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I'm a beginner in EE. Lastly I read a lot about this field and I want to jump in.

I think of a toy PCB design, to experiment essentially. I picked the different hardware so that all the components have digital outputs, which is what I want to stick with (less complex, no type mixing, less costly). As well, the power source is an AC to DC adapter. So no AC in the circuit, apparently.

I get that current is not perfect in the real world. So a capacitor near a Vcc pin of an IC is needed to smooth input current and limit its own impedance.

Given these elements:

  • Is it still needed to put a decoupling capacitor to each ground on this DC circuit? Isn't it useful to filter high frequencies (AC) only? Do we really care about the noise a ground may have?
  • would an ethernet PHY really need a decoupling capacitor in this circuit? Do they generate so much spikes by themselves?

I keep seeing decoupling capacitors in the schematics I read (Raspberry Pi, Arduino, and others). But their constraints are different, I guess.

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  • \$\begingroup\$ Decoupling caps are cheap. Use them even when you think you don't need them. You might be wrong. \$\endgroup\$ – Robert Endl Oct 4 '16 at 19:55
  • \$\begingroup\$ This is way too broad question - people study these fields for months if not years, involve physics of electronic/electrical components, their interaction and interference. Start with this one en.wikipedia.org/wiki/Decoupling_(electronics) and then find some online basic course on electronics and digital circuit design, and carefully study through it. \$\endgroup\$ – Anonymous Oct 4 '16 at 19:57
  • \$\begingroup\$ An Ethernet PHY is an example of a chip that should have decoupling caps (and ferrite beads and/or inductors) that follow the manufacturer's recommendations to the letter as a minimum. There is high speed analog circuitry in there- it is not a strictly digital circuit. Of course even digital circuits are analog in a sense. \$\endgroup\$ – Spehro Pefhany Oct 4 '16 at 20:34
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@Larry you think that because this is a DC circuit then there is no need for decoupling caps. But if you have outputs that are going to do something useful then they have to switch on/off. Each time that an IC switches it creates a momentary increase in the current consumed by the IC, which in turn creates a small voltage spike on the Vcc pin of the IC. The decoupling cap is there to filter out this spike before it gets to the Vcc pin of another IC and perhaps cause it to false trigger.

The same decoupling cap on the Vcc pin of an IC filters out some of the voltage spikes (noise) that inevitably get onto the Vcc supply. Generally, decoupling caps are a very basic but essential part of electronics design. As a starting point, use the values suggested by the IC manufacturer.

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Given these elements: Is it still needed to put a decoupling capacitor to each ground on this DC circuit? Isn't it useful to filter high frequencies (AC) only? Do we really care about the noise a ground may have?

The way I would look at this is: "The chip manufacturers recommend them. What additional information do I possess about their chips that they don't know about?"

The answer is "none" so I do what the manufacturer recommends in the datasheets.

Would an ethernet PHY really need a decoupling capacitor in this circuit? Do they generate so much spikes by themselves?

If it's in the datasheet then you need it.

I keep seeing decoupling capacitors in the schematics I read (Raspberry Pi, Arduino, and others). But their constraints are different, I guess.

It's best practice. Adopt it!

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  • \$\begingroup\$ Thanks Transistor, I get the message : "stick to that f*** datasheet" :) +1 \$\endgroup\$ – Larry Oct 5 '16 at 5:37
  • \$\begingroup\$ I wouldn't ever say it like that. \$\endgroup\$ – Transistor Oct 5 '16 at 6:17
  • \$\begingroup\$ :) I guess, but it bolds the point \$\endgroup\$ – Larry Oct 5 '16 at 6:37

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