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I've noticed that many circuits use 22pF capacitors. For instance, the Arduino uses them to ground the oscillator pins of its AtMega328. Many tutorials call for exactly this value for simple circuits.

Why is specifically 22 pF so common? Why not 20 or 25, which are arguably "rounder" numbers?

dotancohen
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    Because 22 is one of the E-12 numbers. I could write a looong answer but you can find it all in [wikipedia](https://en.wikipedia.org/wiki/E_series_of_preferred_numbers#E12) – Oldfart Mar 01 '20 at 12:02
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    [This question and its answers go into some detail.](https://electronics.stackexchange.com/questions/37717/why-are-the-common-resistors-and-capacitors-rated-the-way-they-are) – JRE Mar 01 '20 at 12:04
  • @Oldfart Thank you. I was hesitant to ask because I was not sure if this was a good question. However the existence of that page reassures me that this was actually a great question with a bit of history and lot engineering. – dotancohen Mar 01 '20 at 12:05
  • Thank you JRE, yes, that is also extremely relevant. – dotancohen Mar 01 '20 at 12:05
  • [What about 47 pF or 4k7 or 470 ohms](https://electronics.stackexchange.com/questions/67975/what-is-the-reason-that-the-value-47-is-so-popular-in-electrical-engineering/67984#67984) – Andy aka Mar 01 '20 at 12:09
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    I do question the comment that the Arduino uses the 22pF capacitors "on the ATMega328 ground pins". The ground pins of the MCU are just part of the ground rail for the whole circuit and the 22pF capacitors are in crystal oscillator circuit happen to also connect to ground. – Michael Karas Mar 01 '20 at 13:37
  • @MichaelKaras Corrected, thank you. – dotancohen Mar 01 '20 at 15:56

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