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?
Answer
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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