PCB Technical - PCB circuit and circuit anti-interference measures

The anti-interference design of the PCB circuit board is closely related to the specific circuit. Here, only a few common measures of the PCB anti-interference design are explained.

1. Power cord design

According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.

2. The principle of ground wire design

(1) The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and leased, and the grid-like large-area ground foil should be used around the high-frequency component as much as possible.

(2) The grounding wire should be as thick as possible. If the ground wire uses a very tight line, the ground potential changes with the change of the current, which reduces the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be 2~3mm or more.

(3) The grounding wire forms a closed loop. For printed boards composed only of digital circuits, most of their grounding circuits are arranged in loops to improve noise resistance.

3. Decoupling capacitor configuration

One of the conventional methods of PCB design is to configure appropriate decoupling capacitors on each key part of the printed board. The general configuration principles of decoupling capacitors are:

(1) Connect a 10~100uf electrolytic capacitor across the power input. If possible, it is better to connect to 100uF or more.

(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the gap of the printed board is not enough, a 1-10pF tantalum capacitor can be arranged for every 4~8 chips.

(3) For devices with weak anti-noise ability and large power changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.

(4) Capacitor leads should not be too long, especially for high-frequency bypass capacitors.

(5) When there are contactors, relays, buttons and other components in the printed board. When operating them, large spark discharges are generated, and RC circuits must be used to absorb the discharge current. Generally, R is 1~2K, and C is 2.2~47UF.

(6) The input impedance of CMOS is very high and it is susceptible to induction, so when in use, the unused terminal should be grounded or connected to a positive power supply.

Five, PCB wiring principles

In PCB design, wiring is an important step to complete product design. It can be said that the previous preparations are done for it. In the entire PCB, the wiring design process is the most limited, the skills are the smallest, and the workload is the largest. PCB wiring includes single-sided wiring, double-sided wiring and multilayer wiring. There are also two ways of wiring: automatic wiring and interactive wiring. Before automatic wiring, you can use interactive to pre-wire the more demanding lines. The edges of the input end and the output end should be avoided adjacent and parallel to avoid reflection interference. If necessary, ground wire should be added for isolation, and the wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is easy to occur in parallel.

The routing rate of automatic routing depends on a good layout. The routing rules can be preset, including the number of bending times, the number of vias, and the number of steps. Generally, explore the warp wiring first, quickly connect the short wires, and then perform the labyrinth wiring. First, the wiring to be laid is optimized for the global wiring path. It can disconnect the laid wires as needed. And try to re-wire to improve the overall effect.

The current high-density PCB design has felt that the through-hole is not suitable for it. It wastes a lot of valuable wiring channels. In order to solve this contradiction, blind and buried hole technologies have emerged, which not only fulfill the role of vias, but also It also saves a lot of wiring channels to make the wiring process more convenient, smoother and more complete. The PCB board design process is a complex and simple process. To master it well, a large number of electronic engineering designers are required. Only by experiencing it yourself can you get the true meaning of it.

1 Treatment of power supply and ground wire

Even if the wiring in the entire PCB board is completed very well, the interference caused by the improper consideration of the power supply and the ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the power supply and the ground wire must be taken seriously, and the noise interference generated by the power supply and the ground wire should be minimized to ensure the quality of the product.

Every engineer engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is described:

2 Common ground processing of digital circuit and analog circuit

Many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground wire.

The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated and they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.

3 The signal line is laid on the electric (ground) layer

In the multi-layer PCB wiring, because there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste and increase a certain amount of work in production, and the cost will increase accordingly. To solve this Contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.

4 Treatment of connecting legs in large area conductors

In large-area grounding (electricity), the legs of common components are connected to it. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding and assembly of components, such as: 1. Welding requires high-power heaters. 2. It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, called heat shields, commonly known as thermal pads (Thermal), so that virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multi-layer PCB board is the same.