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Electrical shielding newbie question

Discussion in 'Electronic Design' started by Yvan, Jul 7, 2005.

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  1. Yvan

    Yvan Guest

    Hi all,

    I have a question about electrical shielding.

    I just read the book "grounding and shielding techniques" and there's
    one thing i don't understand.

    The whole point of electrical shielding is that you provide an easy
    path for unwanted currents to go to ground, instead of going through
    your circuits. Is this right? In that case, it suggests to ground the
    shield where the voltage source is only. I'm trying to figure out the
    problem for an analog sensor connected externally to our device. It
    would share the signal and ground conductors withthe data acquisition

    Our enclosure is in plastic. If the shield is connected to the ground
    of the sensor and then to the enclosure. How can current pass through
    there? I don't see the point of a shield if the enclosure is in
    plastic (open-circuit?). In that case, wouldn't current go through the
    common ground conductor instead and create ground noise? How should I
    connect the shield then?

    Thank you,

  2. That is one purpose for shielding. Shielding deals with local
    magnetic and electric fields and emission of radiated waves.
    Sometimes you contain the source of noise with a shield, and sometimes
    you are not in control of the source of the field, so you contain the
    circuits subject to disturbance from fields. Sometimes you do both.
    Changing electric fields cause displacement currents that connect the
    object to the voltage source. Changing magnetic fields cause local
    eddy currents in conducting loops. Either cause of current will cause
    voltage drops in conductive paths.
    The way I see it, the shield is an extension of the box that contains
    the converter, so that it also contains the sensor and its wiring. It
    should be separated from all wiring, except for a connection at one
    point. In this case, I would pick a point very close to he converter.
    That way, all currents that slosh around in the shield conductor in
    response to electric or magnetic fields produce little voltage drop in
    any of the signal conductors, including the signal common.
  3. Yvan

    Yvan Guest

    I don't totally understand your last paragraph. In our case, our box
    has signal conditioning for a 0-3V analog sensor input. We provide
    with the ground. In other words, the A/D converter is in our plastic
    box and the sensor is in the other one. If I understand well, you
    would make one single connection of the shield with the ground, close
    to the converter, in our box? Every book I have read mentions it has
    to be close to the source of voltage, hence the sensor....

    I know that you either shield from electric and/or magnetic
    disturbances. In our case, I'm more concerned about electric
    disturbances, as the signal frequencies are in the order of 1 kHz.
    The sensor will have little disturbances from external sources as there
    won't be transformers and such close by.

    In that perpective, I don't understand how such a shield would protect
    against electrical disturbances if I have a plastic box. How would it
    change the electrical fields then? Where are currents flowing with and
    without that shield? I wish I could draw what I see in my head... ;)

    Thank you,

  4. What, electrically, do you mean by, "we provide with the ground"?
    I generally connect the shield to the signal common at the point where
    I care about the signal integrity the most. I would wrap that shield
    around as much of the wiring and system as needed for it to be the
    capacitor plate that receives change displacement form external
    electric fields. If capacitive currents are dumped through the
    shield, it will pass these currents to signal common at the connection
    point, and that may bounce the potential at that point around (and all
    the signals, with it). But if all signals and signal common share the
    same noise at that point, the circuits that responds to the difference
    between signal potential and signal common potential at that point all
    share the same noise addition, the differences between them will show
    little of that noise.
    Another source of magnetic interference is power lines running
    parallel to your signal lines.
    Feel free to email any sketch you may have to me. If I have any
    useful comments, I will post them along with your graphic on the
    newsgroup alt.binaries.schematics.electronic, as well as by return email.
  5. Yvan

    Yvan Guest

    Ok thanx, I will come back to you shoftly.

  6. Yvan

    Yvan Guest

    Ok thanx, I will come back to you shortly.

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