Where to connect cable's shielding?

Discussion in 'General Electronics Chat' started by tpny, Dec 13, 2012.

  1. tpny

    Thread Starter Member

    May 6, 2012
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    I have a sensor connected to my pcb at a distance of 15 meters. The connection has 3 wires:
    1. power (12vdc)
    2. gnd
    3. signal (0~5vac)

    Power and ground are tied to pcb's 12V dc power supply. Signal is tied to an op amp.

    The wires are inside a cable with some mesh shielding. Should I connect this shielding to something - power supply 12vdc output's ground? Power supply 110vac input's earth prong? The piece of metal plate my pcb is sitting on? Leave it unconnected (which is what I have right now)...
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is the frequency of your signal?
     
  3. tpny

    Thread Starter Member

    May 6, 2012
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    I think they are pretty slow.. less 100hz..
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Then connect the shield to circuit (signal) ground, only at the circuit board end.
     
  5. tpny

    Thread Starter Member

    May 6, 2012
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    why does low frequency call for connection to signal ground? Thanks! (Signal ground being the dc ground coming out of my 12vdc power supply output right?)

    Is my power supply's dc ground (at output) tied to the main's neutural (at input)?

    Right now I have the shield unconnected and it's not problem since my signal is pretty big. I'm concerned that connecting the shield to the power supply ground (at the output) might somehow cause it to be I don't know..less stable? (Because my delicate circuit shares that same ground.) What if the shield spans 500 meters? Is the supply's dc ground boundless in its ability to turn everything tied to it 0v?
     
  6. ramancini8

    Member

    Jul 18, 2012
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    You were give good advice, take it!
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Without seeing your schematic, I wouldn't want to speculate how your power supply is connected to your circuit board.
    I have no way of knowing.

    A floating shield can pick up noise and capacitively couple it to the signal wire. Grounding the shield prevents this.
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Yes take it!
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    The shield is grounded at ONE POINT ONLY to avoid making it into a loop antenna which can be incredibly sensitive to noise which is capacitively coupled to it. The best place to connect a shield is as close to the power supply as possible. The connection to Earth should also be made at ONE POINT ONLY as close to the power supply ground as possible.
     
  10. tpny

    Thread Starter Member

    May 6, 2012
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    Hi, OK I drew up this schematic attached. Please help clarify where to connect the shield to ground. Thanks!
     
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