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Oscilliscope Restistance

Discussion in 'Electronic Equipment' started by eat411, Oct 6, 2004.

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  1. eat411

    eat411 Guest

    I need to know if the resistance in an oscilliscope is high or not. i
    believe they have a low resistance, but i am not sure why they would
    need to be designed this way. I would appericiate an explanation.

    thanks in advance
     
  2. mike

    mike Guest

    You are correct, they're designed that way.
    I trust my answer was as clear as your question.
    Wanna try again?
    mike

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  3. Keyser Soze

    Keyser Soze Guest

    The answer depends on the kind of signal you need to measure and type of
    oscilloscope used.

    A typical oscilloscope vertical amplifier input will have a 1M ohm input
    impedance. The typical 10x passive probe will have a 10M ohm impedance.

    For signals of 350MHz or less conventional passive probes are fine.

    Oscilloscope designed for signals from 500MHz to 4000MHz will benefit
    from using active probes.

    Active probes have high bandwidth amplifiers at or near the probe tip.

    The amplifiers output is designed to drive a 50 ohm cable to the
    oscilloscope vertical amplifier input.

    Oscilloscope that are intended to use only active probes will usually
    have only a 50 ohm input impedance.

    -----------

    The basic concept here is that it takes a lot of power to accurately
    drive a 500MHz signal down a 1 meter cable. Most circuits that use
    signals at this frequency do not have the needed power and are adversely
    affected when connected to the load of an oscilloscope vertical
    amplifier input.

    The ideal oscilloscope probe would have very high impedance, low
    capacitance and disturb the signal very little.

    As the signal frequency increases it becomes much harder to make a probe
    that can do what is needed.
     
  4. I need to know if the resistance in an oscilliscope is high or not. i
    They should have a high input impedance. You want to load the signal
    under test as little as possible => you want as little current as
    possible to flow through the scope.
     
  5. Mantra

    Mantra Guest

    Now that you've gotten mutually exclusive and contradictory answers,
    here's the real answer. :)

    It depends on your frequency of interest. If you can assume a lumped
    equivalent model applies (generally under ~500 MHz) then you want your
    probe top to look like a high impedance (i.e. invisible) to the
    circuit under test. If you can *not* assume a lumped equivalent model
    applies, then transmission line effects are important and you want to
    assure an impedance match to avoid introducing reflections into your
    scope circuitry or your circuit under test so you'll want the probe
    tip to look like the charactistic impedance (typically 50 ohms) (i.e.
    invisible). In other words: "yes".

    MM
     
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