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PCB News - Knowledge about chassis ground, digital ground

PCB News

PCB News - Knowledge about chassis ground, digital ground

Knowledge about chassis ground, digital ground

2021-11-04
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Author:Kavie

Knowledge about chassis ground, digital ground, etc.


PCB


Q1: Why is it grounded?
Answer: The introduction of grounding technology was originally a protective measure to prevent electrical or electronic equipment from being struck by lightning. The purpose was to introduce the lightning current generated by lightning to the ground through the lightning rod, thereby protecting the building. At the same time, grounding is also an effective means to protect personal safety. When the phase wire caused by some reason (such as poor wire insulation, line aging, etc.) touches the equipment shell, the equipment shell will have dangerous voltages. The generated fault current will flow through the PE line to the ground, thereby playing a protective role. With the development of electronic communication and other digital fields, it is no longer sufficient to consider only lightning protection and safety in the grounding system. For example, in a communication system, the interconnection of signals between a large number of devices requires each device to have a reference ground as the signal reference ground. And with the complexity of electronic equipment, the signal frequency is getting higher and higher. Therefore, in the grounding design, special attention must be paid to electromagnetic compatibility issues such as mutual interference between signals. Otherwise, improper grounding will seriously affect the reliability of system operation. Sex and stability. Recently, the concept of "ground" has also been introduced in the signal return technology of high-speed signals.

Q2: Definition of grounding
Answer: In modern grounding concepts, for line engineers, the meaning of this term is usually'reference point of line voltage'; for system designers, it is often a cabinet or rack; for electrical engineers, it is It means green safety ground wire or connected to the earth. A more general definition is "grounding is a low-impedance path for current to return to its source". Note that the requirements are "low impedance" and "pass
road".

Q3: Common grounding symbols
Answer: PE, PGND, FG-protective ground or chassis; BGND or DC-RETURN-DC-48V (+24V) power supply (electrical
Pool) return flow; GND-working ground; DGND-digital ground; AGND-analog ground; LGND-lightning protection ground

Q4: Appropriate grounding method
Answer: There are many grounding methods, including single-point grounding, multi-point grounding and mixed grounding. The single-point grounding is divided into series single-point grounding and parallel single-point grounding. Generally speaking, single-point grounding is used for simple circuits, grounding distinctions between different functional modules, and low-frequency (f<1MHz) electronic circuits. When designing high-frequency (f>10MHz) circuits, multi-point grounding or multilayer boards (complete ground plane layers) should be used.

Q5: Introduction to signal reflow and cross-segmentation
Answer: For an electronic signal, it needs to find a path for the lowest impedance current to return to the ground, so how to deal with this signal return becomes very critical.
First, according to the formula, it can be known that the radiation intensity is proportional to the loop area, that is, the longer the path that reflow needs to take, the larger the loop formed, and the greater the interference with external radiation. Therefore, when the PCB is laid out It is necessary to reduce the area of the power circuit and signal circuit as much as possible.
Second, for a high-speed signal, providing good signal return can ensure its signal quality. This is because the characteristic impedance of the transmission line on the PCB is generally calculated based on the ground layer (or power layer). There is a continuous ground plane nearby so that the impedance of this line can remain continuous. If there is no ground reference near a segment of the line, the impedance will change. Discontinuous impedance will affect the integrity of the signal. Therefore, when wiring, the high-speed line should be allocated to the layer close to the ground plane, or one or two ground lines should be walked beside the high-speed line to play the function of shielding and providing nearby return.
Third, why do we try not to split across the power supply when wiring? This is also because after the signal crosses different power layers, its return path will be very long and it will be easily interfered. Of course, it is not strictly required that the power supply cannot be divided. It is possible for low-speed signals, because the interference generated can be ignored compared to the signal. For high-speed signals, check carefully and try not to cross over. You can adjust the wiring of the power supply part. (This is for the multi-layer board multiple power supply situation)

Q6: Why should I separate the analog ground from the digital ground, and how to separate it?
Answer: Both analog and digital signals return to the ground. Because the digital signal changes quickly, the noise caused on the digital ground will be very large. The analog signal requires a clean ground reference. If the analog ground and digital ground are mixed together, noise will affect the analog signal.
Generally speaking, the analog ground and digital ground should be treated separately, and then connected together through a thin trace, or connected together at a single point. The general idea is to try to prevent the noise on the digital ground from fleeing to the analog ground. Of course, this is not a very strict requirement that the analog ground and digital ground must be separated. If the digital ground near the analog part is still very clean, it can be combined.

Q7: How to ground the signal on the PCB board?
Answer: For general devices, the nearest grounding is the best. After adopting a multi-layer board design with a complete ground plane, it is very easy to ground general signals. The basic principle is to ensure the continuity of the wiring and reduce the excess Number of holes; close to the ground plane or power plane, etc.

Q8: How to ground the interface components of the board?
Answer: Some boards have external input and output interfaces, such as serial port connectors, network port RJ45 connectors, etc. If their grounding is not designed well, it will also affect normal operation, such as network port interconnection errors., Packet loss, etc., and will become an external electromagnetic interference source, sending the noise in the board to the outside. Generally speaking, an independent interface ground will be separated separately, and the connection with the signal ground will be connected by a thin trace, and a 0 ohm or small resistance resistor can be connected in series. Thin traces can be used to block signal ground noise from passing to the interface ground. Similarly, the filtering of the interface ground and interface power should be carefully considered.

Q9: How to ground the shielding layer of the cable with shielding layer?
Answer: The shielding layer of the shielded cable should be connected to the interface ground of the board instead of the signal ground. This is because there are various noises on the signal ground. If the shielding layer is connected to the signal ground, the noise voltage will drive the common mode current along the shielding layer. External interference, so poorly designed cables are generally the largest noise output source of electromagnetic interference. Of course, the premise is that the interface is also very clean.

Q10: The signal ground of the PCB and the system ground of the chassis should be connected together and then connected to the ground?
Answer: The signal ground of the PCB and the system ground of the chassis must be connected together at the end, and then connected to the ground through the ground wire of the power cord.

The above is an introduction to the knowledge of chassis ground, digital ground and other places. Ipcb also provides PCB manufacturers and PCB manufacturing technology.