I'm trying to use AREF to set the scale of an AI channel. For the AREF input, I'm stepping down the voltage from a 9V battery via a simple voltage divider (27k and 15k resistors). Before I hook up to AREF, I am reading the expected value of about 2.9V from my voltage divider. When I hook this to the AREF pin on my Arduino, that voltage jumps up to about 3.9V. What is happening here? How can I get AREF to see the correct voltage. (I am calling analogReference( AR_EXTERNAL) in my sketch setup.)
They say a picture is worth a thousand words, so here's the relevant section of the R4 Wifi schematic. Pay special attention to R6.
Sorry more information is needed, you have incomplete labels on the processor and signal lines going to?? such as VREFH0. Your divider does not show. What is powering the Arduino? Aref would normally be powered by the processor VCC.
You can look at the full schematic in the link if you like, but I think the fragment is pretty self explanatory. AREF is pulled up to +5V through the 5.1K resistor R6. Any attempt to use a different external voltage reference is going to be messed about by that resistor.
Ahh. Thank you. That makes sense. When I calculate the effect of that pullup resistor on my simple circuit, I get 3.9V, which is what I am seeing.
It sounds like AREF is not as simple and convenient to use as I had expected. I need to change my approach.
This has been a big help.