Let me make that clear: I am not an electrical engineer, but I am a curious person. So,
I was wondering why I need to put a low-pass filter at AVcc so that I can use the ADC (the AVR's manuals say that). I mean, wouldn't low frequencies affect the ADC measurements too? Wouldn't a solid voltage at AVcc be better? So, as I said, I am not an engineer, but I'd really appreciate if someone could give me some pointer on that.
The low pass filter is to keep any noise on the VCC line from going into the AVCC pin if wired up per figure 23-9 on page 258.
This helps keep any switching noise that is picked up out of the conversion process to yield better results.
23-9 of the ATMega328 data sheet.
Summary and complete datasheet available from atmel
Yes, switching generates a lot of high frequencies. The inductor only wants to let DC thru, so it blocks the high frequencies, and the capacitor lets the lower frequencies go to ground as it wants to let AC thru.
Do most folks connect AVCC to VCC? Or just go with the internal 1.1V reference?
I've only done digital with the Atmega's since I started using them in August.
Ok, I think I had myself confused earlier.
The Duemilanove schematic clearly shows AVcc being connected to Vcc (the Promini shows this also), and a decoupling cap. ARef is connected to an IO pin and has a decoupling cap.
So I guess if AVcc is connected directly to Vcc, it would be difficult for us as users to get a 10uH inductor installed per the ATMega datasheet.
Not sure now where I was going with that, so I am not sure if I satisfied myself with anything or not!