Wrong analogRead

Hi guys, I've an issue with the analogread with my arduino uno. If I try to measure the voltage of the 3.3v pin power of the arduino, analogread result is 3.9v.
Somebody could tell me why?

Because there is something wrong with the code you didn't post, or the schematic you didn't post in the wrong forum section.

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I've put in this section because is an issue not releted to a project but look like a problem with arduino. The code is a simple analogRead plus a serialprint to see the results. The schematic is a wire connected to the A0 pin for the analog read and then connected with 5v arduino pin (analogread result 1023), or with the GND arduino pin (analogread result 0) or with the 3.3v, in this last case the analogread result is 800 but should be about 700, right?

No, it isn't a problem with Arduino.

Are you sure the analogue reference voltage is 5V?
If it isn't, it is a bit like saying your house has grown, because your metre stick is only 80cm long.

The code is a simple analogRead plus a serialprint to see the results.

There must be more than that, since calling analogRead() doesn't return a value in Volts. You must be applying some mathematical formula to the analogRead() value to get a value in V, so your formula could be wrong and causing the 'incorrect' reading.

Thanks. I've reported directly the value in Volt just for quick understand, the report of the serialprint is in the scale from 0 to 1023. However I've turned on the arduino for 1 hour and after that now the value is 702, so near the 3.3v . I don't know what happened

I've found the mistake, the first time that I've powered on the arduino uno, I've used the Vin pin with a 7v battery. With this power the 5v pin give 4.9v, but the 3.3v give me 3.9v (checked with multimeter). How is it possible? Somebody has the same issue with the power input through the Vin pin?