Currently, electronic devices are used in various types of electronic equipment and systems still use printed circuit boards as the main assembly method. Practice has proven that even if the circuit schematic design is correct, improperly designed printed circuit boards can have a negative impact on the reliability of electronic equipment. For example, if two thin parallel lines of the printed circuit board are close together, a delay in the signal waveform will be formed and reflected noise will be formed at the terminal of the transmission line. Therefore, it is recommended that when designing printed circuit boards, attention should be paid to grounding as an important method of controlling interference.
If grounding and shielding can be used in the right combination, most interference problems can be solved. The ground structure in electronic devices is roughly system ground, chassis ground (shield ground), digital ground (logic ground) and analog ground. In the ground design should pay attention to the following points.
1. The correct choice of single-point grounding and multi-point grounding in the low-frequency circuit, the signal operating frequency is less than 1MHz, its wiring and inductance between the devices have less impact, while the grounding circuit to form a loop current on the interference impact is greater, and thus a point of grounding should be used. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large, at this time should try to reduce the ground impedance, should be used near the multi-point grounding. When the operating frequency in 1 ~ 10MHz, if a point of grounding, the ground length should not exceed 1/20 of the wavelength, otherwise, 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 far as possible, and the ground of the two do not mix, respectively, with the power end of the ground connected. To maximize the grounding area of the linear circuit.
3. Try to thicken the ground line if the ground line is very thin, the ground potential is changing with the change of current, resulting in electronic equipment timing signal level instability, anti-noise performance becomes bad. Therefore, the grounding wire should be as thick as possible, so that it can be located in the printed circuit board through the three allowable currents. If possible, the width of the grounding wire should be greater than 3mm.
4. will constitute a closed-loop grounding line design only by the digital circuit of the printed circuit board ground system, the grounding line into a closed loop can significantly improve the noise immunity. The reason is that: the printed circuit board has many integrated circuit components, especially in the case of more power-consuming components, due to the limitations of the thickness of the ground wire, will produce a large potential difference in the ground junction, causing a decrease in noise immunity, if the ground structure into a loop, it will reduce the potential difference value, improve the noise immunity of electronic equipment.