About the external reference voltage, the datasheet says, that reference voltage should be in range of 1.4 volts to AVcc. The footnote tells you that lower reference voltage levels (< 1.4 volts ) may be applied with reduced accuracy requirements. The use of the VeRef- input was already explained by Clemens. You can use it for higher voltages than GND and not for lower voltages. Best regards, Tobias
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