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

Former Member

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!

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  • 0

    If you read the datasheet on the ATMEGA328, it states that the Analog VCC should be the same as the Digital VCC, this would be typically 5V on an arduino and if you want max speed, it can operate way lower if needed but at ta slower speed

     

    The analog inputs should not exceed the chip supply voltage byno more than what appears to be 0.5V

  • 0 in reply to Robert Peter Oakes

    Hi Peter- You are the expert in this area as far as I am concerned. Doesn't the use of the analogReference() function allow one to set the ADC to a shorter domain thus giving a better resolution? I don't think that Marc expects to have an input to the analog pin on the UNO greater than 3.3V. Therefore he wanted the 1023 points of resolution to be between 0 volts and 3.3 volts. His concern was that since a voltage of more than 3.3 volts is possible would it harm the UNO and what would the analogRead() return for a value. I am very new to the analogReference () function so any light you can shine on it would be appreciated.

    John

  • 0 in reply to jw0752

    YYou are correct in that by setting Aref to different values you can change the equivalent full scale range, so if you make Aref to 3.3v then you will get 1023 at 3.3v, he also was concerned about when the input went up to potentially 5v or more in extreamly high temp scenarios, this is where you can not exceed supply voltage by more than 0.5v

     

    also you should not really use the supply as the Aref as it will produce a noisy reading and effectively loose resolution

     

    your available options re 1.1v Aref from internal, 5v internal but from supply so will be noisy and dependent on supply of course or an external Aref, I would suggest a low cost band gap reference chip set to say 2.048 volts to vice a nice range to the reading.

    also there is mention of an input offset which will also reduce effective resolution, this could be removed with a simple OpAmp circuit

  • 0 in reply to Robert Peter Oakes

    The voltage will definitely not go over 5V so i should be fine. Would it make sense to put a 10 µF in parallel to the 3.3V and GND close to the AREF pin to smooth the signal?

  • 0 in reply to Former Member


    Hi Marc - Adding a capacitor will effecively average your signal and slow its response. You could experiment with different values to see if you like the effect.

    John

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