decoupling capacitor

Usually decoupling capacitor is used fom VCC to GND in ICs
Can someone plz explain why is necessary to place this capacitor

What i think is it is to cater the power supply fluctuations if it is so then if i will use a normal 1.5v dry cell battery to power an IC lets say mcu.Is this decoupling capacitor is to be used

What is a typical velue for a decoupling capacitor for lets say a 5v supply

darvaish said:

Usually decoupling capacitor is used fom VCC to GND in ICs
Can someone plz explain why is necessary to place this capacitor

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Because it's necessary that the supply rails and GND are at the same AC potential - the decoupling capacitors effectively join the rails together.

Because you have inductance (and resistance) in the PCB tracks (or other types of connections) you should fit capacitors periodically throughout your circuit. For digital circuits it's usual to fit one per IC.

What i think is it is to cater the power supply fluctuations if it is so then if i will use a normal 1.5v dry cell battery to power an IC lets say mcu.Is this decoupling capacitor is to be used

You still need them (more than ever in fact), and you should have a decent size electrolytic across the battery as well - to make the battery lasts longer, as it's internal resistance increases as it goes flat!.

What is a typical velue for a decoupling capacitor for lets say a 5v supply

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Something like 0.1uF is usually fine, for RF circuits smaller ones would be good - you should also have an electrolytic or two as well! (depending on the complexity of the circuit). For low frequency applications (audio etc) you certainly require electorlytics, in order to give a low impedance path at the low frequencies used - preferably with smaller capacitors across the electrolytics.

I might add to Nigel's comments that it is a misconception that decoupling caps are only necessary in dc supplies that are derived from the mains. Not so. Decoupling caps are more for supply variations that are caused by the loads as opposed to taking care of ripple from the source. Clock edges are especially nasty when they get on the supply lines. They're high-frequency glitches that are most easily caught by using smaller disc ceramic caps vs. larger electrolytics.

Dean