Activity Forums Questions & Troubleshooting Hardware [MAXI Automation] Analog IN and internal 5V supply deviation Reply To: [MAXI Automation] Analog IN and internal 5V supply deviation

  • Friedl

    Member
    February 25, 2021 at 8:00 pm

    Dear Jarda!

    Regarding your suggestion of using the AREF pin.

    If you open the microcontroller manual https://ww1.microchip.com/downloads/en/DeviceDoc/ATmega640-1280-1281-2560-2561-Datasheet-DS40002211A.pdf, there on page 9, chapter “2.3 Pin Descriptions” you can read:

    AVCC is the supply voltage pin for Port F and the A/D Converter. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter.

    Therefore we can assume AVCC = VCC = 4.874V as measured before.

    When you scroll down to page 365, chapter ” 31.8 ADC Characteristics” you will discover that:

    Vref_MIN = 1.0V and Vref_MAX = AVCC

    Because we know from above that AVCC = VCC, so AREF must always be less than or equal to VCC.

    If VCC = 4.874V, as demonstrated before, then supplying 5V to AREF without supplying 5V to VCC as well is out of specification.

    This suggestion could be dangerous, don’t you think so?

    Greetings
    Friedl

    • This reply was modified 1 month, 2 weeks ago by  Friedl. Reason: typo