PIC Fixed Voltage Reference?

Discussion in 'Embedded Systems and Microcontrollers' started by spinnaker, Nov 27, 2010.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    My PIC has a Fixed Voltage Reference. But it also has a pin for an external voltage reference and the ability to use VDD as a reference.

    I understand that using VDD as the reference is not entirely accurate. But how does the accuracy of my internal FVR and external voltage reference compare in accuracy?


    If they are the same. in what circumstances would you want to use an external reference?
     
  2. debjit625

    Well-Known Member

    Apr 17, 2010
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    Noise in supply rail i.e.. is noise in VDD is made when different circuit modules switches while the mcu is running.So if your VDD is 5V and you use the ADC feature of your mcu with its fixed VDD as reference voltage then those noise is going to make a wrong adc conversion result i.e.. the adc will see the noise as an analog voltage change rather a noise.So in that case if you use a reference voltage of 3.3V for adc ,the noise will be at much higher level to get detected by the adc while getting a smooth voltage reference level for your ADC module.

    I have used the external ref as 3.3V as its common in digital electronics ,you can use any ref which is supported by your mcu.

    Good Luck
     
  3. spinnaker

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    Thanks. Yes I am fairly aware of the problems using VDD as a reference. My question was what are the advantages of using an external reference over the PIC's internal programmable reference (not VDD, it has an internal reference).
     
  4. nickelflipper

    Active Member

    Jun 2, 2010
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    An external reference voltage comes in handy when battery operation is in play. So in that instance, the internal Vref will vary with the battery voltage. A Vref from say a TL431/LM431 will not vary, if it is less than the lowest expected Vdd from the battery. Accuracy can also be improved over standard voltage regulators.

    EDIT:
    Let me add that it depends on the device selected. Those devices with a fixed voltage reference FVR will do better in battery opreration, but may have lousy regulation over a temperature range, that's where the TL431 shines.
     
    Last edited: Dec 1, 2010
  5. Markd77

    Senior Member

    Sep 7, 2009
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    Which PIC is it? I'd be curious about the results.
    I was looking at the datasheet for the 16F1823 which has internal references of 1.024, 2.048 and 4.096 (so long as Vdd is greater than vref).
    I couldn't find any information on the accuracy apart from the "initial accuracy" which was +/-5% - not good enough for many purposes. Maybe it gets better after a while, I don't know.
     
  6. spinnaker

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    I am using the 18f45k22.
     
  7. Markd77

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    Sep 7, 2009
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    I found this in Table 27.3
    External references are available which are far more accurate, but I suppose this isn't bad for free.

    This is for the 1.024V reference, the others are similar:
    min typ max
    0.92 1.024 1.13
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I recall an app note that used two VRef sources with comparators to get better than 10 bit resolution through oversampling for precision ADC operations.

    Other uses would be a higher precision reference, or one that is continually available to wake up the uC with an interrupt on comparator change.
     
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