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I was watching this Kicad tutorial for an SMT32 chip and at 9:15 it says that the recommendation for each ot the VDD pins on the SMT32 chip is to put 2 100nf capacitors.

On this video: https://www.youtube.com/watch?v=BcJ6UdDx1vg I understood what decoupling capacitors are for, and I also understood why they're needed in parallel and why we can't simply substitute all of them for an equivalent one. It has to do with the curve of impedance. Each capacitance is able to reduce the impedance for some frequency. So that's why we use different capacitances.

However, I can't see a reson why 2 equal ones are needed for each VDD pin. Why not one with equivalent capacitance?

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It is not recommened to blindly believe what is said in random videos on the Internet. There is no such requirement or recommendation for STM32 MCUs bypass caps if you do the fact checking. It might just be a personal opinion or rule of thumb for the person who made the video and it is just mentioned without a disclaimer to confuse others with false facts, or just genuinely wrong without knowing it.

So, two 100nF capacitors are not needed, only a single 100nF capacitor per supply and ground pair is needed.

And even if two 100nF was the recommendation, two 100nF capacitors is not equal to single 200nF capacitor. The capacitance will be equal, but bypassing is not only about capacitance, but impedances. Two caps in parallel have half the ESR and ESL than a single capacitor, so if these parameters are needed then it is common to parallel two or more smaller capacitors to end up with a better capacitor than a single larger one.

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  • \$\begingroup\$ the answer is correct: 2 capacitors in parallel are like 1 capacitor with a better ESR and such supply short spikes of current much better. Also using capacitors of different values can optimize the same thing, as smaller capacitors tend to have lower ESR too, so the smaller one kicks in faster if current is needed from the MCU and the larger one acts as the "long term" resservoir \$\endgroup\$ – schnedan Aug 23 at 9:12
  • \$\begingroup\$ Kemet - a Manufactorer of capacitors - has a tool to view parameters of their parts and calculate their combined behavior: ksim3.kemet.com/capacitor-simulation, just select more than one capacitor and switch the combined mode on... \$\endgroup\$ – schnedan Aug 23 at 9:27

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