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What are decoupling capacitors?

Ben

#23604

What are decoupling capacitors? | 3 March, 2003

We have a customer that is requesting decoupling capacitors, and we are not sure what they are or where to buy them. I know that the customer wants ceramic decaps. Should we just look for multi-layer ceramic capacitors with the rating the customer asked for? Any help will be appreciated.

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MA/NY DDave

#23613

What are decoupling capacitors? | 3 March, 2003

Hi

If you don't know what these puppies are, HAVE the customer specify them. You will be MUCH Safer in the long run since if you choose the wrong value they will COME Back at YOU.

He might even need a network of .1 .01

Piles of places to read; Books on Electronic Packaging Capacitor Suppliers (AVX, , , , even the one you currently buy Motorolla

One caution the Wireless guys are eating these.

Basically these devices take out high frequency ripples on the circuit traces by dumping them to another layer rather than propogating them along. high is in the eye of the designer not the producer.

YiEng, MA/NY DDave

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#23621

What are decoupling capacitors? | 3 March, 2003

genny

#23622

What are decoupling capacitors? | 3 March, 2003

I've never heard of a customer saying just 'we want decoupling caps'. Very interesting. In RF applications - frequencies 50MHz up into several GHz, the location of where they are put is just as important as what the value is. Decoupling caps are part of the circuitry and planned very carefully for both value and location. Spec'ing decoupling caps is not like saying 'I want you to use FR4'.

Definition of coupling and decoupling caps, for interest sake. Caps pass oscillations, but block DC. So a coupling cap is usually put in the signal path, allowing the RF signal pass through to the next point in the circuit, while blocking the DC. A decoupling cap does the same thing but for the opposite purpose - it passes oscillations but blocks DC, so it will be used as an RF ground in this case. It will be put where you don't want oscillations, like a power line, and be connected to ground. This will ground the RF signal but not DC. If the circuit is covering a wide frequency range, you'll probably want several different values of caps, to ground different parts of the frequency range. A large value cap will ground low freq well but not handle high frequency. A small value cap will work better at high frequency, but won't attenuate low freq.

So DDave is right - ask the customer what they mean.

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MA/NY DDave

#23636

What are decoupling capacitors? | 4 March, 2003

Hi

Wow Genny,

I don't think I could have given so much detail so fast.

I have designed some high RF test designs for testing purposes to pass only the High Frequency, yet I didn't want to muddy the water with this other side. Now you did it well and didn't muddy anything, WELL at least to me.

Another thing I didn't note, and thought of later is that in addition to everything you wrote the customer, electronic/RF designer, has to decide if the current PCB design is designed and built well enough to dump the High Frequency and not affect any other functional performances.

Again Thanks Genny

YiEng, MA/NY DDave

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MA/NY DDave

#23640

What are decoupling capacitors? | 4 March, 2003

Hi

Just had to poke back at this one.

Good article David F, Thanks!!

A little old yet still great. Hey ever used Pease Porridge from EDN.

Modelling has come a long way in a short while, yet still the real physical reality and the desired electronic or RF results need to be considered when using decoupling capacitors or capacitors in all their other beauties.

Hey Anyone want on board Storage for that signal Jump Start or a short spot of constant gain.

I hate, (well almost), to repeat myself yet even with all this information we have given the topic launcher or to each other as we enjoyed each other's company I think he still has to turn this choke back over to the engineering designers.

YiEng, MA/NY DDave

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MA/NY DDave

#23655

What are decoupling capacitors? | 5 March, 2003

Hi

I was poking around for awhile and gave up on time. With a little more I could probably find a good article.

Yet here is an article that tells you a little about decoupling capacitors although really more focussed on higher values. I thought about giving you a data sheet with ESR, High Frequency Plots, etc.

Hopefully when done reading you will turn the problem back on over to the Engineering design owner.

http://www.murata.com/articles/01c2.pdf

YiEng, MA/NY DDave

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