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Charge injection on PIC10F220

Discussion in 'Electronic Design' started by MooseFET, Jun 2, 2007.

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  1. MooseFET

    MooseFET Guest

    Perhaps someone has already made this measurement.

    I am going to be doing something a bit odd with a PIC. I will
    configure a pin as an output and set it low most of the time. From
    time to time, I will switch it into the high impedance input state,
    wait a little while while a capacitor gets charged and then trip the
    ADC to measure the voltage.

    Doing this saves me an analog switch. I am a bit concerned about the
    charge that gets injected as the port is reconfigured. Does anyone
    have a number for this? Will it vary with temperature?

    Wild guesses welcome.
     
  2. Guest

    The injection varies with rise/fall time, and that will vary with
    temperature.
     
  3. Donald

    Donald Guest

    My guess would be that every run of chips would be different.

    There would be little stability across chips of the same date code.

    You may get it to work on the chips in your junk drawer today, but going
    into any kind of production would fail.

    I saw a design years ago where a 74c04 was used as a charge pump for the
    -Vdd for an LCD display.

    In production another manufactures 74c04 was used.

    Failed each time and we had to go through each board to replace the
    70c04s to the same date code chips.

    When asked where the original chip come from, the engineer said "my junk
    drawer, all 74c04s are the same".

    Using a digital chip in the analog domain is not good design practice.

    donald
     
  4. MooseFET

    MooseFET Guest

    [... Charge injection on PIC pins ....]
    Yes but how different. I often use devices like the 74HC4053 in
    analog circuit. I know that if I stay with the same list of makers, I
    get good enough repeating of the injection numbers.

    [....]
    That statement, I think you will agree is over broad. If the design
    is insensitive to the injection enough, it will work for all PICs that
    are ever made. The "enough" is what I'm working on defining. The
    PIC's input is intended to be analog when the ADC section is in use so
    I wouldn't expect too many surprises due to that part of the idea.

    The logic output is a CMOS tristate buffer. The parameters of those
    devices can't vary too far and have the circuit still meet its specs.
    as logic.
     
  5. Donald

    Donald Guest

    You use an Triple 2-channel analog multiplexer/demultiplexer as an
    analog device.

    Then you want to use a digital part as an analog device.

    My experience says "don't bother".

    Good luck

    donald
     
  6. joseph2k

    joseph2k Guest

    Total available charge should be on the order of the pin capacitance times
    the supply voltage. The injected charge will be less to far, far less than
    that. If this total available charge is not a problem you should be OK.
     
  7. John Larkin

    John Larkin Guest

    Perfectly reasonable.
    Low 10's of pC likely. It shouldn't vary much with temperature. The
    ADC input will probably kick out/suck up more charge than the
    open-drain.

    Many mux'd ADCs also tend to carry charge from channel N to N+1.
    Sometimes you have to digitize a dummy ground in-between to kill this
    effect.

    What's the capacitance and time span you have in mind?

    John
     
  8. MooseFET

    MooseFET Guest

    Yes I think that about sets the upper limit. In real life, the number
    should be less because the charge comes through the gate to drain
    capacitance. To make a fast device that needs to be lower than the
    gate to source.

    I suspect that the current pulses from the ADC running will be as
    big. This doesn't give too high of a capacitor.

    Assume 50pC
    Assume 8 bits
    Assume 5V = Vref

    5V / 256 = 20mV

    50pC / 20mV = 2.5nF
     
  9. MooseFET

    MooseFET Guest

    I'm scaling the resistors and capacitors to keep the time constants
    involved nearly constant. I have one at

    1000p x 10K = 10uS

    The other is quite short.

    The pin will be high impedance for about 10uS before the convert. I
    will then turn on the output buffer, let an instruction time go by and
    switch on the thing I can't tell you about for about 1uS. There will
    then be a 10uS delay to the time to go tristate again. This other
    10uS is to let things calm down before I measure again.
     
  10. Winfield

    Winfield Guest

    I'll not do your sums for you, but I do feel compelled to make
    one comment before I go downstairs and watch the Red Sox beat
    the Yankees. uS is microSiemens, a unit of conductance, whereas
    us, which you want, is microseconds, a unit of time. And etc.
     
  11. MooseFET

    MooseFET Guest


    The microSiemens unit isn't used any more because it had short
    comings.
     
  12. Winfield

    Winfield Guest

    Very funny, ha-ha, but wrong of course. Meanwhile, in baseball,
    the last-place NY Yankees pulled ahead on a homer by A-Rod, to
    win by one run against the first-place Boston Red Sox. Sigh.
     
  13. "Mine!".


    Best regards,
    Spehro Pefhany
     
  14. MooseFET

    MooseFET Guest

    [.....]
    So did the right or wrong team win?
     

  15. Yes.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  16. John Larkin

    John Larkin Guest

    It's only a game.

    John
     
  17. Marra

    Marra Guest

    Simply put a low value resistor in series with the port to reduce the
    max current into the pin.
    Used this configuration loads of times !
     
  18. John Larkin

    John Larkin Guest

    Charge is charge. A series resistor won't change it.

    John
     
  19. MooseFET

    MooseFET Guest

    :)
     
  20. MooseFET

    MooseFET Guest

    That is not always true. The fraction of the charge that is placed on
    the signal line during the time that the impedance of the transistor
    is still low can be caused to flow to the ground by the existance of
    the resistance in series.

    This is a way that the size of the hold step in a sample and hold
    circuit can be reduced at the cost of spreading out the acquire time.
    The amount of reduction in charge can be quite a bit if the gate must
    move a long way before the device starts to turn off.
     
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