Discussion on circuit board design four layer board and 33 ohm resistor

Discussion on circuit board design four layer board and 33 ohm resistor

The design of printed circuit board is based on the circuit schematic diagram to realize the functions required by the circuit designer. The design of printed circuit board mainly refers to layout design, which needs to consider the layout of external connections. The optimal layout of internal electronic components, the optimal layout of metal wiring and through-hole, electromagnetic protection, heat dissipation and other factors. Excellent layout design can save production costs and achieve good circuit performance and heat dissipation. Simple layout design can be realized by hand, while complex layout design needs to be realized by computer aided design (CAD)

The selection of four layer board is not only a problem of power supply and ground, but also a requirement of high-speed digital circuit for the impedance of wiring. The two-layer board is not easy to control the impedance. 33 ohm resistor is generally added to the driver end, which also plays the role of impedance matching; Data address lines and high-speed lines to be guaranteed shall be laid first when wiring;

In high frequency, the wiring on the PCB should be regarded as transmission lines. Transmission line has its characteristic impedance. Anyone who has studied transmission line theory knows that when there is a sudden change (mismatch) in the impedance on the transmission line, the signal will be reflected when passing through, which will interfere with the original signal and affect the normal operation of the circuit in serious cases. When a four layer board is used, the outer layer usually runs signal lines, and the middle two layers are power and ground planes respectively. On the one hand, the two signal layers are isolated. More importantly, the outer layer runs lines and the plane they are close to form a transmission line called a "microstrip". Its impedance is relatively fixed and can be calculated. It is difficult to do this for two layers of boards. The impedance of this transmission line is mainly related to the width of the line, the distance to the reference plane, the thickness of copper coating and the characteristics of the dielectric material. There are many ready-made formulas and programs for calculation.

The 33 ohm resistor is usually placed at the drive end in series (in fact, it may not be 33 ohm, ranging from a few ohm to 50 ohm or 60 ohm, depending on the specific situation of the circuit). Its role is to match the output impedance of the transmitter with the impedance of the routing line after serial connection, so that the reflected signal (assuming that the impedance of the receiving end does not match) will not be reflected back (absorbed) again, so that the signal at the receiving end will not be affected. The receiving end can also be matched. For example, resistance parallel connection is used, but it is rarely used in digital systems because it is troublesome, and in many cases, one sending and multiple receiving, such as address bus, are not as easy to match as the source end.

The high frequency mentioned here is not necessarily a circuit with a high clock frequency. Whether it is high frequency depends not only on the frequency, but also on the rise and fall time of the signal. Generally, the rise (or fall) time can be used to estimate the frequency of the circuit, generally taking half of the reciprocal of the rise time. For example, if the rise time is 1ns, then its reciprocal is 1000MHz, which means that the circuit should be designed according to the 500MHz frequency band. Sometimes it is necessary to deliberately slow down the edge time. For many high-speed ICs, the driver's output slope is adjustable. The circuit board processing and PCBA processing manufacturers will explain to you: the circuit board design is about four layer boards and 33 ohm resistors.