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Why are the Vcc pin and Avcc pin separated?

Latest Updated:03/08/2012

Question:

Why are the Vcc pin and Avcc pin separated?

Answer:

The reason for separating the Vcc and Avcc pins is to separate/isolate the power supply for the A/D converter from the supply of the other blocks such as the CPU, internal memory, and bus.
This minimizes the effect of supply voltage fluctuation from the other blocks to the A/D converter.

If the Vcc and Avcc inputs share the same pin, the supply voltage fluctuation caused by activity on other blocks, such as port output switching, will also affect the supply voltage for the A/D converter, resulting to deterioration of the A/D converter's precision.

The Vcc and Avcc pins have an internal connection (DC & AC coupling).
Therefore, make sure you apply the same level of voltage to the Avcc pin as to the Vcc pin.
If different voltages are applied to each pin, it may cause the current to flow from Avcc to Vcc or vice versa, resulting to MCU failure.

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