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In the schematic for the Sparkfun Arduino mini pro 3.3v https://cdn.sparkfun.com/datasheets/Dev/Arduino/Boards/Arduino-Pro-Mini-v14.pdf there's a 0.1uF capacitor (C3) just left of the atmega ground pins going from vcc to gnd. In the upper left corner on the power regulator there's also a 0.1uF (C10) cap from the output vcc to gnd which should be essentially the same thing.

What is the design rationale here? I assume they're doing this for power smoothing but why not have a 0.2uF cap or two parallel 0.1uF caps on the regulator output? Are they trying to say that the other one should be closer to the atmega?

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marked as duplicate by Ignacio Vazquez-Abrams, PeterJ, Chetan Bhargava, Daniel Grillo, Ricardo Dec 13 '14 at 10:12

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  • \$\begingroup\$ @pdel For future reference. When discussing schematic components, please refer to components by their designators. That's what designators are for. A good schematic has to have designators, and fortunately this one does. It's a lot easier to say "C3 and C10", than to say "...0.1pF capacitor just left of the atmega ground pins going from vcc to gnd. In the upper left corner on the power regulator there's also a 0.1pF cap from the output vcc to gnd...". (Posting schematic snippets as images doesn't hurt either. That is, in addition to PDF link.) \$\endgroup\$ – Nick Alexeev Dec 13 '14 at 2:48
  • \$\begingroup\$ I understand what's going on in the linked question, the decoupling capacitors are paired as 10/0.1pF and one is for the power while the other concerns the ADC. In this case they're the same and just connect to the same rails \$\endgroup\$ – pdel Dec 13 '14 at 2:51
  • \$\begingroup\$ @pdel I don't see any 0.1pF (picofarad) capacitors in that schematic. However, there are 0.1uF (microfarad) capacitors. No prizes for guessing how many orders of magnitude apart these numbers are. \$\endgroup\$ – Nick Alexeev Dec 13 '14 at 2:52
  • \$\begingroup\$ @NickAlexeev Good point, thanks for the heads up. I'm rather new to this so every bit helps. \$\endgroup\$ – pdel Dec 13 '14 at 2:52
  • \$\begingroup\$ Oh, yea. You're right. I'm talking about C10 and C3. I'll fix the question. \$\endgroup\$ – pdel Dec 13 '14 at 2:53
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That is a decoupling capacitor for the microcontroller. Yes, it is meant to be as close to the VCC and GND pins as possible. Noise can be picked up on the trace from between the regulator and the microcontroller. It is most likely a ceramic capacitor, which has a very low ESR(equivalent series resistance) rating for good decoupling. Smaller capacitance ceramic capacitors have a lower ESR than high capacitance versions generally. The lower the ESR, the better it is at shunting high frequency noise on the VCC rail to the device.

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  • \$\begingroup\$ Looking at the eagle file from their github, they both appear about the same distance away. The problem is (and I know I didn't state this in the question) the board itself is about one square inch total. The cap seems to go to other pads (it looks kind of like it was back annotated to the schematic) but I can't tell what they were populated with. \$\endgroup\$ – pdel Dec 13 '14 at 3:51
  • \$\begingroup\$ Okay, I do not use Eagle, but I was able to map out which caps are which by looking at the board. C1 is between hole #9 and the switch. C3 is between the microcontroller and the switch. C10 is next to the microcontroller and an LED. C2 is the one all the way in the corner near the RAW hole. You are right, C10 and C3 are both the same distance away from the microcontroller. Although, I believe that C10 is acting as the decoupling capacitor, and C3 is primary used to debounce the switch. \$\endgroup\$ – Sergei_Grishin Dec 13 '14 at 5:00

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