Electronic Design - Introduction to the 3 key signal wiring design of pcb

PCB analog signal wiring requirements

1. In order to improve its anti-interference performance, the wiring should be as short as possible.

2. Part of the analog signal can abandon the impedance control requirement, and the wiring can be thickened appropriately.

3. Limit the wiring range, complete the wiring within the analog range as much as possible, and stay away from digital signals.

PCB high-speed signal wiring requirements

1. Multilayer wiring

High-speed signal wiring circuits usually have high integration and high wiring density. The selection of multi-layer boards is not only necessary for wiring, but also a reasonable and effective way to reduce interference. Appropriate selection of the number of superimposed layers can greatly reduce the size of the printed board, can make full use of the intermediate layer to set the shield, can scientifically and reasonably realize the nearest grounding, can reasonably and effectively reduce the parasitic inductance, can reasonably and effectively shorten the signal transmission length, and can greatly Ground to reduce cross-interference between signals, etc.

2. The lead bends as little as possible

The less bending of leads between pins of high-speed circuit devices, the better. The leads of high-speed signal wiring circuit wiring should be fully straight as far as possible and need to be turned. 45° broken line or arc turning can be used. This requirement is only used to improve the fixing strength of the steel foil in low-frequency circuits, while in high-speed circuits, this is met. However, the demand can reduce the external emission and mutual coupling of high-speed signals, and reduce the radiation and reflection of the signals.

3. The shorter the lead, the better

The shorter the lead between the pins of the high-speed signal wiring circuit device, the better. The longer the lead, the larger the distributed inductance and distributed capacitance, which will have a lot of influence on the passage of high-frequency signals of the system. At the same time, it will also change the characteristic impedance of the circuit, causing the system to reflect and oscillate.

4. Alternation between lead layers, the less the better

The fewer the lead layers alternate between the pins of high-speed circuit devices, the better. The so-called "the less the inter-layer alternation of the leads, the better" means that the fewer vias used in the component connection process, the better. It is measured that one via can bring about 0.5pf of distributed capacitance, resulting in a significant increase in the delay of the circuit, and reducing the number of vias can significantly increase the speed.

5. Pay attention to parallel cross interference

High-speed signal wiring should pay attention to the "cross interference" introduced by the short-distance parallel wiring of the signal lines. If the parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce the interference.

6. Avoid branches and stumps

High-speed signal wiring should avoid branching or stub formation as much as possible. Tree stumps have a great influence on impedance, which can cause signal reflection and overshoot. Therefore, we should usually avoid tree stumps and branches when designing. Choosing daisy chain wiring will reduce the influence on the signal.

7. The signal line should go to the inner layer as much as possible

High-frequency signal lines on the surface are likely to cause large electromagnetic radiation, and are also susceptible to external electromagnetic radiation or interference from various factors. Wire the high-frequency signal line between the power supply and the ground wire, and the radiation caused by the electromagnetic wave absorption by the power supply and the bottom layer will be much reduced.

PCB clock signal wiring requirements

In digital circuit design, a clock signal is a signal that oscillates between a high state and a low state, which affects the performance of the circuit. The clock circuit plays an important role in the midpoint of the digital circuit, and at the same time it is the main source of electromagnetic radiation. The clock processing method also needs to pay special attention to the PCB circuit board wiring. At the beginning, sort out the clock tree and clarify the relationship between various clocks, and you can handle it better at the time of routing. In addition, clock signals are often a difficult point in EMC design. Pay special attention to items that require EMC test indicators.

In addition to the conventional impedance control and equal length requirements of the clock line, the following issues must be paid attention to:

1. Select the optimal wiring layer as much as possible for the clock signal.

2. The clock signal should not cross the division as much as possible, let alone route along the division area.

3. Pay attention to the distance between the clock signal and other signals, at least 3W.

4. For the design with EMC requirements, the inner layer wiring should be selected as much as possible when the line is longer.

5. Pay attention to the termination match of the clock signal.

6. Do not use a daisy chain structure to transmit clock signals, but a star structure, that is, all clock loads are directly connected to the clock power driver.

7. All wires connected to the input/output terminals of the crystal oscillator should be as short as possible to reduce noise interference and the influence of distributed capacitance on the crystal oscillator.

8. The crystal capacitor ground wire should be connected to the device with the widest and shortest wire possible; the digital ground pin closest to the crystal should be as small as possible.

9. In digital circuits, the usual PCB clock signals are signals with fast edge changes, which have high external crosstalk. Therefore, in PCB design, it is advisable to surround the clock line with a ground line and use more ground lines to reduce distributed capacitance, thereby reducing crosstalk; for high-frequency signal clocks, use low-voltage clock signals and wrap the ground as much as possible, and pay attention to the package. The integrity of the ground hole.