I have a microcontroller (ATMEGA328) that I'm using to do analog data acquisition. The problem is that I want my ADC values to be referenced to a regulated external source that is ~3V, but that source rises faster than my power supply. Since the microcontroller has ESD protection diodes like shown, I think it is likely to blow up when the circuit starts. Is that true? If so, how can I create a low impedance path (<32 Ohm) between ARef and the external source, but still protect the circuit?
The external source is part of an irreplaceable system, so I don't want to just put a Schottky to VCC since that might overdraw the external regulator.
I thought perhaps an n-channel MOSFET with the gate attached to VCC between the pin and the reference?