I would like to use an external 3.0V reference for A/D conversions.
Looking into the schematics, the related pin on the SAM3X8EA is connected to a (filtered) 3.3V supply. The AREF pin in the standard version of the board (mine is R3) is therefore simply a 3.3V output.
On the other hand, the board seems to have a provision to make a simple change to allow to use the AREF as an input to the SAM3X8EA. There is a small white rectangle drawn on the upper side of the board, with the text "3.3/Ext" above and "AREF" on the right of it, nearby to the ICSP connector.
Can anybody tell me what change has to be made?
From looking at the schematic, you would remove the small 0-ohm jumper located at the "3.3" section of the "AREF" component, and move it to the "EXT" section.
You could then apply your new AvRef input to the AREF-pin on the PWMH header (the header with Pin13 and SDA1).
I like my schematic representation better than what's on the Arduino DUE page. I added a bit in the upper left to describe this (see PDF here):
http://arduino.cc/forum/index.php/topic,146574.0.html
-Chris