Thermistor problem

Hello everybody
I finished making a homemade arduino mega Shield to control a delta 3d printer
I have added three thermistor connectors to the shield

After modefying the pins in the firmware and being sure all are same as the board schematic i tested the board and everything works well but the thermistor always shows 3.1 degrees (it looks like the A0 pin is getting about 5v)

The thermistor is connected between ground and pin A0 and a 4.7k resistor is connected between 5v and A0 pin

So if somebody knows what's the problem please help me

I also tried to connect the same thermistor to an uno board that has a sketch to send the adc on the pin A0 each 0.5 second it gives me about 925 in room temperature (about 27 c)
And i tried the same sketch on the arduino mega used in the printer it gives me about 820 in room temperature (27 c)

Replace the thermistor with a potmeter and see, if the result changes, when you turn the potmeter. That way you can make sure, your code and pin assignment is OK.
If you see diferences between the uno and the mega, measure the 5V regulator of them. When the 5V is not stable, so is the result .
The ATmegas have internal AD-reference voltages for that purpose. ( IIRC 3.3V and 1.5V )
I don´t know, which one Marlin is using?!
-Olaf

Thank you olaf

Finally i found the problem or actually the two problems

The first problem is that i was using an external 5v regulator and it was giving 4.5 and that makes the difference between the uno and the mega so i modified the board to give 5v from the mega voltage regulator instead of the external one and now the mega and the uno gives the same adc numbers.

The second problem was in the configuration of repetier firmware because i added a new thermistor table the same way i used to add it to the marlin firmware but i found that i have to multiply the adc by 4 and temperature by 8 to make the table works with repetier.