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  1. #1
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    output changes when I touch the enclosure

    Hi,

    I have a transmitter board which senses the pressure and outputs 4 to 20mA. The board is mounted in a aluminium enclosure using the Stainless steel standoff. The screws used are Stainless steel.

    Normally, the board is working good and provides 4 to 20mA based on the input pressure. But when I touch the aluminium enclosure, the output current value changes to 3.96 mA from 4mA.

    Why is this behaving strange?

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  2. #2
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    Re: output changes when I touch the enclosure

    i see two possibilities

    by touching the enclosure, you have somehow reduced the pressure and so the current goes down
    0.04 mA is likely well within the design margins of the pressure transducer and the output 4 to 20 mA

    by touching the enclosure, you change the local distributed parasitic impedance

    it is quite possible there is another explanation

    please provide a schematic and mechanical layout



  3. #3
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    Re: output changes when I touch the enclosure

    I would guess that in an industrial environment, with an
    ungrounded or poorly grounded (i.e. "earthed") case, you
    are adding 50/60Hz hum to the local "ground" and some
    rectification / charge pumping (like with input ESD diode
    networks) is adding to the real signal.

    You might repeat the experiment with the case tied hard
    to a known good earth ground, and in a location without
    much electrical equipment, fluorescent lighting and so on.
    You could put an oscilloscope to the internal input point
    and eyeball the waveform as you touch / un-touch the
    case, to get quantitative. If you've ever just touched the probe
    tip on a 'scope you have an idea of how large human body
    line frequency "pickup" can be against a high impedance
    sense-point.



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