External reference voltage for Arduino Due

Q1.

As documentation says, Arduino Due power supply is based on NCP1117ST33T3G regulator, which is quite precise (1%). But for scientific measurement higher precision can be required. I want to understand is it possible to provide custom reference voltage but further documentation is a bit unclear:

The analogReference() function is ignored on the Due. [ArduinoBoardDue]

Does this mean that reference voltage can not be changed? Or, the board will configure it automatically?

The AREF pin is connected to the SAM3X analog reference pin through a resistor bridge. To use the AREF pin, resistor BR1 must be desoldered from the PCB.

Can I connect precise external power supply after unsoldering the resistor (which will be quite challenging task)?

Q2.

Will change of the reference voltage affect all other voltages on the board?

Yes, if you desolder the resistor, you can connect anything you like to the Aref.

This will not affect anything else, except possibly the DAC outputs.

I must rise this topic again. I'm going to select an appropriate reference element. What current is required?

Does JR1 need to be removed as well?

@brian: Yes, JR1 needs to be desoldered off the board if you are connecting any input to AREF. I'd just slide it over onto the other side of BR1 so that it connects the external pin.

@KVA: Page 1403 in the datasheet: 250uA. It also has a specification for impedance, 14kOhm, which should be very easy to achieve. It may not be obvious, but AREF can't exceed the supply voltage of 3.3V or be negative.

Thank you, @MorganS! You opened my eyes.

Since you say that 14kOhm should be easy to achieve, can you explain how?
Currently I'm looking at this element:

Well, one use of AREF is if you have 5V sensors (like the great MPX pressure sensors from Freescale) and you have a voltage divider from the 5V rail providing the reference voltage. Then you shouldn't choose 15K resistors for your voltage divider. The reason why you would want 15K resistors is then you would waste less power providing the reference. You might then add a capacitor to lower the DC impedance, which also has the beneficial effect of increasing the AC impedance, reducing noise.

That Linear chip looks perfectly adequate. It will have an extremely low impedance, probably under 100 ohms at a guess. Fortunately there is no such thing as "too low" for this application.

Oh for just a pullable jumper and pins like the good old days.

Jumpers and pins are incompatible with 12 bit ADCs. The noise they pick up like little antennae is too much.

@briantee, I think if you desolder JR1, you can return everything back by connecting AREF and VCC+3.3 with external wire. I'm not sure if I right.