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Discussion in 'Microcontrollers, Programming and IoT' started by Rajinder, Nov 21, 2019.

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  1. Rajinder


    Jan 30, 2016
    I have a 2.2uF cap and 330K resistor in parallel going into the A/D channel of the PIC at the emitter of a phototransistor.The Top part of the circuit is part of a light sensor. The PIC has a 10 bit A/D.
    However I am not seeing a full scale of 0V to 2.8V. Note. Using VDD (3.3V) as VREF for A/D. I am seeing around 690mV.

    Do i need to lower the resistor value as i think the impedance is greater than the 10K input impedance of the PIC A/D. Hence the holding cap can't sample for long enough. Also any A/D leakage current multiplied by the 330K may cause a voltage drop. Or do i try snd reduce the 2.2uF to a lower value.

    I cant seem to upload a schematic at all.

    I would be grateful for any help.
    Thanks in advance.
  2. davenn

    davenn Moderator

    Sep 5, 2009
    don't even know why you would have a cap in there ?? never seen that on any of the A/D's I have had anything to do with

    use jpg or gif 1024 x 768 as absolute max, preferably much smaller
    and compress any jpg to 100kb or less in size
  3. Rajinder


    Jan 30, 2016
    Is the 330K causing the issue as it increases the impedance. Or is the 330K x ADC leakage causing a voltage drop. Or should I reduce or remove the 2.2uF cap?
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Which issue? Your data is incomplete. You "see 690 mV" at which input voltage? Where? On the ADC input or on the ADC's digital output?
    No schematic, no reply (at least very probably not).

    What exactly are you trying to do? Where do you experience problems? Looking at the ADC input circuit is only part of the job. What is your signal source? The parameters of the signal source (e.g. voltage level, output resistance) need to be known to calculate the effect of the ADC input resistance on the circuit. Show us the ful circuit from voltage source to ADC input.
  5. BobK


    Jan 5, 2010
    Are you saying that measuring the voltage at the input you get a full range of 0 to 2.8V, but the ADC is reading a range of 0 to 0.690V?

    If so, one possibility is that you are not sampling for long enough. Did you go by the recommendations in the datasheet?

    The capacitor will provide low impedance for reading it since the hold capacitor is much smaller than the one in your circuit. Of course, it will limit high frequency response. What kind of response time do you need?

  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Yep! Tell us what you are trying to DO then let us suggest one or more ways to do it. What PIC are you using? Why can you not upload a schematic? A hand-drawn schematic is preferable to none at all. What is the part number of your photo-transistor? Have you considered using a reverse-biased photo-diode instead? These have very linear current versus intensity responses over at least seven decades of intensity. Some analog signal conditioning may be required to match your range of intensity requirements to the PIC A/D input range, but the necessary circuits are quite simple. There may be op-amps involved, but these are inexpensive components and you should learn how to use them... they come in handy for interfacing the real world with digital electronics.

    Good luck with your project, WTF it is.
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