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electronics low/high impedance

Discussion in 'Electronic Design' started by c5f8, Mar 13, 2007.

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  1. c5f8

    c5f8 Guest

    I have a general question. Electronics articles sometimes state that a
    circuit or electronic device such as a sensor has either low input
    impedance or high input impedance. The articles never state why it's
    important to know what the input impedance is.

    I've seen both cases where a product is advertised with the low/high
    impedance statement and suggesting that there product is somehow
    better because of that statement.

    Can someone explain when it's beneficial to have low input impedance
    and when it's beneficial to have high input impedance?
     
  2. Can someone explain when it's beneficial to have low input impedance
    For signals encoded as voltage levels you want a HIGH INPUT impedance and a LOW OUTPUT impedance.
    For signals encoded as currents, you want the opposite.

    For voltages, if your source is high impedance then your device input should be even higher
    impedance. A 1 MegOhm input is NOT high impedance if your source impedance is also 1 Meg.

    The question you ask is really so broad that at least a monograph could be written in answer,
    but the stuff above should supply you a start on it.
     
  3. c5f8

    c5f8 Guest

    Original question (I deleted it)
    Circuits are described as having either low input impedance or high
    input impedance. The articles imply that their circuit is somehow
    better because of it. Can someone explain when it's beneficial to have
    low input impedance and when it's beneficial to have high input
    impedance?

    Kevin, Thanks for the response.
     
  4. Rich Grise

    Rich Grise Guest

    That's what he just did. Maybe you'd better take a remedial reading course.

    Good Luck!
    Rich
     
  5. Robin

    Robin Guest

    If an input is "high impedance" it means that is easy to "drive" i.e.
    easy to push around, in the sense that powered steering is easy to
    handle, you don't need much muscle power to move it.

    So generally speaking, inputs will tend to be high impedance because
    then they will have more tendency to do what they are told as opposed
    to a lower impedance input e.g. unpowered steering.

    A "high impedance" output is a "weak" output, a weak drive. If you
    used a high impedance output to drive a low impedance input (old man
    trying to turn non-powered steering) then the movement transferred
    will be reduced or to use the jargon, the ouput will be loaded by the
    input, and subsequently the signal will be loaded and reduced in size.

    So it is usual to have e.g. the output of a HiFi amplifer (0.0001
    Ohms) drive a loudspeaker of 4 Ohms. By this design, the loudspeaker
    will do exactly as it is told being driven by an impedance very much
    lower than its own.

    Now a big confusion is "matched impedances". When a strong guy turns
    powered steering, he does not expect to get tired and he won't because
    his low impedance is not loaded by the high impedance reaction of the
    wheel. There is no intention to depend on the man's strength. The
    power is provided by the petrol. But what if there is no petrol? As in
    e.g. in a bicycle now it is important to get *power* from the man to
    the machine. When the road speed is low, a gear selection is required
    to match the maximum output of man to machine i.e. the impedances are
    "matched" but as he road speed increases so the peddle speed reaches a
    maximum that the man can manage and no power is transferred (the
    impedances have become mis-matched) so a new gear is selected to match
    the impedances again, to reduce the pedal speed to the "power zone" of
    the man and so on, up through the gears.

    Under certain circumstances it is necessary to match impedances to get
    the maximum power from input to output. Different requirements might
    need maximum voltage transfer (low driving high) while others require
    maximum current transfer ( high driving low ).


    Cheers
    Robin
     
  6. chuck

    chuck Guest

    To see the answer to your question, imagine you are measuring the
    voltage in some part of an electronic circuit (maybe a battery) with a
    voltmeter. Now suppose the input impedance of your meter is one ohm.
    That would almost always be considered a low impedance (but since low
    and high are relative terms, compared to an impedance of 0.001 ohms, one
    ohm is high). You'd have a hard time locating a voltmeter with a one ohm
    input impedance.

    Not many circuits will continue to function with a one ohm impedance
    across them, and some might even be destroyed.

    Now imagine a voltmeter with a 10 megohm input impedance. It would be
    much less likely to interfere with the normal operation of a circuit.
    Some voltmeters have an input impedance as much as a million times
    higher than 10 megohms for precisely that reason.

    I encourage you to continue your study of electronics, and in
    particular, the notion of impedance. You will find some interesting
    information dealing with maximum power transfer.

    Chuck
     
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