http://www.schramm-software.de/tipps/adc-referenzspannung/
http://www.mikrocontroller.net/articles/AVR-Tutorial:_ADC
http://www.mikrocontroller.net/topic/103378
http://forum.arduino.cc/index.php?topic=56750.0
http://www.atmel.com/Images/doc8444.pdf
http://www.arduino-tutorial.de/2010/06/arduino-schematics
http://electronics.stackexchange.com/questions/30549/atmega328-external-aref-connection