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Trace Impedance Basics

by: Dec 07,2023 535 Views 0 Comments Posted in PCB Design & Layout

PCB Design Impedance Trace

Summary:       What is trace impedance and how to work with it

Trace impedance is a characteristic of PCB traces that can have significant effects on signal integrity. Since all wires have some resistance and capacitance (we don't know of any perfect conductors), all PCB traces have impedance. Often we can ignore this impedance because it is minimal and the signals/power we are working with aren't adversely affected by it. However, as signal frequency increases, along with decrease in amplitude voltage, this small impedance can disturb signals. Component manufacturers will often describe in data sheets the acceptable (and even desirable) trace impedance. For example, in a recent project of mine, the chip ADV7280 mentioned in the data sheet that the analog signal inputs should have a trace impedance of 75 ohms.

In order to achieve specific trace impedances, discrete components can be added such as resistors and capacitors. However, choosing proper trace widths can also have an important effect. Using PCB parameters given by PCBWay (such as copper weight, substrate thickness, and dielectric coefficient), the ideal trace width can be calculated. Online calculators (such as Digikey's and PCBWay's) are available for this. Following manufacturer guidelines will help ensure signal integrity.

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