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52 DESIGN007 MAGAZINE I AUGUST 2022 Controlling impedance is critical to signal integrity and board performance in devices powering everything from high-speed digital applications to telecom and RF communica- tion. It is common practice for designers to include impedance-related notes with their PCB designs and rely on the manufacturer to determine the proper trace parameters. is inherently passive methodology oen leads to delay, cost overrun, and even batches of use- less boards. Designers can save time, money, and effort if they are aware of the impedance math as they design their board. Impedance math is heavily influenced by the distance an electrical signal must travel between components. Comprised of a signal trace and a return path, a PCB transmission line transmits an electri- cal signal from one component to another. To function as a transmission line, the line length must be at least one quarter of the signal wave- Controlled Impedance: the Devil is in the (Math) Details length. For most transmission lines, the return path is a ground plane located on a layer above or below the signal trace. Impedance comes into play when trans- mission lines are part of the PCB design. It is equivalent to the measure of resistance in a DC circuit and is also measured in ohms. e higher the impedance, the higher the input voltage must be to achieve a desired current in the circuit. For high speeds greater than 100 MHz, fre- quencies and noise sensitive signals are used to control the characteristic impedance and are important to maintaining signal integrity. Mis- matches in the impedance of these high-speed signals can cause signal reflections—meaning a portion of the signal is traveling in the opposite of the intended direction. e greater the dif- ference in impedance, the greater the amount of signal reflected in one portion of the circuit, which will cause the signal to be noisy and could even cause a disruption of the signal. Connect the Dots by Matt Stevenson, SUNSTONE CIRCUITS Figure 1. Figure 2.

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