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Provide a stable supply to the device by filtering any noise or interference , providing fast the needed energy to the device since they are connected very close to the IC.
A decoupling capacitor provides a low impedance to by-pass unwanted signals or noise. It may be across a voltage supply or it may be between some point in a circuit and a supply line - usually the 0V line, but could equally be a positive or a negative rail.
A good voltage source has an internal resistance/impedance tending to zero, so it shouldn't matter to which line of a supply a decoupling C is connected. In signal analysis, the supply lines a considered to be shorted together.
Decoupling capacitors are used to ground AC signal ( or noise ).
In power supplies we wanted to have a smooth constant DC output and we don't want any noise (which are high frequency AC signal) to reach the load. One way of doing this is to put a capacitor (mostly are ceramic type) in parallel with the DC output line. So that any noise on the DC output line will be shorted out by the capacitor (remember that capacitors have low impedance on AC) and wont reach the load. That's basically what a decoupling capacitor do ( actually this decoupling capacitor is more properly called as 'filter capacitor' if used in power supply output voltage regulation).
Hope it helps, Iam a beginner too.
Apart from the glitch/noise /transference of data thru the power supply ,there is another element.
.For the input stage of DC converters ,we need to provide for surge currents needed by the input MOSFET stage ..
The tank capacitance at input is a must for clean operation.
Similarly , the output stage of Switching converters need a storage to lower down the ripple content and storage for current supply during current flow..
Shorting out AC variation (like noise) on the main power supply output line (+V to 0V) where we want constant and clean DC voltage.
I mean the same as you're reply, maybe I had used confusing terms or grammar, what i meant with ground is 0V.
With respect, that doesn't explain what is so magic about "ground".
You will see from the third posting in this thread that the (voltage) supplies of any circuit have, or should have, an impedance tending to zero. It is the shunting of unwanted a.c. components to one of the supply rails (not necesssarily 0V in the case of another supply rail) or necessarily to the 0V rail for any other circuit node - it can equally, and as effectively, be to a positive or negative rail.
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