ADC measuring between VREF- and VREF+

Discussion in 'Embedded Systems and Microcontrollers' started by Potato Pudding, Oct 17, 2010.

  1. Potato Pudding

    Thread Starter Well-Known Member

    Jun 11, 2010
    684
    92
    Before I put a lot of work into this can somebody give me a quick check answer regarding some PIC features.

    Confirm that I can constrain the ADC measurements between VREF- and VREF+. I would like to use that to limit measurements to within a 1 volt range and need to know if doing that will give a full 10 bit accuracy across that 1 Volt of measurement so that in that range I will get measurements to the Millivolt.

    Have I misunderstood the documentation? (PIC16f887 checking 16f88# family datasheet F version dated 2009).

    I was expecting to set up a complicated external window amplifier to fill the normal 5volt input range from that 1 volt window to get a similar effect, until I looked at the PIC data again.
     
  2. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    There is this table which suggests the error may be above 1 Lsb with 1 V.
    There is no mention of Vref- in the tables, which isn't very helpful.

    [​IMG]
     
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  3. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
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    As the data sheet notes you still need from 2.2 to 2.7V between VREF+ and VREF-
    You need an OpAmp to multiply the input voltage to more useable levels.
     
  4. Potato Pudding

    Thread Starter Well-Known Member

    Jun 11, 2010
    684
    92
    Interesting.

    So it doesn't say that it won't work as far as I can tell.

    Minimum error is +-1least significant bit at 2.7 Volts so +-2.7 mV.

    I think that it could still be worth locating that 5.4mV error bar with 1 mV accuracy if the min and max Vrefs work the way that I am thinking they do.

    I am just worried that I am missing another meaning for them, and assuming they do something different than intended.

    I searched the Vrefs in the PDF and none of the references for them seemed to show anything else.

    I would not bother with an external amplifier. I can get a limit to a 2.5 Volts range located in the middle or even at the top end of the 5 Volts supply range and that will be plenty. If I really wanted more accuracy I would use a device with higher resolution ADC, and there are plenty of those to choose from.

    Being able to locate my sample range where the input naturally falls is actually more important.
     
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