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ADC Pin of PIC 18F4685 Loading Circuit Unacceptably

Discussion in 'Microcontrollers, Programming and IoT' started by macaulay.flower, Feb 25, 2012.

  1. macaulay.flower

    macaulay.flower

    1
    0
    Feb 25, 2012
    Hello there!

    For a school project, I am creating a simple data acquisition system which uses a frequency to voltage converter to sample a frequency and then display it on an LCD screen on my PIC 18F4685 board.

    I have created the frequency to voltage converter in such a way as that it outputs a voltage from 0-5V for an input of 0-1000Hz.

    When I attach this frequency to voltage converter to the ADC pin (RA0) of the board, the voltage drops from 5V to 4.7 volts (as seen on a multimeter and the LCD screen). To fix this, I added a buffer circuit which consists of a voltage follower (with an op amp). However, this buffer actually makes the problem worse - that is, it drops from 5V to about 0.6 V.

    How can I create this circuit so that the PIC board does not load it so much?

    Thanks for any ideas.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,367
    1,903
    Nov 17, 2011
    This shouldn't happen.
    1) What is the output voltage of the voltage follower without the PIC connected?
    2) Did you make sure to use a rail-to-rail OpAmp for your analog circuit? If you power a non rail-to-rail OpAmp from 5V, the output will not be able to reach +5 V or 0V (see OpAmp datasheet for exact values).
    3) Did you ensure that the port pin of the PIC is configured as analog input to the ADC?

    Regards,
    Harald
     
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