Coupling,decoupling,bypass...how do I differentiate?

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Hi,

A coupling capacitor in in series with a signal and blocks DC, either a residual
DC offset of a signal nominally symmetrical around ground for bipolar power
supply rails or blocks half the voltage of single power rail to allow symmetry.

Decoupling capacitors generally go between the rails or from a circuit point
to one of the rails to smooth supply ripple at that point. They are not in
series with the signal.

Bypass caps can be used in both cases. Usually smaller higher quality
capacitors placed in parallel with bigger lower quality types, usually
to improve the high frequency performance of the capacitor.

rgds, sreten.
 
Coupling, decoupling and bypass are broad terms. Useful when not pushed too far. Some components may combine these applications.

Generally, a coupling capacitor passes an AC signal from one stage to the next where, in the absence of that capacitor, there would otherwise be little coupling between them.

A decoupling capacitor does the opposite: it reduces an unwanted coupling between stages (typically via the supply rail). More generally, it reduces the signal voltage at a point (e.g. emitter decoupler).

A bypass cap offers a current an alternative route. Nowadays often used to describe a lowish value film cap placed in parallel with a larger value electrolytic, but also used to describe some decouplers (e.g. emitter bypass = emitter decoupler).

Don't worry too much about the names but instead think about the function of a particular cap in a particular circuit position. The names are useful when describing the circuit.
 
The presentation was on EMI of PCB. He mentioned in his slide, "scattering bypass capacitors (not decoupling) throughout the PCB, will reduce overall PDN impedance and minimize plan bounce."

I started to wonder later after the presentation what is the different.
 
neoinc said:
The presentation was on EMI of PCB. He mentioned in his slide, "scattering bypass capacitors (not decoupling) throughout the PCB, will reduce overall PDN impedance and minimize plan bounce."
It is possible he meant 'bypass' to mean middling value ceramics (say, 1-10nF) and 'decoupling' to mean electrolytic or tantalum. Many years ago, when chips were much slower, people used to put a ceramic by each chip and an electrolytic at the end of each row of chips.

Overdream said:
I mean, what is the best caps ??
Best for what? There is a world of difference between decoupling for a 741 op-amp (10's of kHz) or a video amp (10's of MHz) or a microwave amp (many GHz). Component value and type is chosen to meet particular requirements. You haven't given any requirements.

If I asked you "What is the best way to travel?" what would you say? You might want to know where I was going and how quickly I wanted to get there. Air travel is very good for visiting other countries, but a poor way of getting to the other end of my suburban garden. Conversely, walking is a good way to travel in my garden but a bad way to cross an ocean.
 
There is a world of difference between decoupling for a 741 op-amp (10's of kHz) or a video amp (10's of MHz) or a microwave amp (many GHz). Component value and type is chosen to meet particular requirements.

To choose the correct capacitor for a given requirement, can i safely use the formula 2*pi*F*C to find the cutoff freqency i want to decouple? It should acts a like a low pass filter to filter away the high frequency.
 
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