Can you damage the ADC with a voltage above Vref?

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1

Hi,

Is it safe to applying a voltage to an ADC pin that is less than Vcc, but higher than the analog reference voltage, be that 1.1v, or an externally applied reference?

Thanks.

2

IIRC you should not apply voltages above Vcc, voltages between AREF and Vcc are mapped upon MAXVAL/1023

To be sure check the Atmel datasheet

3

paulrd:
Hi,

Is it safe to applying a voltage to an ADC pin that is less than Vcc, but higher than the analog reference voltage, be that 1.1v, or an externally applied reference?

Thanks.

Yes it is safe. The electrical limit is the same as any I/O pin (except the reset pin) equal or less then the chip's operating Vcc voltage and no negaitive voltages greater then -.5vdc (from my memory, check datasheet). And if you apply a voltage greater then the actual voltage reference (1.1, 2.5, Avcc, external Aref) then the analogRead() function will just return the maximum 1023 count value, but no damage will occur.

Another 'damage mechanism' is if you apply an external voltage to the chip's Aref pin but select other then external reference as the voltage source. This can cause high current flow on the Aref pin based on the difference between the external voltage applied to the Aref pin and the actual internal reference selected. This is covered in the reference for the AnalogReference() function in the WARNING section:

Bottom line is to be very careful when planning on the use of an external reference voltage applied to the Aref pin. On standard arduino boards this pin is unconnected with just a bypass cap wired between it and ground for extra noise filtering for ADC readings.

4

Thanks, that's good to know.