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In Arduino UNO R3 official schematic the AREF pin is not connected to anything:

AREF pin

While in most online Arduino schematics (such as this one) it's connected to a 100nf capacitor and then to the ground, the other two differences are a 100nf capacitor between VCC pin 7 and GND pin 8 and a 10uH inductor between AVCC and 5V.

  1. Why in some schematics it's recommended to connect AREF to a 100nf capacitor?

  2. Why there's a capacitor between pin 7 and 8?

  3. Whats the use of that inductor on pin 20?

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  1. the AVR datasheet recommends a capacitor to ground to stabalise the AREF pin, It recommends 10nF, but 100nF sems to be the most common decoupling capacitor value used on most designs for other things, so likely just choosing to use the same value on that schematic you chose,

    AREF is the analog voltage the ADC is measuring relative to, it can generally be either some internal bandgap reference, e.g. 1.2 or 2.5V, the AVCC power rail, 5V in this example. or may be fed from outside, for the internal reference options a small capacitance makes it more stable across conversions

  2. This is called a local decoupling capacitor, the arduino can switch its outputs very fast, when it does it tries to draw a large pulse of current from its supply pins, by having a capacitor to supply this current very close to those pins, it prevents the noise from that large current spike from getting out to the rest of your circuit, e.g. anything else connected to the 5V supply

  3. This is to filter out those current spikes from the main digital supply pins a little more, its called an "LC Low pass filter", the inductor prevents fast current spikes from getting through it, keeping the analog supply as clean as reasonable,

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  • \$\begingroup\$ side question;so the Arduino UNO AREF is connected to 5v, what happens if i connect the 3.3V output to AREF pin? \$\endgroup\$ – Hamed Aug 26 at 8:53
  • \$\begingroup\$ You have to make sure that you never try and switch AREF's source to anything but external, due to this, it is generally wise to have some resistor between it and 3.3V, leaving the capacitor to keep it smooth, e.g. 1000 ohms \$\endgroup\$ – Reroute Aug 26 at 9:02

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