Hello, I am supplying regulated 5V at the AREF pin to use it instead of the internal reference for voltage measurements. However, I have noticed that when 5V is connected to the AREF pin it powers the Arduino Uno even if there is no voltage supplied to the VIN or 5V pin. Is this normal?
Sounds like you are "back powering" the Arduino through the Vref pin when Vcc is off. That is never a good thing.
groundFungus:
Sounds like you are "back powering" the Arduino through the Vref pin when Vcc is off. That is never a good thing.
How to avoid that?
You should never connect a voltage source to any pin of an unpowered Arduino other than Vcc/Vdd. Power all connected devices up and down at the same time.
There are a very few circumstances where it is difficult to avoid doing so. For input pins, put a 10K resistor in series, to limit the current into the unpowered Arduino.
I am supplying regulated 5V at the AREF pin to use it instead of the internal reference for voltage measurements.
Use that same regulated 5V to power the Arduino. Problem solved.
jremington:
You should never connect a voltage source to any pin of an unpowered Arduino other than Vcc. Power all connected devices up and down at the same time.There are a very few circumstances where it is difficult to avoid doing so. For input pins, put a 10K resistor in series, to limit the current into the unpowered Arduino.
So should I put 10K resistor in series with the AREF to avoid "back-powering" when the board is off?
No. Find another solution, like using the regulated 5V supply to power the Arduino.
Hello, I am supplying regulated 5V at the AREF pin to use it instead of the internal reference for voltage measurements.
The ADC might not be accurate if AREF is greater than Vcc (and if they are both supposed to be 5V you can't be sure). I couldn't find anything in the datasheet but you might want to do an experiment to check that.
Power to Aref when VCC/AVCC are not powered allows current to go thru the input protection diodes and appear to 'phantom' the chip/board.
My issue is that I cannot use the internal reference for my voltage measurements, since the Arduino VCC won't stay always at say 5 or 4.8V for my current setup and this won't give me precise ADC measurements.
The 10K resistor does not prevent back powering. It limits the current through the protection diodes to a level that will not cause damage.
What is the source of the regulated 5V that connects to Vref? Why can't that source be used to power the Arduino as well as be the analog reference?
I don't have the details. You need to learn about
analogReference() and how it works.
Herb
I would suggest an external Vref source then, such as
https://www.digikey.com/product-detail/en/texas-instruments/REF1941AIDDCT/296-38445-1-ND/5015869
https://www.analog.com/media/en/technical-documentation/data-sheets/REF19XSERIES.pdf
Pretty precise 4.096V output from a varying 5V input.
My issue is that I cannot use the internal reference for my voltage measurements, since the Arduino VCC won't stay always at say 5 or 4.8V for my current setup and this won't give me precise ADC measurements.
The problem is solved if you power up and down the Arduino and the voltage reference at the same time.
Very simple.
garabetov:
So should I put 10K resistor in series with the AREF to avoid "back-powering" when the board is off?
No because it requires a low impedance source for the ADC to be accurate.
You have to arrange that the pin is never driven until the Arduino powers up. Its your responsibility.
What voltage measurements.
It's common to use one of the more stable internal Aref settings, and divide the input voltage down to that.
Connecting an external 5volt to Aref is a bad idea.
Leo..