Alll this stuff is in the data sheet, of course:

First, there has to be at least a 2V difference between Vref+ and
Vref-. Vref+ can range from Vdd - 2.5V to Vdd + 0.3V, Vref- can range
from Vss - 0.3V to Vref+ - 2.0V.

Dave Johnson

>Hi, all.
>
>
>   I try  1.1.94V on vref+, and vref- is set to gnd and there are on an0
>two resistor connected. One is 15k to vcc and second is a silicon temp
>sensor (900-2000 ohm)which is connected to gnd. The voltage on an0 is
>.877V and the adc measure 1023 on this pin. No matter what is the input
>voltage (gnd or other). If I set the vref+ to vcc then the measured
>value is good, but the resolution is unusable.
>
>I would like to know what is the minimal useable voltage on vref+? Or
>has someone met this problem?
>
>udv
>Csaba
>
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