According to the rule of thumb, four-layer boards are usually used in high density and high frequency occasions, and are more than 20 DB better than two-layer boards in terms of EMC. In the four-layer board condition, it is often possible to use a complete ground plane and a complete power plane, in this condition only need to be divided into several groups of circuit ground and ground plane connection, and the working noise will be special treatment.
There are many practices that can be taken to connect the ground to the ground plane from the ground lines of individual circuits, including:
Single point and multi-point grounding
① Single point grounding: The ground wire of all circuits is connected to the same point of the ground plane, which is divided into series single point grounding and parallel single point grounding.
② Multi-point grounding: the ground wire of all circuits is grounded nearby, and the ground wire is very short and suitable for high-frequency grounding.
③ Hybrid grounding: the single point grounding and multi-point grounding are mixed.
In low frequency, low power and the same power layer, single point grounding is the most suitable, usually used in analog circuits; In this case, a star connection is generally used to reduce the possible effect of series impedance. High frequency digital circuits need to be grounded in parallel, here generally through the way of the ground hole can be relatively simple processing, generally all modules will use two grounding methods, using a hybrid grounding method to complete the circuit ground and ground plane connection.
Hybrid grounding system
If you do not choose to use the entire plane as a common ground wire, such as when the module itself has two ground wires, you need to split the ground plane, which often interacts with the power plane. Pay attention to the following principles:
① Align each plane to avoid overlapping between unrelated power plane and ground plane, otherwise it will lead to the failure of all ground plane segmentation and interference between each other;
② In the case of high frequency, the interlayer parasitic capacitance through the circuit board will produce coupling;
③ The signal lines between the ground planes (such as the digital ground plane and the analog ground plane) are connected using the ground bridge, and the nearest return path is configured through the nearest through hole.
④ Avoid running high frequency lines such as clock lines near the isolated ground plane, causing unnecessary radiation.
⑤ The ring area of the signal line and its loop is as small as possible, also known as the loop minimum rule; The smaller the ring area, the less external radiation, the less interference received from the outside. In the ground plane segmentation and signal routing, the distribution of ground plane and important signal routing should be taken into account to prevent the problems caused by the ground plane slot.
The connection method between the ground, refer to the practice of some articles, here for some sorting.
① The use of this method can ensure reliable low-impedance conduction between the two ground wires, but only the connection between the low-frequency signal circuit.
② Large resistance connection between the ground: the characteristics of the large resistance is that once the pressure difference appears at both ends of the resistance, it will produce a very weak conduction current, and after the charge on the ground wire is discharged, the pressure difference at both ends is finally zero.
③ Capacitive connection between the ground: the characteristics of the capacitor are DC cut-off and AC conduction, which are used in floating systems.
④ The magnetic bead connection between the ground: the magnetic bead is equivalent to a resistance that changes with the frequency, and it shows the resistance characteristics. It is applied to the ground to ground of weak signals with fast small current fluctuations.
⑤ The inductance between the ground: the inductance has the characteristics of inhibiting the change of circuit state, and can be cut and filled, which is usually used between two ground and ground with large current fluctuations.
⑥ Small resistance connection between the ground: the small resistance increases a damping, hindering the overshoot of the rapid change of the ground current; When the current changes, the rising edge of the impulse current is slowed down.
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