LM75 placement

[eeh]

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!

 

[PeteS]

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