selection of decoupling cap

hi
I have a question:
while designing the decoupling cap required for a ckt. why do we use a cap which has voltage rating twice the input voltage?

can anyone of you help to share teh reasoning behind that?

thanks and regards
sita

Decoupling cap is generally rated above the input voltage and not necessarily double the input voltage. This would protect the circuit from any unwanted spike, switching noise

Decoupling is not the process of placing a capacitor adjacent to the IC to supply the transient switching current, rather it is the process of placing an L-C network. It is very important to remember this fact -- we are placing an L-C network between the power and ground, not a capacitor

Some more info
Decoupling is the isolation of two circuits on a common line. The decoupling network is usually a low pass filter and the isolation is rarely equal in both directions. Decoupling is used to prevent transmission of noise from one circuit to another.

When choosing component values for a decoupling network, two parameters are important. First, of course, is isolation versus frequency. Second, and often overlooked, is the impedance seen by the load. This is important because a large output impedance will cause the load noise current to be translated into a large noise voltage.