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Just a quick question about decoupling capacitors here, hopefully will be something nice and easy!

Let's say you are designing a circuit with 4 IC's that all need a decoupling capacitor. Some datasheets specify the minimum value of decoupling capacitor that should be on the PCB. Let's assume all of the datasheets to these IC's say a minimum of 100nF is required.

If all of the IC's are powered from the same rail, and each one has a decoupling capacitor of 100nF, will this not then mean they are in parallel, and hence lower the capacitance? With 4x 100nF capacitors, will this mean that each IC now has a decoupling capacirot of around 25nF value?

If a datasheet says their IC requires a minimum decoupling capacitance of 100nF, will there be any issues from this now?

As a visual aid:

schematic

simulate this circuit – Schematic created using CircuitLab

If we just ignore the function of the IC's and just focus on the input, we see all 4 are connected in parallel between V+ and GND, is this not the exact same configuration as this:

schematic

simulate this circuit

This, as we all know will give us a total capacitance of 25nF(ish). Am I right in thinking that this is how it would work? And are there any instances where this could cause issues?

Apologies for this question. As soon as I read the answer by Andy aka, I realised how silly it was! Must have been a long week this one!

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    \$\begingroup\$ Beyond the mistake of parallel caps -> larger capacitance, you should keep in mind that each 100nF decoupling cap really only affects the IC its connected to (even though, on a schematic, they're all connected together). In reality, the traces between the caps and the ICs look slightly inductive/resistive as well, which is why decoupling capacitors are supposed to be placed as close as possible to the VCC/GND pins on an IC. It would be mistaken to believe that more capacitance depicted in the schematic is necessarily "better" decoupling, since physical placement is critical. \$\endgroup\$
    – Shamtam
    Sep 15, 2017 at 15:06

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Paralleling capacitors means the net value is the sum total so 4x 100 nF capacitors in parallel means the net capacitance is 400 nF.

If the capacitors are in series then the net capacitance would be 25 nF but this isn't the case in the scenario you described.

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  • \$\begingroup\$ Thanks for the quick reply. I have just realised how stupid that was! Was just a random thought that came through my head! Must have been a long week! \$\endgroup\$
    – MCG
    Sep 15, 2017 at 14:36
  • \$\begingroup\$ Apologies for the absolute ridiculousness of this! (and embarassment of such a simple mistake). Once you acknowledge you have read this, I'll delete the question! \$\endgroup\$
    – MCG
    Sep 15, 2017 at 14:38
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    \$\begingroup\$ Strangely enough, my answer has collected three votes so some people must think the question and answer was useful. I don't see any down votes on your question - everyone can get things back to front now and then. \$\endgroup\$
    – Andy aka
    Sep 15, 2017 at 14:40
  • \$\begingroup\$ I think the votes were all about your answer as it answered the question completely. The question was just a silly one! Unsure if you keep reputation if the question is deleted but I'll accept the answer and leave it up for a few minutes more, any longer and I may die of embarrassment!! \$\endgroup\$
    – MCG
    Sep 15, 2017 at 14:45
  • \$\begingroup\$ Ahh, won't allow me to delete the question.... Guess I'll have to leave it up! \$\endgroup\$
    – MCG
    Sep 15, 2017 at 14:54
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"IC requires a minimum decoupling capacitance of 100nF"

I have always had issue with that number/statement. If you have a 7400 sitting there hanging onto some manual switch input that gets toggled every third Friday in June, 100nF is severe overkill..............

The truth is the amount of decoupling you need is significantly impacted by the frequency of voltage changes going on in the device. In some cases, like my extreme example, you wont need any decoupling, in others you may need three of four decouplers of different values and types to give you the frequency range you need.

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  • \$\begingroup\$ I was actually just using that as an example, as it was an easy number to work with. The actual value was irrelevant to the question \$\endgroup\$
    – MCG
    Sep 15, 2017 at 16:35
  • \$\begingroup\$ @MCG yes I assumed you were, but I figured the relevance of the value chosen depending on application was still worth a mention here. \$\endgroup\$
    – Trevor_G
    Sep 15, 2017 at 16:36
  • \$\begingroup\$ Fair point. Was useful information! \$\endgroup\$
    – MCG
    Sep 15, 2017 at 16:43

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