Why do you add decoupling capacitor between VDD and GND?

[yen]

decoupling capacitor

Dear everyone,
Why do between vdd and gnd add decoupling capacitor? How much do I add?
Add between analog power or digital power or both?
Thanks.

 

[IanP]

Re: decoupling capacitor

Goto this site:
http://www.williamson-labs.com/480_byp.htm
It will tell you why we use decopupling capacitors (decoupling) and also on bypassing, grounds etc.

Decoupling has to be done on both circuits: analog and digital. Usually they have also separate grounds, so decoupling for analog section will be done between Van and AGND, whereas in digital section it will be between Vdig and DGND ..
Regards,
IanP

 

[huanchou]

Re: decoupling capacitor

It is used to avoid the bounding wire noise.
Usually the capacitor is the more the better, but it will occupy your core circuits area. Use the capacitor to your analog power is better.

 

[alchip]

Re: decoupling capacitor

Be careful not to add much, otherwise it will destroy recovery time.

 

[aldevan]

Re: decoupling capacitor

I use 1 capacitor 100nf by chip, if you have 10 chip in your circuit you have to put 10 capacito 100nf, you have to put as near as possible from every chip

 

[mosis]

Re: decoupling capacitor

we use decoupling capacitor to seperate two sections/circuits, mostly they are analog and digital. One should decide the C of capacitor due to the frequency domain. And you should calculate the resistor and capacitor to eliminate the wrong high-frequency signals. usualy the resistor and capacitor here acts as a filter-maybe low-pass-filter or high-pass-filter.

 

[danda821]

Re: decoupling capacitor

Usually the more the better. You can add it during the layout. Use the spare space you have. For some processing, you need to add some dummy metal layers to satisfy the local density rule, so you can use the space to draw multi-layer MIM cap.

 

[chuan556]

decoupling capacitor

how can we calculate the capacitor and inductor for the slot?let say we have 2 reference plane with slot in the center,the transmission line(microstripe) is on the top.

 

[alok_ky]

decoupling capacitor

the more the decoupling cap the better.
The main purpose of the dcap is to isolate the power supply surges caused by the circuit transitions from other parts of the circuit.

 

[chuan556]

Re: decoupling capacitor

the more the decoupling cap the better.
The main purpose of the dcap is to isolate the power supply surges caused by the circuit transitions from other parts of the circuit.

may i know how many capacitor is considered enough?10?20?.......