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Scope Probe Resistance

Discussion in 'Electronic Design' started by Andreas Schmidt, Sep 8, 2003.

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  1. Hi all,

    I've got a X1/X10 switchable probe that came with my scope which seems
    to work fine, but ... if I measure the resistance from the probe tip to
    the BNC connector when set to X1, it's exactly 480 ohms. Shouldn't it be
    very close to zero ohms? In X10 mode, the resistance is 9.55 megaohms,
    which is also weird because with the scope's 1 meg input resistance,
    shouldn't the probe have a resistance closer to 9 megaohms?

    The probe does seem to give accurate readings though, e.g. when onnected
    to the 2V and 0.2V 1kHz/1MHz probe calibration outputs of the scope.

    But the main issue for me is the resitance in X1 mode. Are all X1 probes
    like that?


  2. John Larkin

    John Larkin Guest

    Scope probes use special high-resistance coax to dampen ringing;
    that's probably what you're measuring.

  3. Are scopes callibrated to allow for this, then? It strikes me that 480
    ohms is a heck of a lot of series resistance which is bound to the
    play havock with voltage level readings.
  4. Somehow, an additional .05% DC error does not equate to playing havoc
    to my mind. To how many significant digits can you read voltage from
    a scope screen?
  5. Michael

    Michael Guest

    The Resistance of the center conductor in a cro lead will not damage a
    signal generator. Maybe you are confused with higher power levels of
    RF not being terminated correctly.
  6. Mike Page

    Mike Page Guest

    I'd be using a cheapo signal generator - LF to 2MHz, choice of 50 or 600
    Ohm outputs. If I didn't use a scope probe, I'd use a BNC to croc clip
    lead, or BNC to 4mm sockets.
  7. Er, are you trying to suggest that because this 480 ohms appears in
    series with the scope's own input impedance of 10Mohms || 30pf it may
    safely be ignored (since it's so much smaller?). I'm sorry, I'm not a
  8. Jim Yanik

    Jim Yanik Guest

    It makes a voltage divider,but 480/ONE megohm,not TEN meg.
    A really negligible divider ratio.

    Scope input Z is generally One Megohm,not 10Mohm.
  9. James Meyer

    James Meyer Guest

    I don't think so. Please open one up and double check. The resistance
    of the center conductor is made high on purpose, about 40 Ohms per foot, but I
    have never seen one made with a helix. Helix center conductors are used in
    delay lines. Perhaps that's what you're remembering and confusing with scope
    probe cables.

  10. James Meyer

    James Meyer Guest

    Tightly wound? As in each turn touching its neighbor? Was the inner
    conductor made of insulated wire so the turns didn't short out?

    A spiral wound inner conductor might be a lot more flexible than a
    straight wire of equivalent diameter.

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