What is the purpose of a decoupling capacitor?

[allegro]

First you put a capacitor in order to guard the input pin from a voltage drop in PSU, but then why would you want to add another capacitor?

I've read all about the fact that a capacitor is actually RLC series component, but i dont understand whats the purpose of the B.P cap.

Thanks in advance.

 

[tresca]

Whats the application ?

caps in dc circuits are either used to stabalize voltage levels or prevent them from changing too fast or to block out DC components of a signal.

 

[Ubergeek63]

bypass capacitors supply power pulses to the components that they bypass.

this both allows the component to function normally since the pulses no longer get dropped across the power bus impedance and protects the rest of the system from the power pulses that the component requires.

You often add another bypass capacitor since the different capacitors work for different frequencies. You usually need at least two in critical high frequency systems, say a 0.01uF and a 100pF. If the system sees low frequency pulses as well as would be the case if you were taking a microcontroler in and out of power down you should add a 1-10uF as well.

The lowest values need to be at every pair of power pins as close as possible to the pins with as short traces as possible. larger values do not need to be as numerous or as close the the components as smaller ones.

Dan

 

[spldart]

Super simply put.

A decoupling capacitor keeps a portion of a circuit from effecting another portion.

One active circuit causes variations in the common power supply.

You wouldn't want your 'oscillator' to effect some high sensitivity comparator in
the same circuit would you?

Hope that made sense.

 

[allegro]

Thanks!

Are we talking about the same capacitors?
I was talking about capacitors that are connected in parallel with the input pin, not in series, so how do they protect the PSU from the pulses?

 

[Mikebits]

Bypass caps just love to eat transitions on a DC voltage path. They gobble em right up. Once they smell a DC change they put on their dinner coat. Bypass caps keep a DC voltage nice and smooth. Integrated circuits on the other hand hate transitions on their supply line. It makes them seasick and queasy, they throwup in the form of oscillations, eventually they get a fever, burn up and die. Seems like a symbiotic relationship...

 

[allegro]

Bypass caps just love to eat transitions on a DC voltage path. They gobble em right up. Once they smell a DC change they put on their dinner coat. Bypass caps keep a DC voltage nice and smooth. Integrated circuits on the other hand hate transitions on their supply line. It makes them seasick and queasy, they throwup in the form of oscillations, eventually they get a fever, burn up and die. Seems like a symbiotic relationship...

You're talking on placing a single capacitor in parallel to the input pin.

Why do you need the second capacitor?

 

[Mikebits]

You're talking on placing a single capacitor next to the input pin.

Why do you need the second capacitor?

Ahhh, I see your question now, and this is a good one, let me see if I can give a well deserving response.

So we are taught that caps in parallel add, so one would think that putting a 100pf cap in parallel with a 1uf cap would serve no purpose. But this is not the case.

Going back to my earlier analogy, caps like to eat, but 100pf caps only like fast small glitches, large caps being big and clunky as they are are somewhat slower, they can't catch those fast glitches, but in one fell swoop they can swallow one fat slow glitch. So little caps fast glitches and noise, big fat caps for big glitches and slow noise. End result nice quiet DC line.

Somewhat a childish explanation and I do not mean to insult your intelligence, but sometimes simple explanations are best.
-------------------edit---------------------------

For most designers, they use decouple caps like salt and pepper, just sprinkle them on a board and no longer analyze if they are needed or not. Why? because they are cheap and can't hurt and in most cases prevent possible problems from creeping up

 

[Ubergeek63]

Going back to my earlier analogy, caps like to eat, but 100pf caps only like fast small glitches, large caps being big and clunky as they are are somewhat slower, they can't catch those fast glitches, but in one fell swoop they can swallow one fat slow glitch. So little caps fast glitches and noise, big fat caps for big glitches and slow noise. End result nice quiet DC line.

Didn't I just say that?

 

[Mikebits]

Didn't I just say that?

Yeah you did, but my way was more colorful
And I like to hear myself type... j/k

What can I say, did not mean to steal your thoughts...

 

[spldart]

Ah... Yeah. Different decoupling than I was referring to. sry

 

[Ubergeek63]

Yeah you did, but my way was more colorful
And I like to hear myself type... j/k

What can I say, did not mean to steal your thoughts...

not a problem...slow minds such as mine just confuse easily.

Dan