Why do people sometimes put multiple decoupling cap on the same node?

[david90]

Why do people sometime put multiple decoup. cap with same or different value on the same node? Can't they just combine them together and have one equvilent cap?

 

[Kral]

cap decoupling

david90,
Ceramic capacitors, which are often used for decoupling have the property that for higher capacitance, the ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance) is also higher. To provide adequate decoupling for both low and high frequency components, a high value capacitor (with high ESL) is used to take care of the low frequency components for which the higher ESL is insignificant. Another lower capacitance capacitor is connected in parallel to take care of the high frequency componets for which a lower Xc is acceptable, but a low ESL is required.
.
Two capacitors of equal value in parallel, will, in general, have lower ESR and ESL values thatn a single capacitor with twice the capacitance as each of the lower value capacitors.
Regards,
Kral

 

[cesare]

decouple cap how much

A real life capacitor is the series of an ideal capacitor a resistor (ESR) and an inductor (ESL) and behaves like a series resonant and starting from its resonant frequency its impedance start to rise.

In order to mitigate this effect different capacitors are put in parallel in order to keep impedance low on a wider bandwidth

 

[raka200]

decouple cap

We need a high value capacitor to provide 'low frequency' decoupling.
But, as Kral said, this high value capacitor have a bad high frequency response (Ex for a 100nF X7R C0805 cap, the resonnant frequency could be around 10MHz
for a 1nF COG Cap, the resonnant frequency could be around 100MHz

 

[watertreader]

cog cap esr

Some additional interesting reading on the topic

**broken link removed**

 

[ddkrishna]

decoupling and cap

If you use a electrolytic capacitor, one will be quite enough, but the size will go up. so if yu are using ceramic capacitor, use more of them

 

[laststep]

size decoupling cap

ya, i have the same question too...
i think what he try to say is if we wanna 1nF capacitor, why dun we just put one 1nF cap instead sometime they put ten 10pF cap in series or when they need 10nF cap, why dun just put one 10pF cap instead of putting 10 1pF cap in parallel

 

[raka200]

use of decouple caps

Hi !!!

To answer to laststep, I think the ESR of a cap is mainly related to the package.
So a 1pF cap has quite the same ESR of a 10pF cap.
So when you put 10x 1pF Cap in parrallel to obtain the same value of 10pF cap, you will have the ESR divided by 10....

everybody would like to use a perfect cap with high value, small package and perfect response. But it doesn't exist. We only have some cap which are good, but for a limited region (value, size, high frequency response, precision, temperature, or piezo-electric response...).
So we use several cap, each one are good in a limited region, but the combination is good for a wider range...

 

[laststep]

decoupling cap size

ya thanksraka200 i think i get ur idea.

 

[arnarendra]

frequency decoupled by using a 100nf cap

To add one more point, ceramic decoupling capacitors are used mainly for the decoupling switching noise. You can see that at least one cap (normally) will be used for one power pin of the IC so that it serves as instantaneous power source for the IC during switching. The value of the decoupling also depends on the frequency of operation taking into account the ESR and ESL of the caps.

 

[springf2000]

Decoupling cap

Because the lower CAP, the more higher self resonant frequency

 

[funnynypd]

Re: Decoupling cap

Why do people sometime put multiple decoup. cap with same or different value on the same node? Can't they just combine them together and have one equvilent cap?

They are there for different purpose. So cannot be put together or just use one.

 

[saas123qa]

Re: Decoupling cap

Normally, decouple capacitor is used to stabilies the voltage if there is afluctuation.

 

[rania_hassan]

Re: Decoupling cap

The place is too important as the nearer the Dec.cap the better performance we have.

 

[vijayanand_ME]

Re: Decoupling cap

I will put in other way.
Any signal is made of harmonics ( different frequency components).
If u put a single capacitor its is filters a particular harmonic only.
u need to put some capactior which is of different values for filtering the different harmonics.
hope it helps
-VJ