Table 35-33 (page 719) of the SAM3N00B datasheet appears to say that the current drawn from the ADVREF pin by the micro's ADC module is 250uA.
Is this correct?
If it really is that much then that means the micro's external voltage reference circuitry (based around a TLV431) will not be able to keep the reference constant at 3.064V (due to the volt drop across the 220R resistor in the external voltage reference circuit)
The following is the external reference circuitry connected to the ADVREF pin of the SAM3N00B microcontroller.......
External Voltage reference circuit feeding the ADVREF pin (also attached in pdf)
Table 35-35 on page 720 says that a further 300uA is drawn into the ADVREF pin by the micro's DAC module.
This makes things even worse.
Does this much current really get drawn into the ADVREF pin by the microcontroller?
SAM3N00B microcontroller datasheet: