Do you fully understand the role of impedance matching and zero ohm resistance in PCB design?

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

According to the access mode impedance matching has serial and parallel two ways; According to the source frequency impedance matching can be divided into low frequency and high frequency two.

High frequency signals generally use serial impedance matching

The resistance value of the serial resistor ranges from 20 to 75Ω. The resistance value is directly proportional to the signal frequency and inversely proportional to the PCB wire width.

In an embedded system, serial matching resistors are required for signals with a frequency greater than 20M and a PCB route length greater than 5cm, such as clock signals, data signals, and address bus signals in the system. Serial matching resistors serve two purposes:

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

The series resistance forms a RC circuit with the distributed capacitance of the signal line, the load input capacitance, etc., which reduces 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 short. When the wavelength is short enough to match 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, resulting in self-excited oscillation. The low-frequency signals in PCB board can be directly connected, generally do not need to add serial matching resistance.

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 Class 5 twisted pair input matching resistance of 100~120Ω; The matching resistance of the video signal is 75 or 50Ω using coaxial cable and 300Ω using flat cable.

The resistance value of parallel matching resistance is related to the medium of transmission cable, independent of the length, and its main function is to prevent signal reflection and reduce self-excited oscillation.

It is worth mentioning that impedance matching can improve the EMI performance of the system. In addition, in addition to the use of string/parallel resistance to solve the impedance matching, transformers CAN also be used to do impedance transformation, typical examples such as Ethernet interface, CAN bus, etc.

Zero ohm resistance

The simplest is to use jumper, if a section of the line is not used, directly do not weld the resistance can (does not affect the appearance).

When the matching circuit parameter is uncertain, it is replaced by zero ohm. When the actual debugging, the parameter is determined, and then the specific value of the component is replaced.

If you want to measure the working current of a certain part of the circuit, you can remove the zero ohm resistance and connect the ammeter, so as to measure the current easily.

In the wiring, if the cloth is not in the past, you can also add a zero ohm resistance to play a bridging role.

In high frequency signal networks, acts as inductor or capacitor (acts as impedance matching, zero ohm resistance also has impedance). When acting as inductor, mainly to solve EMC problems.

Single point grounding, such as analog ground to digital ground single point joint common ground.

Configure circuits that can replace jumpers and dip switches. Sometimes users will mess with the Settings, easy to cause misunderstanding, in order to reduce maintenance costs, the application of zero ohm resistance instead of jumper welding on the board.

For system debugging, for example, the system is divided into several modules, and the power supply between modules is separated from the ground zero ohm resistance. When the power supply or ground short circuit is found during debugging, the zero ohm resistance can be removed to narrow the search range.

The above functions can also be replaced by "magnetic beads". Zero ohm resistance and magnetic bead although somewhat similar in function, but there are essential differences, the former is impedance characteristics, the latter is inductive characteristics. Magnetic beads are generally used in power and ground networks, and have filtering effects.

If you want to do a good job, you must first improve the device, better understanding of impedance matching and zero ohm resistance, so that PCB design and manufacturing more simple.