Microwave Tech - How does the circuit board design standardize the ground wire design to control interference?​

In electronic equipment, grounding is an important method to control interference. If the ground wire design can be standardized in the circuit board design and the grounding and shielding can be used in combination, most of the interference problems can be solved. The ground structure of electronic equipment roughly includes system ground, chassis ground (shield ground), digital ground (logical ground), and analog ground.

1. Correctly choose single-point grounding and multi-point grounding

In the low-frequency circuit, the working frequency of the signal is less than 1MHz, its wiring and the inductance between the devices have little influence, and the circulating current formed by the grounding circuit has a greater influence on the interference, so one point grounding should be adopted. When the signal operating frequency is greater than 10MHz, the ground wire impedance becomes very large. At this time, the ground wire impedance should be reduced as much as possible, and the nearest multiple points should be used for grounding. When the operating frequency is 1~10MHz, if one-point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be used. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and thick, and a grid-like large-area grounding copper foil should be arranged around the high-frequency component as much as possible.

2, separate the digital circuit from the analog circuit

There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they should be connected to the ground wires of the power supply terminal. Try to increase the grounding area of the linear circuit as much as possible.

3. Make the ground wire as thick as possible

If the ground wire is very thin, the ground potential will change with the current change, 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 three times the allowable current of the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

4, the ground wire of a single-layer PCB

In a single-layer (single-sided) PCB, the width of the ground wire should be as wide as possible, and should be at least 1.5mm (60mil). Since star wiring cannot be implemented on a single-layer PCB, the change in jumper and ground wire width should be kept to a minimum, otherwise it will cause changes in line impedance and inductance.

5, the ground wire of the double-layer PCB

In double-layer (double-sided) PCBs, ground grid/dot matrix wiring is preferred for digital circuits. This wiring method can reduce ground impedance, ground loops, and signal loops. As in a single-layer PCB, the width of the ground and power lines should be at least 1.5mm

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Another layout is to put the ground plane on one side and the signal and power lines on the other side. In this arrangement, the ground loop and impedance will be further reduced. At this time, the decoupling capacitor can be placed as close as possible between the IC power supply line and the ground plane

6. the copper filling of the ground

In some analog circuits, the unused circuit board area is covered by a large ground plane to provide shielding and increase decoupling capabilities. But if the copper area is suspended (for example, it is not connected to the ground), then it may behave as an antenna and will cause electromagnetic compatibility problems