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Decoupling

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gtr_stitch

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Okay...
MAJOR N00B question here...
I have heard about using a capacitor to decouple a circuit...but what exactly does that mean? What is it's purpose?
I know I could have googled it, but I prefer actualy human interation rather than a pompous search engine ("did you mean...?").
And I know that my need for human interaction on this subject may directly lead to my untimely death by sniper (who is commissioned to take out n00bs like me) so a prompt reply would be appreciated :p
 
Hello there,

To "couple" usually refers to using a capacitor to inject an AC signal into a circut while eliminating the DC component of the signal.
To "decouple" is the contrary, to reduce the AC signal while maintaining the DC component.

Decoupling is usually used to get rid of noise, which is an AC signal of varying frequencies and/or amplitudes, or to reduce the effects of a signal in one part of the circuit from interfering with another part of the circuit.
 
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Decoupling caps are filter caps that provide surge reserve to a specific functional circuit. Another application is to provide a short path RF bypass for an RF circuit.

Example of decoupling for a microprocessor is several 0.1 uF caps connected close to the power pins to ground (Vss) terminals. CMOS processor draw very high current peaks for short period of time when logic changes states. The decoupling caps give up some of their charge during the high current spikes. This reduce the voltage ripple on the supply line.

The actual value depends on application. An RF bypass decoupling cap has to take into account the path inductance so it provides a low impedance path at the functional frequency of the circuit. Ceramic caps have lower internal resistance.
 
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So, basically they add stability and eliminates any unwanted noise...right?
 
I think you can think of it like you said, but replace "eliminates" with "potentially reduces". You should also realize that ceramic caps are much better for this than most other types of caps. My senior design project several years ago had 100uF electrolytics sprinkled around the board for decoupling caps... all I needed was about 6 0.1uF ceramics and it would have worked MUCH better.
 
Awesome! Much appreciated! Now to venture for a smoke break and await the sniper...
 
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