“When designing a PCB, we often rely on previous experience and tips found online. Some rules are general at design time, and some rules can only be used for a specific design.
When designing a PCB, we often rely on previous experience and tips found online. Some rules are general at design time, and some rules can only be used for a specific design.
For example, analog-to-digital converter PCB rules do not apply to RF and vice versa. However, certain guidelines can be considered general to any PCB design. Today, I will introduce some methods and techniques that can significantly improve the basic problems of PCB design.
1. Power and signal distribution
Power distribution is a critical element in any electrical design. All components rely on power to perform their functions. Depending on your design, some components may have good power connections and some components on the same board may have poor power connections.
For example, if all components are powered by one trace, each component will observe a different impedance, resulting in multiple ground references. For example, if you have two ADC circuits, one at the beginning and one at the end, and both ADCs read an external voltage, each analog circuit will read a different potential relative to themselves.
We can summarize the power distribution in three possible ways: single point source, star source, multipoint source.
1. Single point power supply
The power and ground traces of each component are separated from each other, and the power traces of all components only meet at a single reference point, which is considered suitable for power. But for complex or large/medium sized projects this is not feasible.
The star source can be seen as an improvement of the single point source. It differs due to its key characteristic: the same length of traces between components.
Star connections are often used for complex high-speed signal boards with various clocks. In a high-speed signal PCB, the signal usually comes from the edges and then to the center. All signals can go from the center to any area of the board, with delays between areas.
3. Multi-point source
Considered a poor design type in any case, but it’s easy to use in any circuit. Multiple point sources can create reference differences between components, as well as in common impedance coupling.
This design style also allows high-frequency switching ICs, clocks, and RF circuits to introduce noise in nearby circuits that share connections.
Of course, in our daily life, we will not always be able to have a single type of distribution. The compromise we can achieve is to mix single point sources with multi point sources. It is possible to put analog sensitive equipment and high speed/RF systems in one point, while all other less sensitive peripherals are in one point.
Second, the power plane
Have you ever wondered if you should use a power plane? The answer is yes. Power planes are one of the ways to deliver power and reduce any circuit noise.
The power plane shortens the ground path, reduces inductance, and improves electromagnetic compatibility (EMC) performance. Thanks also to the fact that the power planes on both sides also create a parallel plate decoupling capacitor, preventing noise propagation.
The power plane also has a distinct advantage: due to its larger area, it allows more current to pass through, thereby increasing the operating temperature range of the PCB.
It is important to note, however, that power planes can improve operating temperature, but routing must also be considered. The tracking rules are given by IPC-2221 and IPC-9592, for PCBs with RF sources (or any high-speed signal application), it is necessary to have a complete ground plane to improve the performance of the board. The signals must be on different planes, and it is nearly impossible to meet both requirements with a two-layer board.
If you want to design an antenna or any low complexity RF board, you can do it with two layers. The image below shows an illustration of how a PCB can make better use of these planes:
In mixed-signal designs, it is often recommended to separate the analog ground from the digital ground. Sensitive analog circuits are susceptible to high-speed switching and signals, and if the analog and digital grounds are different, the ground planes will be separated.
But there are disadvantages as follows, we should pay attention to the crosstalk and loop areas that divide the ground mainly due to discontinuities in the ground plane.
The figure below shows an example of two separate ground planes. On the left, the return current cannot pass directly along the signal trace, so there is a loop area that is not designed on the right.
3. Electromagnetic Compatibility and Electromagnetic Interference (EMI)
For high frequency designs (such as RF systems), EMI can be a big disadvantage. The ground planes discussed earlier help mitigate EMI, but depending on your PCB, ground planes may introduce other issues. In laminates with four or more layers, the distance to the plane is critical.
When the inter-plane capacitance is small, the electric field will spread across the board. At the same time, the impedance between the two planes is reduced, allowing return current to flow to the signal plane. This will create EMI for any high frequency signals that pass through the plane.
A simple solution to avoid generating EMI is to prevent high-speed signals from traversing multiple layers. Add decoupling capacitors; and place ground vias around signal traces.
The image below shows a good PCB design with high frequency signals:
4. Filter out noise
Bypass capacitors and ferrite beads are devices used to filter noise from any component. Basically, any I/O pin can be a source of noise if used in any high speed application.
In order to make better use of these, we also need to pay attention to the following points:
● Always place ferrite beads and bypass capacitors as close as possible to noise sources;
● When we use automatic placement and automatic routing, the distance should be considered for checking;
● Avoid vias and any other traces between filters and components;
● If there is a ground plane, use multiple vias to properly ground it.
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