Digital Circuit PCB Design

High Speed Design of Digital Circuit PCB Based on SI

In recent years, with the rapid development of integrated circuit technology, the speed of its work is getting higher and higher. This brings about a problem. The smaller the volume, the larger the layout and wiring density of the circuit, the faster the output switching speed of the integrated circuit, and at the same time the signal operating frequency continues to increase. Therefore, how to deal with high-speed signals and ensure system design performance has become a design capability. key factor for success.

With the rapid increase of the clock frequency of the electronic system and the steepening of the signal edge, the influence of the trace interconnection and layer characteristics of the printed circuit board on the electrical performance of the system is becoming more and more important. For low-frequency designs, the influence of trace interconnection and board layers can be ignored. When the operating frequency of the system exceeds 50MHz, on the one hand, the influence of the transmission line must be considered in the interconnection relationship, and on the other hand, the electrical parameters of the printed circuit board should also be considered when evaluating the system performance. Therefore, the design of high-speed systems must face timing issues caused by interconnect delays and signal integrity (signal quality) issues such as crosstalk and transmission line effects. How to take into account the signal integrity factor in system design and plate design, and take effective control measures, has become a hot topic among system design engineers and PCB design industry.

1.The method to ensure the signal integrity of the PCB board

1.1 Isolation

Devices on the PCB have various edgerates and various capacitive noises. The most direct way to improve signal integrity is to physically separate them on the PCB according to their different margins and sensitivities.

1.2 Impedance, Reflection and Termination

Impedance control and termination are fundamental issues in high-speed design. It is also a core issue in every RF circuit design. However, the operating frequency of some digital circuits exceeds that of RF circuits, and the impedance and terminal load are still not considered in the design. Impedance mismatch will have the following fatal effects on digital circuits:

(1) The digital signal will be reflected between the input terminal of the receiving device and the output terminal of the transmitting device, and the reflected signal will bounce back and propagate along the two ends of the line until it is completely absorbed at last;

(2) The reflected signal causes the ringing effect of the signal passing through the transmission line, and the ringing will affect the voltage and signal delay or even completely deteriorate the signal;

(3) The mismatched signal path may cause the signal to radiate to the environment; the problems caused by the impedance mismatch can be reduced by the terminal load. Usually one or two separate terminal loads are placed on the signal line close to the receiver, and the simple way is to connect a low-value exclusion in series. Termination limits the rise time of the signal and can partially absorb reflected energy. It is worth noting that termination does not completely eliminate the destructive effects caused by impedance mismatches. However, with careful selection of the proper device, termination can effectively control signal integrity. Not all wiring needs impedance control, it is up to the designer to decide whether to match. There are various rules in various applications, but generally follow the rules between the wiring length and the rise time of the signal, that is, the general impedance control rule is that when the wiring length is greater than 1/6 of the rise time, impedance matching must be performed .

1.3 Layers and Layer Segmentation

One issue that is often overlooked by digital designers is current propagation in the loop. For example, assuming a unidirectional signal is transmitted between two gates (as shown in Figure 2), the current will propagate in the loop from gate A to gate B, and then return to gate A through the ground connection. Here There are two potential problems:

(1) The ground should be connected by a path with low impedance value. If a path with a high impedance value is used, then there will be a voltage drop at the ground pin of Figure 2, which will destroy the ground reference of all devices and reduce the input noise margin;

(2) The loop area caused by the current loop is as small as possible. The loop is equivalent to the antenna. Generally speaking, a large loop area will increase the chance of loop radiation and conduction. Every PCB designer hopes that the return current is directly along the signal line, so that the smallest loop area can be obtained;

Using a large-area ground layer can solve the above problems at the same time. The large area ground provides low impedance between all ground points while allowing loop currents to travel directly through the respective signal paths.

A common mistake made by PCB designers is to open a slot in the ground plane (as shown in Figure 3a). Figure 3(a) shows the current flow when the signal line bypasses the slot in the ground plane. The loop current will be forced to bypass the slot, which will necessarily create a large circulating loop. Figure 3(b) shows the current flow when the ground plane is not slotted. Generally speaking, the ground plane cannot be slotted. However, there are also situations where slotting cannot be avoided. When it occurs, the designer of the PCB must ensure that no signal path passes through the slotted part.

Attention should also be paid to the area between layers in the power layer with mirror image differences. The power layer and ground layer of the PCB have radiation at the edge of the board. Electromagnetic energy radiating from the edges may damage adjacent connection plates. The solution is to shrink the power plane so that it overlaps the ground plane by a fixed distance. This can reduce the electromagnetic radiation energy value in the direct area outside the board, and reduce the influence of electromagnetic leakage on adjacent boards.

1.4 Signal wiring

The most important thing to ensure signal integrity is the physical routing of the signal lines. High-speed signals cannot propagate in discontinuous signal lines. The right corner shown in Figure 4(a) is a problematic wiring method that is usually easier to commit. Such wiring has no problem at low frequencies, but it will radiate at high frequencies. It should be replaced by a 45o in Figure 4(b) or two 45-degree corners in Figure 4(c).

In high-speed circuit design, if there is no special reason for signal wiring, all short-circuits should be eliminated as much as possible. Short-circuits are like radiation caused by impedance mismatch of signal lines. In addition, special attention should be paid to the wiring of differential pairs in the wiring of high-speed circuit design. The differential pair is driven by two completely complementary signal lines, and the differential pair can well avoid noise interference and improve the S/N rate. However, the differential pair signal line has particularly high requirements for wiring: (1) the two lines must be as close as possible to the wiring; (2) the two lines must be exactly the same length;

1.5 Overcoming Crosstalk

In PCB design, crosstalk is another problem worthy of attention. When the spacing between signal lines is too small, the electromagnetic regions between signal lines will affect each other, resulting in signal degradation and crosstalk.

Crosstalk can be resolved by increasing the spacing of the signal lines. However, PCB designers are usually constrained by the increasingly tight wiring space and narrow signal line spacing. Since there are no more choices in the design, it is inevitable to introduce some crosstalk problems into the design. Many relevant rules for reliable spacing are given in the literature. The commonly used rule is the 3W rule, that is, the spacing between adjacent signal lines should be at least 3 times the width of the signal line. However, the actual acceptable signal line spacing depends on the actual application, working environment and design redundancy and other factors. Therefore, when the crosstalk problem is unavoidable, the crosstalk should be quantified, and the designer can determine the signal integrity effect and evaluate the crosstalk effect of the system through computer simulation.

in conclusion

Signal integrity is one of the most important issues throughout the design of high-speed digital circuits. Here are several methods to ensure signal integrity in digital circuit design: (1) Physically isolate sensitive components from noise devices; (2) Impedance control, reflection and signal terminal matching; (3) Use continuous power and ground plane layers; (4) Try to avoid using right angles in wiring; (5) Differential pair wiring lengths are equal; (6) In high-speed circuit design The problem of crosstalk should be considered; (7) The power supply should be decoupled.

In the design process of the PCB board, the factors of signal integrity are fully considered, and effective control measures are taken, so that a safe and reliable high-speed circuit can be designed. The author's innovation point: signal integrity (SI) has become a new hot topic in today's PCB design industry. This paper expounds typical signal integrity problems in high-speed PCB circuit design. It describes the manifestations of signal integrity problems, and focuses on analyzing several common problems that affect signal integrity - crosstalk, electromagnetic interference and reflection. And pointedly put forward specific solutions to the problems. In circuit design, taking corresponding measures can effectively improve signal integrity.

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