that the interference in this case is from A1 to A2, and since A2 operates at a much higher signal strength than A1, the interference from A1 is unlikely to have adverse effects.
The most rational wiring approach is to remove N and then connect the lower end of M directly to the “ground signal zero point.”
Grounding Techniques in PCB Design for Electronic Products
The above explains the causes of grounding interference. Below, we will introduce several common grounding methods. In PCB design for electronic products, suppressing or preventing grounding interference is a primary concern.
Single-point grounding means connecting all the circuits and devices in a system to one reference ground point, which serves as the zero potential reference for everything. This type of grounding can be divided into two forms: series single-point grounding and parallel single-point grounding.
Series Single-Point Grounding: This method connects all grounds in a series to a single point. If the circuit carries high power, it can cause significant current to return through the ground, creating voltage drops due to the limited impedance. This drop can lead to voltage differences between the circuit and the reference ground, potentially disrupting how the system functions. This method is not suitable for circuits with varying power levels, as high-power circuits can adversely affect low-power devices. The most sensitive circuits should be placed near the power input and away from low-power components. If the grounding wires are short and the impedance is low, this method can be effective if the grounding levels of all circuits are similar.
Parallel Single-Point Grounding: In this setup, each circuit or device has its own ground wire connected to the same ground point. The advantage is that each circuit’s ground only carries its own current and impedance, unaffected by other circuits. This is effective at low frequencies to prevent low impedance interference between circuits. However, it has drawbacks, mainly the need for multiple ground wires, which increases the total length and impedance of the ground wires, complicating the structure and making it less suitable for high-frequency circuits due to increased impedance, inductance, and capacitance.
Multi-point grounding: It involves connecting each circuit or device that needs to be grounded directly to the nearest ground plane. This setup minimizes grounding length and reduces ground impedance.
Why Use Multi-Point Grounding?
When an electronic system operates above 1 MHz, the working wavelength becomes comparable to the length of the ground leads. In this situation, the ground wire acts like a short-circuited transmission line, where the currents and voltages distribute in standing waves, making the ground wire behave like a radiating antenna instead of just a ground conductor. To prevent this and reduce ground impedance, the length of the ground wires should be less than 1/20th of the wavelength.
Advantages and Drawbacks:
Multi-point grounding reduces the phenomenon of high-frequency standing waves on the ground lines, simplifying the circuit structure. However, it can lead to many ground loops within the device, which can cause interference in sensitive parts of the system. Generally, single-point grounding is preferable for frequencies below 1 MHz, multi-point grounding is suitable for frequencies above 10 MHz, and a hybrid grounding approach is often used for frequencies between 1 and 10 MHz. This helps optimize grounding effectiveness across different operating conditions.
Hybrid grounding: It combines elements of both single-point and multi-point grounding techniques. It’s commonly used in PCBs that operate with a mix of high and low frequencies.
How It Works:
- In low-frequency applications, the grounding structure resembles a single-point grounding, where all circuits are grounded to a single reference point.
- At high frequencies, the system shifts to a multi-point grounding setup. This shift is due to capacitors that divert the high-frequency currents to the ground, helping to manage the distribution effectively.
Advantages and Key Considerations:
- The success of hybrid grounding depends on clearly understanding the frequencies in use and the anticipated flow of grounding currents.
- Using capacitors and inductors in the grounding topology allows for optimized control of RF (radio frequency) currents. This careful control helps to route PCB layouts efficiently, preventing issues related to RF loop misunderstandings, which could lead to emissions or sensitivity problems.
Visual Guides:
- Diagrams (referred to as Figures 5 and 6 in the original document) illustrate the two methods of hybrid grounding, providing a clear visual understanding of how the grounding shifts between the two modes based on frequency changes.
Floating Grounding: It refers to the electrical insulation of a device’s ground system from its chassis or casing to prevent electromagnetic interference (EMI) from being conducted into the device. However, because the device is not connected to a common ground, this type of grounding can lead to the accumulation of static electricity. When enough charge builds up, the potential difference between the device and the common ground can cause severe electrostatic discharge (ESD), generating disruptive discharge currents. Floating grounding is not suitable for communication systems where stable and consistent grounding is crucial.
Practical Tips for PCB Grounding
When designing PCBs based on the theories of grounding, it is important to layout the ground lines thoughtfully, keeping in mind the following points:
Separate Digital and Analog Grounds: Digital and analog circuits should have separate grounds to prevent noise from digital circuits affecting the more sensitive analog circuits.
Avoid Closed Loops in Digital Grounds: Digital circuit grounds should not form closed loops, which can act as antennas and pick up or radiate interference.
Layer Placement in Multilayer PCBs: In multilayer PCBs, try to place the ground layers and power layers adjacent to each other. This setup minimizes the loop area between the power supply and its return path, reducing the impedance and the susceptibility to interference.
Key Points on PCB Design for High-Speed Circuits
Appropriate Wire Widths: It’s essential to design the widths of ground wires, power wires, and signal wires appropriately. This helps to manage the impedance and minimize noise and interference effectively.
High-Speed Circuit Design: Proper grounding is one of the most effective techniques for electromagnetic compatibility (EMC) in high-speed circuits. Incorrect wiring and grounding are responsible for about 90% of EMC problems. Good wiring and grounding not only improve noise resistance but also reduce interference emissions, potentially solving many electromagnetic interference issues at lower costs.
Ground Layer Use: It’s generally recommended that both power and signal currents return through the ground layer. This layer also serves as a reference point for converters, voltage sources, and other subcircuits. However, even with widespread use of the ground layer, it doesn’t guarantee a high-quality ground reference for AC circuits.
Understanding Current Flow in Ground Layers: Knowing how currents flow from one via to another in the ground layer helps identify real-world issues and eliminate high-frequency grounding noise. The inductance and the area of the current loop are directly proportional; a larger loop area means greater inductance, storing more magnetic energy and thereby increasing impedance, leading to higher voltage generation at given frequencies.
Avoiding Layout Problems: Once the current return paths in the ground layer are understood, common layout issues can be identified and rectified. For instance, key paths should be kept the shortest, away from digital lines, and without vias. If a path needs to cross another, it may require cutting the ground layer below and rerouting through vias, but this can introduce inductance due to the increased loop area. A simple remedy for this is to add a wire over the cut in the ground layer to maintain a small loop area.
Power Line Interference: The characteristic impedance of power lines should be as low as possible to reduce inductance and increase capacitance. Strategically placing bypass capacitors at critical locations can further enhance capacitance. For instance, placing a 0.1 µF capacitor at the power pin can significantly reduce its impedance, leading to a damping oscillation of about 3 MHz after each transient.
