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Old 22nd June 2012, 02:52 PM   #1
neoinc is offline neoinc  Singapore
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Default Coupling,decoupling,bypass...how do I differentiate?

Not sure if it is a dump qns, but i find the terms coupling, decoupling and bypass of capacitors confusing. How do I use the terms correctly and how to differentiate them from one another.
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Old 22nd June 2012, 03:07 PM   #2
sreten is offline sreten  United Kingdom
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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.
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Old 22nd June 2012, 03:20 PM   #3
DF96 is offline DF96  England
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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.
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Old 23rd June 2012, 06:26 AM   #4
neoinc is offline neoinc  Singapore
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Thank you Sreten and DF96, it is much clearer now. Once I heard a presenter said not to mix bypass and decouple function. Is he correct when making such statement?
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Old 23rd June 2012, 10:22 AM   #5
DF96 is offline DF96  England
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As 'bypass' and 'decouple' adjectives are sometimes used interchangeably I have no idea what he means. The modern usage of 'bypass' often involves adding an extra cap to a decoupling cap (which is essentially what a smoother is) so the two functions are inextricably linked. Can you remember an example he gave?
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Old 23rd June 2012, 02:59 PM   #6
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So what do you guys suggest a decoupling caps for an ic ???
Ceramic , tantalum, multi layer, or electrolyt caps ???
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Old 23rd June 2012, 03:22 PM   #7
DF96 is offline DF96  England
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Any IC, or one particular IC used for a particular purpose?
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Old 23rd June 2012, 03:33 PM   #8
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Let say one ic for a circuit
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Old 23rd June 2012, 03:38 PM   #9
DF96 is offline DF96  England
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Wow, you are really narrowing down the possibilities. I would say that one IC used for a circuit would probably need one or two decoupling caps per supply rail, probably using different dielectrics if two are used. Does that answer your question?
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Old 23rd June 2012, 03:42 PM   #10
neoinc is offline neoinc  Singapore
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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.
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