As a rule of thumb, four-ply boards are usually used in high density and high frequency situations and are more than 20 DB better than two-ply boards in terms of EMC. It is often possible to use a complete ground plane and a complete power plane under the condition of four layers. In this condition, only the ground plane of the circuits divided into several groups is connected to the ground plane, and the working noise is specially treated.
There are many practices that can be taken to connect to the ground plane from the ground of various circuits, including:
Single-point and multipoint grounding modes
(1) Single point grounding: the ground 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 of all circuits is nearby grounding, grounding is very short suitable for high-frequency grounding.
(3) Mixed grounding: single point grounding and multi-point grounding are used together.
In low frequency, low power and the same power layer, single point grounding is the most suitable, usually used in analog circuits; Here, star connection is generally adopted to reduce the possible influence of series impedance, as shown in the right half of FIG. 8.1. High frequency digital circuits need to be grounded in parallel, here generally through the way of ground holes can be relatively simple processing, as shown in the left half of the figure; Generally, all modules will use a combination of the two grounding modes, using the mixed grounding mode to complete the connection between the circuit ground and the ground plane.
Hybrid grounding system
If you do not choose to use the whole plane as a common ground wire, for example, when the module itself has two ground wires, you need to segment the ground plane, which often interacts with the power plane. Pay attention to the following principles:
(1) All planes should be aligned to avoid overlapping of irrelevant power plane and ground plane, otherwise all ground plane segmentation will fail and interfere with 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 ground planes (such as digital ground plane and analog ground plane) are connected by 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, which may cause unnecessary radiation.
(5) The loop area formed by the signal line and its loop should be 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 slotting.
The connecting method between the earth, referring to some practices in the article written by Wu Ye-qing, is arranged here.
(1) Common wire connection between the ground circuit board: This method can ensure reliable low-impedance conduction between the two ground wires, but it is limited to the connection between the low-frequency signal circuits.
(2) Large resistance connection between the ground: the characteristics of large resistance is that once there is a pressure difference at both ends of the resistance, it will produce a very weak conduction current. After the charge discharge on the ground wire, the pressure difference at both ends will eventually be zero.
(3) Capacitive connection between the ground: the characteristics of the capacitor are DC cut-off and AC conduction, which is applied to the floating ground system.
The magnetic bead connection between the earth: the magnetic bead is equal to a resistance with the frequency change, it is the performance of resistance characteristics. Applied to ground to ground of weak signals with fast and small current fluctuations.
(5) Inductance connection between the ground: inductance has the characteristics of suppressing the change of circuit state, which can be used to cut peaks and fill valleys. It is usually used between two ground with large current fluctuations.
⑥ Small resistance connection between the ground: small resistance adds a damping, hinder the rapid change of the ground current overshoot; When the current changes, the rising edge of the impulse current becomes slow.