PIC18F1220 AVdd and Vdd

Discussion in 'Embedded Systems and Microcontrollers' started by jwilk13, Jul 9, 2012.

  1. jwilk13

    Thread Starter Member

    Jun 15, 2011
    228
    12
    So I've been doing a bit of reading on the reasoning for separate Vdd/AVdd and Vss/AVss pins, and I've come across lots of different solutions on how to use them.

    I'm currently using a PIC18F1220 and I'm using the ADC with internal voltage references selected. I've read that if you're doing that, it's good to put a filter on the analog power supply pins. Others say you can direct connect the digital and analog supply pins together. Others say that as long as you're using bypass caps properly, then you're good.

    Despite all of these differences, one thing everyone says is "According to the datasheet...". I can say that I've been through all 300+ pages of the PIC18F1220 datasheet and can't find anything about what to do with the analog/digital supplies. A separate Microchip document said that this type of information would be in the "Programming Specification" document, which they state is different from the data sheet. I haven't been able to find this document for the PIC18F1220.

    So, the question is: What the heck should I do with the analog supply pins? I know they need to be connected in order for the device to operate, just not sure how.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,362
    All the above suggestions are correct. It depends on what you are trying to measure.

    How critical are your measurements?

    Here are the answers in order of simplicity.
    The resolution and accuracy gets better as you go down the list.


    1. Connect AVdd to Vdd and AVss to Vss.
    2. Same as 1. but make sure you put 0.1μF and 10μF caps between AVdd and AVss.
    3. Put 100Ω or ferrite bead between Vdd and AVdd. Make sure you keep the caps as in 2.
    4. Same as 3. but make sure you have separate solid ground planes for Vss and AVss. Connect the ground planes at one point only, at the power input.
    5. Same as 4. but provide separate voltage regulator and filter for AVdd.
     
    Last edited: Jul 9, 2012
    jwilk13 likes this.
  3. jwilk13

    Thread Starter Member

    Jun 15, 2011
    228
    12
    They're not extremely critical. I'm measuring the output of a hall effect sensor, but that voltage is very rarely going to stay stable. My ADC is 10 bits, and if that value changes by 5 or so (about 25 mV I think), it shouldn't be a big deal. Would connecting them directly be a decent solution? My PCB has a bottom ground plane and is using a 5-V LDO as the power source.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,362
    Then go with solution #2.
     
  5. jwilk13

    Thread Starter Member

    Jun 15, 2011
    228
    12
    Oh, I was already doing that :)

    Thanks!
     
  6. JohnInTX

    Moderator

    Jun 26, 2012
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    Whatever you do, make sure that AVdd does NOT rise before Vdd on power up. The PIC will latch up. Been there! Gets HOT. In my case, it didn't fry the chip, though. Tough stuff.
     
  7. jwilk13

    Thread Starter Member

    Jun 15, 2011
    228
    12
    Interesting...they should rise at almost the exact same time in my case. They're pins 15 and 16, and they're tied together with the capacitor configuration that MrChips suggested in there. I guess I'll find out...
     
  8. JohnInTX

    Moderator

    Jun 26, 2012
    2,347
    1,029
    Our issue surfaced when AVdd was powered by a precision linear regulator and Vdd was powered by a 'simple switcher'. AVdd rose before the switcher stabilized and applied Vdd. Its the only time I've seen it happen but thought I'd get it out there.
     
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