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Impedance and EMI/RFI

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  1. Jul 16, 2015 #1

    cda

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    Question in relation to audio.

    I read online that a low impedance system is less susceptible to EMI and RFI than high impedance. How? I thought that high impedance = high resistance, therefore higher resistance to EMI and RFI. Would love to be updated with the explanation.

    Thanks.
     
  2. jcsd
  3. Jul 16, 2015 #2
    This is the case when the interferer has mainly an electric field. For instance, the AC mains supply, or maybe a computer. The audio cable acts as an antenna, and develops a voltage as a result of the interfering field. If we terminate the "antenna " with a low impedance, its efficiency as an antenna is greatly reduced. We can choose what impedance we use for our wanted audio, so it makes sense to make it low.
    In more detail, a few metres of wire acting as an antenna at audio frequencies is like a small capacitor, so it has a very high impedance (a high capacitive reactance). If we terminate it with a low value resistor, there is potential divider action and the interfering voltage is greatly reduced.
    In some cases, magnetic fields are a problem, and using a low impedance will not then be effective as a cure.
     
  4. Jul 16, 2015 #3

    meBigGuy

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    Gold Member

    To say it another way:
    The EMI fields can only induce a small current. If the circuit is high impedance, that means a high voltage, and vice versa.
     
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