5V on ADC that is set to a reference of 3.3V? What happens?

Question

Hi,

Normally you can put up to 5V to the analog-pins. Let's say you set your reference to 3.3V by connecting the 3.3V to the AREF-pin and call analogReference(EXTERNAL).

What happens now if you put 5V to the pin? Will it still put out 1024 as MAX-value or will it burn the arduino?

Background:

I have a thermal sensor that has a range from 500 to about 1700 Ohm for a temperature range from 0°C to 200°C. Combined with a 1kOhm resistor I want to use this as a voltage divider for the analog-in. With these settings the voltage will be between 1.6V and 3.15V, so i can't use the full range of the ADC but by setting the reference voltage to 3.3V I can increase the useable area from about 30% to 50% which gives me a better temperature resolution (0.4°C/step instead of 0.6°C).

BUT when the temperature increases above 235°C the input voltage will increase over 3.3V!

Thx in advance for your answers!

jw0752 · Accepted Answer

Hi Marc - You are correct. Any voltage above the 3.3 volts will not harm the Arduino but all values will return a 1023. Here is a link to Arduino home that covers this. http://forum.arduino.cc/index.php/topic,119109.0.html

Robert Peter Oakes · Answer

So a couple of things

A cap on the ARef will not slow down measurement (Assuming you are referring to putting it there to provide a bit of smoothing) it will help a bit as long as your not running anything else on the 3v3 supply it should remain stable but don't expect the best quality reading

there is an internal Band Gap reference of 1.1V that will be better, all you would need to do it use a resistor divider to bring your analog signal into the right range and this should be more stable than the 3v3 rail

Another option is to use an inexpensive 12Bit ADC from TI or Microchip connected via SPI or I2C if you need better resolution