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STLM20W87F Output Values Do Not Change

Question asked by roofie01 on Jun 8, 2017
Latest reply on Jun 8, 2017 by roofie01

Hello:

I am using the STLM20W87F temperature sensor in an application, basically, on a PCB to measure ambient temperature.

Referring to page 10 of the datasheet, I have the sensor connected with a filter of 1000p / 10K. That output goes directly to the ADC input of the controller. My STM32F4 is sampling at 200 samples/sec. The Vref pin of the controller is at Vdd = 3.3V. At an ambient of 25C, the ADC values are around 2235 - 2260. This translates to a voltage of (2250/4095) x 3.3 = 1.81V. It seems to be high for the value in the datasheet that should be around 1.55 for T=25C. If I measure the actual voltage at the output of the STLM20W, it's pretty much spot-on at 1.59V.

If I put my finger on the sensor, the counts should decrease, but barely moves. I even tried warm air from a hair dryer but nothing significant, maybe 100 counts lower. Ditto for taking some canned air and spraying on the device, the counts don't increase much, maybe 50 at most.

 

Questions:

1 - Why the discrepancy between a DMM measurement and the ADC? I can see 10's of millivolts, not hundreds.

2 - Why is there no obvious ADC count changes with warm air and cool air placed to the device?

 

I hope someone can help with this dilemma, thanks so much!

Roger

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