The higher the frequency, the more a capacitor looks like a short.
The lower the frequency, the more a capacitor looks like an open.
We use those little 0.1uf ceramic caps next to fast switching IC's
because those IC's insert fast transients onto the power rail.
Capacitors that are placed between VDD and ground have the effect of
making VDD "more DC like".
Another way of thinking of it is that your chips want a low-impedance
power supply, one that doesn't change voltage when the load changes.
The high frequency switching transients that cause trouble (since they
can momentarily draw a couple of amperes of current) see the capacitor
as a low impedance, which stabilizes the voltage against these current
surges. Wire between the capacitor and the IC contributes resistance
and (importantly) inductance, which isolates the capacitor from the IC -
the opposite of what you want. So you want low inductance connections
between the capacitor and the IC. In practise, this amounts to short
wires, the wider the better (a wide flat wire has lower impedance than
the same amount of copper in a round wire).
- John KG4L
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