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Digital Circuit Boards: Mach 1 GHz by Ralph Morrison

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4.7 Decoupling Capacitors

Capacitors that are used for power decoupling in digital circuits are usually surface-mount types. This means that there are no connecting leads. The leads that must be considered are connections to pads and connecting traces. Capacitance values vary from a few hundred picofarads to 10,000 pF. Most of the capacitors that are used for decoupling use a ceramic dielectric. Manufacturers will usually provide information on the equivalent series resistance (ESR) and the equivalent series inductance (ESL), as measured at the terminals.

For high frequency sinusoids, capacitors are usually considered series resonant circuits. At their natural resonant frequency, the series impedance is a resistance. The series resistance at the resonant frequency is largely controlled by skin effect. A typical resistance is in the range 0.1–1.0 ohm. For a capacitor of 0.001 μF and a series inductance of 1 nH, the resonant frequency is 159 MHz. This is well below the frequencies involved in gigahertz operations. At frequencies above the resonant frequency of a capacitor, the impedance appears to be inductive. All of these ideas are related to sine wave analysis. In fast digital applications the transmission line view is preferred. This is the view point taken in this book. This view makes it easier to consider the response of a capacitor to step changes in load.

In designs involving high clock rates, it is customary to place many decoupling capacitors on the circuit board. To optimize ...

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