Telford Dorr shared a tutorial on how to apply a precision voltage to the AtMega Aref pin without accidentally blowing up the processor.
There’s a better way: feed the Aref pin resistor with an op-amp, and connect the op-amp negative feedback input directly to Aref. The op-amp will generate whatever output voltage is needed (within its limits, of course) to keep its negative input pin the same as its positive input pin (tied to a stable reference voltage), and thus, the Aref voltage the same as the stable reference.
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