Description

Impedance is the result of the interaction between the resistance and reactance in a circuit and expressed in Ohms. Impedance is an AC characteristic, which means it is related to frequency. A controlled impedance is important to maintain signal integrity, especially for high-frequency products.

When a signal must have a particular impedance in order to function properly, controlled impedance should be used. In high frequency applications matching the impedance of PCB traces is important in maintaining data integrity and signal clarity. If the impedance of the PCB trace connecting two components does not match the components’ characteristic impedance, there may be increased switching times within the device or the circuit. There may also be random errors.

While it might not be obvious to the casual or to those who think the mathematics underlying PCB design is largely settled, there is plenty of disagreement regarding the right formula to calculate trace impedance. This disagreement extends to online trace impedance calculators, and designers should make themselves aware of the limitations of these tools.

Controlled Impedance should be considered for PCBs used in fast digital applications such as:

Most controlled impedance PCBs undergo 100% testing. However, it is not uncommon for the actual PCB traces to be inaccessible for testing. In addition, traces may be too short for accurate measurement and may well include branches and vias which would also make exact impedance measurements difficult. Adding extra pads and vias for test purposes would affect performance and occupy board space. PCB testing is therefore normally performed, not on the PCB itself, but on one or two test coupons integrated into the PCB panel. The coupon is of the same layer and trace construction as the main PCB and includes traces with precisely the same impedance as those on the PCB, so testing the coupon affords a high degree of confidence that the board impedances will be correct.

Andwin Circuits as a industry leading of PCB manufacturer since 2003.

Specializing in multilayer PCB, Rigid flex PCB, HDI PCB, controlled impedance PCB and Radio Frequency circuit, from quick turn prototype to mass products.