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Scope Probe Enhancing Performance of Circuit

Discussion in 'Electronic Design' started by [email protected], Feb 10, 2006.

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  1. Guest

    I have a circuit that acts stangely enough when I connect a ground
    connection of a scope probe to it.

    The circuit has a TI DSP 280x Series Processor, an Isolated Switching
    Power Supply All the AD inputs and GPIO are isolated to the AC Line. In
    my evaluation of the performance of the circuit I found that the
    systems is able to perform accordingly when the scope probe ground is
    connected to the circuit. I don't even need to have the probe connected
    to the circuit, just the ground. The probe must be connected to the
    scope in order for this to happen. If the this connection is not made
    the device sometimes delays to react to the input accordingly. I'm
    using a portable scope (THS720 Tektronix with isolated ground
    connections). The behavior also happens with benchtop scopes that are
    not isolated.

    I have all my unsuded GPIO set as inputs and with internal pullups
    enabled to avoid any parasitic effects.

    This is a multi-layered board with Separated Power Planes (3.3V and
    1.8V) and a single ground plane.

    Any suggestions on where to start?
    Thanks for all your help on this in advance.

  2. The Phantom

    The Phantom Guest

    I had something like this happen once. We had to ship scope probes with
    the product. :)
  3. Jim Thompson

    Jim Thompson Guest


    I had a circuit that would only work when my middle finger was
    touching a particular point.

    So I bridged my finger and added that much capacitance ;-)

    ...Jim Thompson
  4. James Beck

    James Beck Guest

    How many pF is your middle finger?
  5. gt5513e's problem sounds even more expensive. He will have to ship a
    Tektronix THS720 portable scope with each unit;-)
  6. Jim Thompson

    Jim Thompson Guest

    Don't remember now... 40+ years ago, but IIRC just a few pF.

    ...Jim Thompson
  7. John Fields

    John Fields Guest

  8. Guest

    I did consider this option, but only as a quick fix for the first
    million boards. Any changes will be for rev 2
  9. Tim Wescott

    Tim Wescott Guest

    If you don't need a scope attached to the probe perhaps you should just
    ship each unit with a probe attached :).
  10. Guest

    I do need the scope! I can fit the probe in the design budget but the
    whole scope, that's a different story! :)
  11. I threatened to do this once. The circuit (an ECL logic circuit,
    at that) only worked with a scope probe attached.
    Tried that (adding the scope capacitance); no joy. Turned out that
    the wire to the pin was broken. The scope probe added enough
    weight that it made contact. I chased that one for a while!
  12. No, that's unaccounted for "parasitics". ;-)

    Sometimes you just gotta drain the swamp.
  13. What more do you expect? After all, it is J.T.

    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
  14. John Fields

    John Fields Guest

  15. David Harmon

    David Harmon Guest

    On Fri, 10 Feb 2006 09:08:05 -0700 in, Jim
    Alternately, a drop of super glue between the part and your finger
    would have done.
  16. amdx

    amdx Guest

    I once got a job at a consumer electronics repair shop because
    of scope probe capacitance!
    I walked in to put in an application, showed my resume, and he said he
    had just hired someone. He saw I had some Sony experience and said
    he had a projection TV that no one had been able to fix, he ask if I'd be
    interested in looking at it. The projection TV had no raster, no output
    from the tubes. I sat it up and started probing around and after
    about ten minutes he looked over and said "hey you got it going". I had
    hung the probe on part of the high voltage osc. circuit and it fired up.
    I was on the backside and hadn't noticed light was coming out of the tubes.
    Turns out someone previously had replaced a cap. with one only 1/10
    of the correct value. The scope probe added enough capacitance to
    keep it running. So he started thowing repair work my way.
  17. Jim Thompson

    Jim Thompson Guest

    Reminds of a true story... over in Sun City, our local retirement
    community, a gal suspected hubby of infidelity.

    While he slept she super-glued his "privates" to his leg. When he
    awoke, in need of a pit stop, there was sheer panic... made the
    newspapers because he called 911 ;-)

    ...Jim Thompson
  18. Fred Bartoli

    Fred Bartoli Guest

    I'll fix it for free, then you give me half the scope price for each of the
    first million boards.
  19. It happens.

    A while ago I hung an old Heathkit signal tracer onto a test point in
    a fancy Fluke signal generator. The test point was in a PLL lock

    Turns out the PLL would lock only with the signal tracer probe on the
    test point. Very weird.

    After a little investigation it turned out the signal tracer had a
    leaky input capacitor. Substituting a resistance decade box in place
    of the probe showed the circuit needed between 11 and 12 megohms on
    that point to achieve lock.

    The capacitor leakage: 11.6 Megohms. How cool is that?

    But bvack to your problem-- first I'd put the scope PROBE on the
    circuit board ground, and leave the scope ground lead dangling.

    Apparently there's a fair amount of hash on that, if just hanging a
    piece of metal onto it makes a difference.
  20. I've seen PLL designs that would work better with the application of a
    finger. From memory, one would reduce its phase noise by about 10dB
    when touched. It turned out that the leakage current would cause a
    steady state phase shift that moved the phase detector away from its
    dead zone.

    I hate the 4046.

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