power supply decoupling...

Hi guys,
Can anyone explain me what is Power supply decoupling and how is it done using capacitors in a circuit?
Thanks...
Surya

noise generated along the wire from power supply to the power pin, this could cause the IC to oscillate
capacitor drain the ac noise into ground
hence, it should be connected as near as possible to the power pin

Power supply decoupling basically refers to filtering the supply at the point where it is used. This is to eliminate some ripple, transients from other components in the system switching, noise on the supply etc, from reaching a particular component (usually an op-amp, micro, etc). To do it, you place a small value capacitor physically as close as possible to the device you want to decouple. If you look at a modern board, you will usually notice a tiny cap sitting right next to each IC on the board, those are the decoupling caps. Typical values are 0.1uF.

In other word, the idea is to create a low inductance path for high frequencies from VCC to GND. Smaller physical capacitors are better because they have a lower internal inductance.

well,
Thank u both for explaining me. Actually i am building a project based on the principle of IR reflection for proximity detection. For this purpose, i am using an IR LED as a transmitter and receiver units are the ones available in our TVs and VCRs(that receive the IR signal from the remote control). Any ideas about these receiver units??? The one that i have for now is from SHARP (GP1U26R series). Can anybody explain how to use it for proximity detection along with the IR LED? I have less time for its completion.
Thanks again,
Surya

Is it possible that in some applications the use of the word "decoupling" suggests that the capacitor might serve to separate stages or sections of a circuit - the capacitor would allow AC to pass but would not pass DC - to essentially decouple the circuits - possibly isolating the power supplies from each stage? That is how I've looked at it but my knowledge on this subject is limited.

surjo_dm said:

well,
Thank u both for explaining me. Actually i am building a project based on the principle of IR reflection for proximity detection. For this purpose, i am using an IR LED as a transmitter and receiver units are the ones available in our TVs and VCRs(that receive the IR signal from the remote control). Any ideas about these receiver units??? The one that i have for now is from SHARP (GP1U26R series). Can anybody explain how to use it for proximity detection along with the IR LED? I have less time for its completion.
Thanks again,
Surya

Click to expand...

I have details of doing this using a PIC 12C508 on my website, it's very simple to use giving simple right and left outputs, have a look at which includes a link to the original website with the source code.

If you want to see the effect of decoupling capacitor construct this circuit on a bread board and check the output square wave with and without decoupling capacitor C1 (0.1uF).

Without C1 you will see a heavily distorted square wave and with C1 you'll find very little or no distortion.

This is the first circuit in which I saw the effect of not using decoupling capacitor.