Printed circuit board ground wire design
At present, electronic equipment is used in various electronic devices and systems, and printed circuit boards are still the main assembly method. Practice has proved that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the electronic device. For example, if the two thin parallel lines of the printed board are in close proximity, a delay in the signal waveform is formed, and reflected noise is formed at the end of the transmission line. Therefore, when designing a printed circuit board, care should be taken to use the correct method.
In electronic equipment, grounding is an important method of controlling interference. If the grounding and shielding are properly combined, most of the interference problems can be solved. The ground wire structure in the electronic device is roughly systematic, chassis ground (shielded ground), digital ground (logically), and analog ground. Pay attention to the following points in the ground line design:
1. Correctly select single point grounding and multi-point grounding
In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and the influence of the inductance between the wiring and the device is small, and the circulating current formed by the grounding circuit has a great influence on the interference, so a grounding should be adopted. When the signal operating frequency is greater than 10MHz, the ground line impedance becomes very large. At this time, the ground line impedance should be reduced as much as possible. When the operating frequency is between 1 and 10 MHz, if a grounding is used, the grounding length should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be used.
2. Separate the digital circuit from the analog circuit
The circuit board has both high-speed logic circuits and linear circuits. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they are connected to the power ground. Try to increase the grounding area of the linear circuit.
3. Try to thicken the ground wire
If the grounding wire is very thin, the grounding potential changes with the change of the current, causing the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass the three allowable currents on the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.
4. The grounding line constitutes a closed loop
When designing the grounding system of a printed circuit board consisting only of digital circuits, making the grounding line into a closed loop can significantly improve the noise immunity. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are many power-consuming components, due to the limitation of the grounding wire thickness, a large potential difference will be generated on the ground junction, causing the noise resistance to decrease. If the ground structure is looped, the potential difference will be reduced to improve the noise immunity of the electronic device.