ATMEGA AVcc pin - low pass filter I am a newby to the world of electronics and IC chips, and am looking to make an ATMEGA328 based board to monitor 12v batteries. The board will be powered from the batteries being monitored, with a regulated power supply taking 12v input and outputting 5v to the board. 
The data sheet for the ATMEGA328 chip says that AVcc should be connected externally to Vcc via a low-pass filter. A bit of internetting tells me the low-pass filter will filter out noise above a specific frequency, and provides links to calculators that show the affect of different resistor / capacitor pairs.

My problem is that, I have no idea what frequencies I need to filter! All / any advice much appreciated.

Further info: voltage measurements will be via voltage divider circuits, amp hour consumption will be via sensors, all of which will be connected to analog pins for ADC conversion.
The filter is only needed if low noise is important to your application.
For monitoring batteries, you probably don't need the accuracy level at
which you would want the filter. For what it's worth, the Arduino Uno
doesn't have such filter: it's AVCC is connected straight to VCC.

If you really want the filter, the exact characteristics are not
critical. The datasheet suggests 10&nbsp;µH + 100&nbsp;nF, so this
should be a safe choice.



Edit: To answer the question in the comment, yes, it is generally
advised to use decoupling capacitors, usually something like
100&nbsp;nF, even when powered from a voltage regulator.

Notice that both the VCC and the AVCC pins are very close to a ground
pin. Best practice is to use one decoupling cap for each (A)VCC/GND
pair, and to place that cap as close as possible to the pins.