PCB board wiring
The rules of PCB wiring are as follows:
1) line length
The copper film line should be as short as possible, especially in high frequency circuits. The corners of copper film wires shall be rounded or beveled, while the right angle or sharp angle will affect the electrical performance in the case of high frequency circuit and wiring density. When double sided wiring is made, the wires on both sides should be perpendicular to each other, skewed or bent to avoid parallel lines, so as to reduce parasitic capacitance.
2) linewidth
The width of copper wire should be based on the requirement of electrical characteristics and convenience for production. Its minimum value depends on the current flowing through it, but generally it should not be less than 0.2mm. As long as the plate area is large enough, the best choice for the width and spacing of copper film line is 0.3mm. In general, the line width of 1~1.5mm allows current flowing through 2A. For example, it is better to select a linewidth larger than 1mm for ground wire and power wire. When two wires are laid between pads of integrated circuit base, the pad diameter is 50mil, the line width and line spacing are 10mil. When one wire is laid between pads, the pad diameter is 64mil, and the line width and line spacing are 12mil. Pay attention to the conversion between the metric system and the British system, 100mil=2.54mm.
3) line spacing
The distance between adjacent copper wire lines should meet the electrical safety requirements, and for the convenience of production, the spacing should be wider and better. The minimum spacing shall be able to withstand the peak value of applied voltage at least. When the wiring density is low, the spacing should be as large as possible.
4) shielding and grounding
The common ground wire of the copper film wire shall be placed on the edge part of the circuit board as far as possible. The copper foil should be reserved as much as possible on the PCB as the ground wire, so as to enhance the shielding ability. In addition, the shape of the ground wire is preferably circular or grid shaped. Multilayer PCB board can be used as a power supply and ground wire special layer, so it can play a better shielding effect.
PCB plate grounding
1. Common impedance interference of ground wire
The ground wire on the circuit diagram represents the zero potential in the circuit and is used as the common reference point of other points in the circuit. In the actual circuit, due to the existence of ground wire (copper film wire) impedance, it will inevitably bring common impedance interference. Therefore, when wiring, the points with ground wire symbols cannot be connected together at any time, which may cause harmful coupling and affect the normal operation of the circuit.
2. How to connect the ground wire
Usually, in an electronic system, the ground wire is divided into several parts, namely, the system, the chassis (shielding), the digital ground (logic) and the analog ground.
1) correctly select single point to ground and multi-point grounding.
In the low frequency circuit, the signal frequency is less than 1MHz, the inductance between the wiring and the components can be ignored, while the voltage drop produced by the resistance of the ground circuit has a great influence on the circuit, so the single point grounding method should be used. When the frequency of the signal is greater than 10MHz, the influence of the inductance of the ground wire is greater, so the multi-point grounding method of the nearest grounding should be adopted. When the signal frequency is between 1~10MHz, if the single point grounding method is adopted, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise multi-point earthing should be adopted.
2) digital and analog separation
There are both digital circuits and analog circuits on the PCB, so they should be separated as far as possible, and the ground wire should not be mixed, and should be connected with the ground wire end of the power supply respectively (preferably the power supply end should also be connected separately). The area of linear circuit should be increased as much as possible. The noise tolerance of TTL circuit is 0.4 ~ 0.6V, and that of CMOS digital circuit is 0.3 ~ 0.45 times of the power supply voltage. However, if there is microvolt level noise in analog circuit, it is enough to make it work abnormally. Therefore, the two types of circuits should be arranged and wired separately.
3) try to polish the ground as much as possible.
If the ground wire is very thin, the grounding potential will change with the change of the current, resulting in the interference of the signal of the electronic system, especially the analog circuit part. Therefore, the ground wire should be as wide as possible, generally more than 3mm.
4) make the ground wire into a closed loop
When there are only digital circuits on the PCB, the ground wire should form a loop, which can significantly improve the anti-interference ability, because when there are many integrated circuits on the PCB, if the ground wire is very thin, it will cause a large ground potential difference, and the ring ground wire can reduce the ground resistance, thus reducing the ground potential difference.
5) grounding of the same level circuit.
The location of the same level circuit should be as close as possible, and the power filter capacitor of the circuit should also be connected to the grounding point of the same level.
6) grounding method of general ground wire
The total earth wire must be strictly connected with the high frequency, medium frequency and low frequency order from the weak current to the strong electricity. The high-frequency part is better to adopt large area surrounding ground wire to ensure good shielding effect.
PCB anti-interference
The electronic system with microprocessor, anti-interference and electromagnetic compatibility is a problem that must be considered in the design process, especially for systems with high clock frequency and fast bus cycle; systems containing large power and large current driving circuits; and systems with weak analog signals and high-precision A/D conversion circuits. In order to increase the anti electromagnetic interference ability of the system, the following measures shall be taken:
1) select microprocessor with low clock frequency
As long as the performance of the controller can meet the requirements, the lower the clock frequency, the better. Low clock can effectively reduce the noise and improve the anti-interference ability of the system. Because the square wave contains all kinds of frequency components, its high frequency components can easily become noise sources. Generally speaking, the high frequency noise with 3 times clock frequency is the most dangerous.
2) reduce the distortion in signal transmission
When the high-speed signal (signal frequency high = signal with fast rising edge and falling edge) is transmitted on the copper film line, the signal will be distorted due to the influence of the inductance and capacitance of the copper film line, when the distortion is too high
