decopuling capacitor in power circuit

[sxg]

why usually this circuit has two decopuling capactiors side by side ,a big one
and a small one.
why has two ? Isn't the big one enough?

 

[wholx]

Maybe the small decoupling capacitor is used to improve the high frequency characterristics. for instance, the small one is a film and the big one is a electrolytic one.

 

[IanP]

The main reason for using several different decoupling capacitors on the power lines is to provide very low impedance for AC/RF currents.
Ideally it should be 0Ω (or as low as possible) accross the whole frequency spectrum.
As the inductance of a capacitor decreases with its size so does its impedance.
So pretty common combination is 100µF + 100nF +100pF ..
This below graph shows how caps impedance changes with frequency ..

The "big" one (µF), as you called it is only good for some MHz, rest has to be done by a "smaller" one (nF), and if it is a RF circuit, then another "small" one (pF) is required ..

 

[hock]

electrolytic capacitors are good for 100's of Hz.
tantalum capacitors are good for 10's of kHz
ceramic capacitoes are goof for 100's of KHz to MHz range
mica and paper are good for MHz range.
amount of capacitance required goes down along with the increasing frequency.

 

[supra924]

Hi ,
the reson behind using two capacitor is to attain the required voltage at two diffrent operating frequencies of your digital IC.
Say for an example the IC requires more cureent at 5 MhZ due to swithing and 7 Mhz on another place... so based on this capacitor choosen
.
Siva

 

[newbie101]

It allows us to obtain low impedance for a wide range of frequencies ( since impedance of caps are effectively like that of inductors at high frequencies ).

 

[kgeorge123]

Say I have one regulator from where I am supplying the power. And I have three gounds Digital(<50MHz) ground Analog(<20Khz) ground and RF(<1GHz) ground. And say that I want to design a low pass filter which filters out all the frequencies if possible above 0Hz.What caps should be best?

Now my belief is that the type(Electrolytic/Ceramic/tanalum) of capacitor does not matter what matters is only their values.

What are your views??

 

[kender]

Ian when you were generating your log(Z) vs. log(f) plot, how did you model the capacito? RLC all in series?