abdulahad100,
In digital applications, ICs draw significant amounts of current while switching. The sudden increase in current creates a voltage drop across the inductive reactance presented by the power supply traces. Decuplin g capacitors serve to provide a low impedance voltage source to supply these transient power supply currents.
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In analog applications, the inductive reactance of the power supply traces provides a potential positive feedback path from the output of the analog IC throught the power supply. This feedback is a potential source of osillations. The decoupling path reduces the voltage that is induced into the power supply traces, by providing a low impedance path to signal ground.
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In both applications, the simplified equation to use is
(Delta V) = (i * (Delta T))/(C). where
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Delta V is the magnitude of the voltage change
i is the magnitude of the transient current.
C = decoupling capacitance
Delta T is the duration of the current pulse.
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This is an over-smplification, since the transient current is not a nice square pulse. However, you can come up with a conservative design using this formula. It does, however, require a knowledge of the transient current amplitude and duration.
Regards,
Kral