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I often see those two terms explicitly used as synonyms, but sometimes from the context I would say they're not. So, is a bypass capacitor exactly the same as a decoupling capacitor?

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    \$\begingroup\$ The two terms are used interchangeably en.wikipedia.org/wiki/Decoupling_capacitor. But sometimes bypass is IC supply bypass capacitor and decoupling capacitor is when it is a filter with resistor. \$\endgroup\$ – G36 Dec 11 '16 at 14:52
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    \$\begingroup\$ It's the same thing, from two different viewpoints. If you look at it from a "stable voltage" viewpoint, you want to provide an alternative source of power that can handle sharp transitions in power demand. If you look at it from a "no noise on the power supply lines" viewpoint, you want to decouple the ICs generating noise from the distribution rails. However, "stable voltage" and "no noise" are really the same thing -- just whether you set your scope to DC or AC mode for measurement. \$\endgroup\$ – Simon Richter Dec 11 '16 at 15:34
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Yes, decoupling capacitor and bypass capapcitor are the same thing.

These refer to a capacitor located physically close to something drawing power. The capacitor holds the local voltage up for the short time until the current feed can catch up. Due to inevitable inductance of longer lines back to the power supply, the current in these lines takes a little while to change. The purpose of a bypass or decoupling capacitor is to provide the little extra energy during sudden current demands until more current can be supplied from further away.

Since decoupling or bypassing is a high frequency issue, the caps for this purpose must be chosen for low impedance at high frequencies. Their bulk storage capability is not of much importance. In a practical sense, this means they are usually ceramic.

In ye olde days (1980s, or even back into the pleistocene like the 1970s), such capacitors were usually 100 nF ceramic disks. That was about the largest ceramic that was small and affordable. Nowadays, SMD multi-layer ceramic 1 µF capacitors have better characteristics, and are cheap and readily available.

For ordinary use, like around a microontroller or digital chips connected to one, 1 µF ceramic is a good choice. If you're doing RF, then you have to look at the capacitor impedance charts more carefully. I once used a specific model of 100 pF cap for bypassing a RF chip because they had lower impedance at the frequency of interest than other caps with higher values.

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    \$\begingroup\$ Bypass capacitor is also a term used in analog circuits e.g. the emitter bypass capacitor in the common emitter transistor amplifier. In this case I would say that bypass and decoupling mean different things. Bypass provides a separate path for the AC signal, while decoupling is used for power integrity. \$\endgroup\$ – user110971 Dec 11 '16 at 15:38

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