Why some ADC pins of ATMEGA328p uses Anlog power while other uses digital power

[yasinzaii]

I was reading the datasheet of atmega328p and I found that PC3 is using analog power while PC4 is using digital power what does it mean actually.

"PC3 can also be used as ADC input Channel 3. Note that ADC input channel 3 uses analog power."
and
"PC4 can also be used as ADC input Channel 4. Note that ADC input channel 4 uses digital power."

[markd833]

I had a quick look at my 2018 version of the 328p datasheet and searching the PDF for the phrase "analog power" resulted in only 4 hits -> pins PC0, PC1, PC2 & PC3. My search of the same datasheet for the phrase "digital power" resulted in only 2 hits -> pins PC4 & PC5.

I did find a question on StackExchange in the electronics section posing the same question. See here. The gist of the was that maybe the analog front end electronics were powered from 2 different sources. I think they were talking about Vcc and AVcc.

I also came across an Atmel note AT15007 that described the differences between the 328p and the 328pb. On page 3 of that note it says that "ADC input channel 7 uses analog power AVCC".

[yasinzaii]

In arduio pro mini and other arduino board, Vcc is shorted with aVcc pin. So in that case we cannot have separate voltage source on aVcc

[MrMark]

It's common practice to separate the power source for analog circuitry from digital circuitry. The reason is so that switching of the digital circuitry can cause voltage transients (noise) on the power supply which would then leak through as noise added to the analog measurements. Thus on mixed analog and digital integrated circuits, like the ATMEGA328P, the IC designer will sometimes separate the analog and digital voltages on separate input pins and in distribution on the chip.

A board level designer may chose to provide some isolation between the voltage rails, perhaps a low pass filter or independent voltage regulators. In non-critical applications, the board designer may simply tie the AV_cc and V_cc pins together, which is the implementation on the Arduino Uno reference design, for instance.

In summary, Atmel separates the analog and digital circuitry power (and ground) to enable somewhat better performance of the analog functions, but the Arduino Uno (at least) circuit board does not exploit this feature, trading a little bit of ADC performance for somewhat lower board cost.