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LM75 placement

Discussion in 'Electronic Basics' started by eeh, Apr 24, 2005.

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  1. eeh

    eeh Guest

    Hi,

    I am going to use LM75 to measure temperature. However, I have other
    electronics inside the same PCB such as MCU and LEDs which generates
    heat during operating. Though they are not so hot, I want to ask
    generally how to place the LM75 so that it can sense environmental
    temperature more accurate. Or I need to place it to another PCB and
    link it by cable?

    Thanks!
     
  2. PeteS

    PeteS Guest

    Is there any airflow in the unit? If not, is there any point where
    outside air has an entry point?

    Once more, if not, place the sensor well clear of the MCU - should be
    fine.

    There is one very specific precaution you should take with the LM75,
    however: The ground pin is extremely susceptible to any noise, even
    when the power and ground in are properly decoupled with a cap close to
    the device. The problem shows up as off the wall temperature readings
    (which is hardly surprising really, considering the low current levels
    involved).

    Ensure the ground you connect the ground pin to is very quiet, with no
    high speed noise or eddy currents (basically, make sure the grund path
    is not inline with other ground return paths).


    I know this sounds wierd, but I have had this exact problem with this
    device (although that was in a 32 port InfiniBand switch). I ended up
    haveing to decouple the local ground to a quiet ground (which threw up
    all sorts of DRCs (two pins of a capacitor connected to the same
    logical net).

    Cheers

    PeteS
     
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