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Q: Why capacitor across power pins

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thaking:
Can anyone explain in few words why we use capacitor across power pins?

KTP:
One use is a low impedance path for signals.  Z = 1/(j*2*pi*f*C)

(I mean such that a signal return does not have to travel a long path but can go through the capacitor)

Simon:
it os known as a decoupling capacitor and has a similar function to a filtering capacitor afer a rectifier but in reverse, it supplies peaks of power when required so as to help the power circuit be stable. The line from the MCU to the power supply will have a resistance and an instant power requirement (peak current pulse) will cause a voltage drop accross the path from the supply and so reduce the voltage supplied and cause an unstable supply, the capacitor help deal with this by supplying the initial surge of power required giving the supply to ajust to the change. in simple terms it's a power buffer.

as an example get a pic to deliver a stream of pulses to even a few Kohms of load and observe the power line to the pic with an oscilloscope, you will see negative and possibly positive (where the regulator over compensates) spikes at the same frequency as the output frequency

Zero999:
Yes it's a decoupling capacitor.

Google has lots more information
http://www.google.co.uk/search?hl=en&safe=off&client=opera&hs=Z5j&rls=en&&sa=X&ei=4U-nTMqmMqKS4gbFq4z2DQ&ved=0CBQQvwUoAQ&q=decoupling+capacitor&spell=1

thaking:
Many thanks ;)

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