Do you fully grasp the role of China PCB board design impedance matching and 0 ohm resistance? Impedance matching

Do you fully grasp the role of PCB board design impedance matching and 0 ohm resistance?

Impedance matching

Impedance matching refers to a suitable matching method between the signal source or transmission line and the load.

According to the access mode, there are two ways of impedance matching: serial and parallel; according to the frequency of the signal source, impedance matching can be divided into low frequency and high frequency.

1.png

High-frequency signals generally use serial impedance matching

The resistance value of the serial resistor is 20 ~ 75Ω, and the resistance value is proportional to the signal frequency and inversely proportional to the PCB trace width.

In an embedded system, when a signal with a frequency greater than 20M and a PCB trace length greater than 5cm, serial matching resistors must be added, such as clock signals, data, and address bus signals in the system. The function of the serial matching resistor has two functions:

1. Reduce high frequency noise and edge overshoot. If the edge of a signal is very steep, it contains a lot of high-frequency components, which will radiate interference, and it is also prone to overshoot.

The series resistance and the distributed capacitance of the signal line and the load input capacitance form an RC circuit, which will reduce the steepness of the signal edge.

2. Reduce high frequency reflection and self-excited oscillation. When the frequency of the signal is high, the wavelength of the signal is very short. When the wavelength is shorter than the length of the transmission line, the reflected signal superimposed on the original signal will change the shape of the original signal.

If the characteristic impedance of the transmission line is not equal to the load impedance (that is, does not match), reflection will occur at the load end, causing self-oscillation. The low-frequency signal of the wiring inside the PCB board can be directly connected, and generally no serial matching resistor is needed.

2.png

Parallel impedance matching is also called "terminal impedance matching"

Generally used in the input / output interface, mainly refers to the impedance matching with the transmission cable.

For example, LVDS and RS422 / 485 use Category 5 twisted pair input terminal matching resistance is 100 ~ 120Ω; video signal using coaxial cable matching resistance is 75Ω or 50Ω, using flat cable is 300Ω.

The resistance of the parallel matching resistor is related to the medium of the transmission cable and has nothing to do with the length. Its main role is also to prevent signal reflection and reduce self-oscillation.

It is worth mentioning that impedance matching can improve the EMI performance of the system. In addition, to solve the impedance matching, in addition to using series / parallel resistors, transformers can also be used for impedance transformation. Typical examples are Ethernet interface, CAN bus, etc.

Zero ohm resistance

The easiest is to use a jumper. If a certain section of the line is not used, just don't solder the resistor (it will not affect the appearance).

When the matching circuit parameters are uncertain, replace with zero ohms. When actually debugging, determine the parameters and replace with specific value components.

When you want to measure the operating current of a part of the circuit, you can remove the zero-ohm resistance and connect an ammeter, which is convenient for measuring the current.

When wiring, if it can't be passed, you can also add a zero ohm resistor to play a jumper role.

In the high-frequency signal network, it acts as an inductance or capacitance (impedance matching, zero ohm resistance also has an impedance). When used as an inductor, it is mainly to solve EMC problems.

Single-point grounding, for example, a single-point connection between analog ground and digital ground.

Configuration circuit can replace jumper and DIP switch. Sometimes the user will set up disorderly, which is easy to cause misunderstandings. In order to reduce maintenance costs, zero-ohm resistors should be used instead of jumpers to solder on the board.

For system debugging, for example, the system is divided into several modules, and the power supply between the modules is separated from the ground with a zero-ohm resistance. When the power supply or ground is shorted during the debugging phase, removing the zero-ohm resistance can narrow the search range.

The above functions can also be replaced with "magnetic beads". Although the zero-ohm resistance and the magnetic bead are somewhat similar in function, there is an essential difference. The former has an impedance characteristic and the latter has an inductive resistance characteristic. Magnetic beads are generally used in power and ground networks and have a filtering effect.

If workers want to do their best, they must first use their tools to better understand impedance matching and zero-ohm resistance, making PCB design and manufacturing easier.