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PIC micro ringing power supply

Discussion in 'Electronic Basics' started by [email protected], Sep 19, 2005.

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  1. Guest

    I am playing with PIC microcontroller 16F628.

    I am seeing high frequency ringing on my power supply with nothing else
    hooked up, except the voltage regulator and the PIC. The noise goes
    away when I pull the PIC so its not the power supply. (Lab supply -->
    7805 regulator)

    The waveform is:
    around 15 mHz
    15mV peak to peak

    I have input capacitance on my regulator 100uF and output 4.7uF and a
    bypass cap next to Vcc at the PIC of .1uF The circuit is wire wrapped,
    and am taking everything back to a common ground pin. The waveform has
    been observed on two different scopes. Also, I pulled out one of my
    older projects, using a PIC16F628 and the waveform is visable on the
    rails there too.

    Any idea where this waveform is coming from and better yet how do I
    flat line it? I realize 15mV is not that large but I am trying to
    amplify a small DC voltage with an op-amp, and I also have a 12 bit ADC
    that will use the positive side of this rail as voltage reference.

    The waveform looks like a decaying Sine wave, amplitude large then
    slowly decaying and then amplitude spikes large and then slowly decays
    over and over at about 15mHz.

    I may be picking nits, but would like to get rid of it, I guess I could
    build another separate rail for my Vref and Op Amp but it seems
    wasteful.
    Thanks for any help
    ED
     
  2. mike

    mike Guest

    Are you sure it's not the power supply? Removing the transient load
    will eliminate the transient load response of the power supply.

    Are you sure it's not in your measurement technique?
    If you expect to see substantially less than 15mV p-p on your power
    supply, you'd better be using power and ground planes, lots of
    decoupling, ferrite beads, etc...
    And you'll need a dedicated socket for the probe. You can't just hook
    a long ground lead up anywhere, poke the probe anywhere and expect to
    make low level measurements.

    Your processor may have a mode where it stops the internal clock during
    the A/D measurement interval.
    mike

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  3. Rich Grise

    Rich Grise Guest

    Move the .1 to as close to the regulator output as is physically possible,
    and put the 4.7 on the board, and add at least one .1 uF for every chip,
    preferable more sprinkled around the power/ground buses.

    What's the clock freq. of the PIC?

    Good Luck!
    Rich
     
  4. Guest

    Thanks for the reply:
    This makes sense, I guess it could be the power supplies response to
    the PIC
    I am grounding the scope at the circuit. I don't know much about how
    to add more filter caps or beads. I've tried larger and smaller caps,
    whatever is ringing, rings right through. The interesting thing is the
    signal does not look like 15mV random noise, its very clearly a sine
    type signal, repeating, and fast.
    Care to elaborate on the dedicated socket?
    This is a good idea, I will check.

    I thought possibly its just something common in high speed digital or
    the PIC family. I am using an internal oscillator at 4mHz. Its a very
    small and insignificant amount of voltage, but it is interesting and
    puzzling to me. I just spent a week futzing about with an op amp
    circuit with high gain, so I am particulary focused on the small right
    now.

    I know just enough to make me dangerous!

    again thanks for the quick response,
    ED
     
  5. mike

    mike Guest

    Where is the cap? If it's a cap with wires, how long are the wires from
    the VCC to the Ground pin on the pic? That inductance resonates with the
    capacitor. There's also significant resistance in the cap.
    So, if you don't have ground and VCC planes on your circuit board and
    surface mount caps with proper, low inductance, connection to the
    planes, you'll never get rid of this low level ringing.
    There are probe sockets designed to mount on the board and accept your
    probe tip. Put one RIGHT AT the cap.
    Probe has the same problem as the cap did. Inductance seriously
    degrades the measurement. Also picks up stray signals. Stick the probe
    tip on ground near where your ground clip is hooked. I betcha you still
    see most of the ringing. Move the probe a little bit and you'll see
    more ringing.
    I can connect the ground clip to the probe tip, waive it in the air and
    see FM radio and TV stations ten miles away.
    ACCURATE probing is extremely difficult.
    Your op amp has a spec on power supply rejection. That's often a big
    number. Bigger problem is the bias resistor that you hooked to the
    noisy power supply. Filter that connection. Not much you can do about
    the PIC 'cause it uses the same chip pin for both digital and analog
    stuff. You often have to average many readings to get decent precision
    with a PIC A/D.

    mike



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    FS 512MB 45X SD Flash memory.
    Return address is VALID but some sites block emails
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    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
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  6. Guest

    I will try these changes later today, moving the 4.7uf out to the
    middle of the board, and putting a .1uF right at the regulator, this
    makes sense. Probably I will put a .1uF right at my ground pin and
    power pin since this is wire wrap.

    Begginning to realize I am just being to nit picky and that this is
    something that I will just have to embrace and work around in other
    ways.

    Thanks for the suggestions!
    ED
     
  7. Noway2

    Noway2 Guest

    With a wirewrap implementation you will probably not be able to get rid
    of the ringing. For that matter, your circuit logic will be limited to
    a fairly low speed (not clock speed) but signal rise / fall times due
    to wiring inductance. As a result, depending on what devices you have
    in your circuit, it may or may not work. Try to stay with slow logic
    devices. As I understand it, Microchip PICs tend to be very easy to
    work with in this regard, so you have a good chance. Instead of a
    wirewrap, you may wish to consider a one or two sided PCB. There are
    free tools available that will do the job.

    If you do stay with wirewrap, keep the wiring as short and neat as
    possible and place a good bypass capacitor as close to every power pin
    as possible. Avoid using Aluminum Electrolytics, a better choice would
    be ceramics.

    Good Luck!
     
  8. Guest

    I can see a little more clearly now, thanks.
    Oh Man! you are so right! Grounding on regulator ground, and then
    looking just a few centimeters away at another ground pin, while I
    don't see the same waveform its just as nasty. In fact probing to the
    ground pin of the scope itself, if I hold my hand just right, I see a
    very similar waveform to what I have in the circuit of almost the same
    amplitude. I am beggining to see the light.
    Understood, I believe you, thanks so much for your feedback,
    I also read up a bit over in sci.electronics.design on bypass caps, so
    understand now more about surface mount bypass and ground and power
    planes. All of it slowly creeps in.

    Will, just quench it as much as I can and, design around the rest of
    it.
    Thanks again,
    ED
     
  9. Guest

    These are all very good suggestions,
    I have taken note. Eventually I will make a board for this project, but
    right now I am still in the prototyping stage and experimenting.
    *Obviously*
    Good suggestion about slower logic devices this makes sense and I
    probably would have done the opposite.

    Again thanks to all for the comments,
    this has been an eye opener, and will help improve my technique as I
    move on to more complex projects. I appreciate the feedback.

    _later
    ED
     
  10. Guest

    Rich Grise wrote
    I will try these changes later today, moving the 4.7uf out to th
    middle of the board, and putting a .1uF right at the regulator, thi
    makes sense. Probably I will put a .1uF right at my ground pin an
    power pin since this is wire wrap

    Begginning to realize I am just being to nit picky and that this i
    something that I will just have to embrace and work around in othe
    ways

    Thanks for the suggestions
    E
     
  11. Noway2

    Noway2 Guest

    With a wirewrap implementation you will probably not be able to ge
    ri
    of the ringing. For that matter, your circuit logic will be limite
    t
    a fairly low speed (not clock speed) but signal rise / fall times du
    to wiring inductance. As a result, depending on what devices you hav
    in your circuit, it may or may not work. Try to stay with slow logi
    devices. As I understand it, Microchip PICs tend to be very easy t
    work with in this regard, so you have a good chance. Instead of
    wirewrap, you may wish to consider a one or two sided PCB. There ar
    free tools available that will do the job

    If you do stay with wirewrap, keep the wiring as short and neat a
    possible and place a good bypass capacitor as close to every power pi
    as possible. Avoid using Aluminum Electrolytics, a better choic
    woul
    be ceramics

    Good Luck
     
  12. Guest

    I can see a little more clearly now, thanks.
    Oh Man! you are so right! Grounding on regulator ground, and then
    looking just a few centimeters away at another ground pin, while I
    don't see the same waveform its just as nasty. In fact probing to the
    ground pin of the scope itself, if I hold my hand just right, I see a
    very similar waveform to what I have in the circuit of almost the same
    amplitude. I am beggining to see the light.
    Understood, I believe you, thanks so much for your feedback,
    I also read up a bit over in sci.electronics.design on bypass caps, so
    understand now more about surface mount bypass and ground and power
    planes. All of it slowly creeps in.

    Will, just quench it as much as I can and, design around the rest of
    it.
    Thanks again,
    ED
     
  13. Guest

    These are all very good suggestions,
    I have taken note. Eventually I will make a board for this project,
    but
    right now I am still in the prototyping stage and experimenting.
    *Obviously*
    Good suggestion about slower logic devices this makes sense and I
    probably would have done the opposite.

    Again thanks to all for the comments,
    this has been an eye opener, and will help improve my technique as I
    move on to more complex projects. I appreciate the feedback.

    _later
    ED
     
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