adcstm32stm32f10x
There seems to be a badgap voltage reference built-in in the STM32F1 microcontrollers (on ADCx_IN17).
Coming from AVRs it seems to me very strange that it's not possible to use this reference as the ADC VREF, at least I think it's impossible. Am I right ? Why is it there than ?
You must not connect a negative voltage to VREF-. The "-" only denotes that it's the lower voltage. If you check the manual, p. 209 it will state:
VREF-: The lower/negative reference voltage for the ADC,VREF- = VSSA
and for VSSA:
VSSA: Ground for analog power supply equal to VSS
In the same datasheet , indeed, on the same page is the following diagram:
The diagram you've included above is captioned "capacitively coupled"
Given you're only capacitively coupling a DC signal you should only see "noise".
Best Answer
It is possible to use the internal reference, albeit indirectly. You measure it using the Vdd, and then measure the external voltage you are interested in, using the same Vdd as ADC reference. You can then calculate what the supply must be, knowing Vref and the counts it gave. Then you know more accurately what the ADC is using, and can calculate the external channel voltage you want. The assumption here is that Vdd does not move much between the two readings. If this is a concern, measure Vref before and after measuring the channel with your data on, and use an average. For battery operated systems, this gives you the battery voltage as well, which is probably of interest. This is not as accurate as an external precision reference, but is cheaper, uses less power and does not tie up extra pins...