PCB News - Grounding mode in multi-layer PCB board

As a rule of thumb, four-layer boards are usually used for high-density and high-frequency applications and are more than 20 DB better than two-layer boards in terms of EMC. In the condition of four layers, 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 grounding and ground plane connection, and work noise special treatment.

There are a number of ways to connect the ground from the individual printed circuit boards to the ground plane, including:

Single point and multi-point grounding modes

(1) single point grounding: all circuit ground wire is connected to the same point of the ground plane, divided into series single point grounding and joint single point grounding.

(2) Multi-point grounding: all the ground cables of the circuit are grounded nearby, and the ground cables are very short and 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 supply layer, single point grounding is suitable, usually used in analog circuits; The star connection is generally adopted here to reduce the impact of possible series impedance, as shown in the right half of. High frequency digital circuit will need parallel grounding, here the general way through the ground hole can be relatively simple processing, as shown in the left half of the figure; Generally, all modules use the two grounding modes, and the hybrid grounding mode is used to complete the connection between the circuit ground and the ground plane.

Mixed grounding mode

If you do not choose to use the entire plane as a common ground, such as the module itself has two ground lines, you need to divide the ground plane, which often interacts with the power plane. Note the following principles:

(1) Align each plane to avoid overlap between irrelevant power plane and ground plane, otherwise it will lead to failure of all ground plane segmentation and interference between each other;

(2) In the case of high frequency, the parasitic capacitance between layers through the circuit board will produce coupling;

(3) The signal line between ground planes (such as digital ground plane and analog ground plane) is connected by ground bridge, and the return path near Z is configured through the nearest through hole.

(4) Avoid high frequency cable walking such as clock line near the isolated ground plane, which may cause unnecessary radiation.

(5) The ring area formed by the signal line and its loop is as small as possible, also known as loop Z small rule; The smaller the ring area, the less external radiation, the less interference received outside.During ground plane segmentation and signal routing, the distribution of ground plane and important signal routing should be considered to prevent problems caused by ground plane slotting.

How to connect ground to ground, so let's do a little bit of rearranging here.

(1) ordinary wiring connection between the circuit board: the use of this method can ensure reliable low impedance conduction between the two ground lines, but is limited to the connection between the low and medium frequency signal circuit.

(2) large resistance connection between the ground: the characteristic 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 on the ground is released, the pressure difference at both ends is zero.

(3) Capacitance connection between the ground: the characteristics of the capacitor is DC cut-off and AC conduction, used in the floating ground system.

(4) between the magnetic bead connection: magnetic bead is equal to a resistance with frequency change, it is the performance of resistance characteristics. Applied to ground to ground for weak signals with fast small current fluctuations.

Inductive connection between the ground: the inductor has the characteristics of restraining circuit state change, can cut peak valley, usually used in two large current fluctuation between ground and ground.

Small resistance connection between the ground: the small resistance increases a damping, blocking the overshoot of the rapid change of the ground current; When the current changes, the surge current rising edge becomes slow.