LM75 placement


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?


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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).



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