decoupling capacitors

hi, i would like to know how to choose the number of capacitors to use for decoupling and what values. for example i have a board now with source VCC and 3 decoupling cap, 1uf, 68pf, 1.2nf. how are the values chosen ?

the values r chosen using this formula C = I / (Vripple x 100) basically it is used to stabilize the voltage. they are connected in parallel to power supply mainly u can use an approximate value but it should be greater than the required value either by arbiterly selecting or by calculating using formula

Decoupling capacitors have nothing to do with "stabilinzing the voltage".
They are used in digital circuits as "local energy storage devices" because of the possibility of many simultaneously switching outputs. Even devices that are connected to the most stable of power supplies needs them to prevent power integrity issues.

yes i agree to ur point the reason y i told is when i u have less voltage then the required voltage it uses the capacitor to maintain the voltage

Here are a nice tutorial on the subject and a discussion from AVX:

http://www.analog.com/static/importe...als/MT-101.pdf
http://www.avx.com/docs/techinfo/dcplbsc.pdf

This third article provided by forum member marce adds some perspective on selection and placement:
http://pcdandf.com/cms/fabnews/8194-...grity-insights
As the author of the article suggests, practical considerations may outweigh theoretical.

John

I didn't get answer from above discussion to choose the right value of decoupling capacitor...I understood other parameters like reduced lead inductance and IR drops. But still not clear that what is the exact value of Capacitor which is used for decoupling...

Yes, you got the answer. Just read the papers posted above and you will find the values that have to use.