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8 years, 1 month ago.
Strange behaviour with several analog sensors
Hi, I was working with a Nucleo f411re in order to read several analog sensor. I started testing a single temperature sensor (LM35) and all was fine. When I added a linear potentiometer for reading its extension, a strange fact happened: temperature read increased proportionally to the extension of the potentiometer. So I mesaured the output voltage from LM35 with multimeter and it was stable even when I was extending the potentiometer; while, if read from Nucleo, the temperature was "following" the potentiometer read.
I solved this problem adding a capacitor between ground and then V_out of the LM35. But I'm not satisfied, because I would like to know the reason of the strange temperature read behaviour. Any Idea?
Thank you for your attention!
1 Answer
8 years, 1 month ago.
This is a known issue with the LM35 (or similar) and STM32 ADC.
Simply explaining. When converting ADC converter temporarily changes its input impedance (capacitor switching) and not all signal sources can tolerate it (eq. LM35 as you can see).
The explanation for this problem is in the document (Section 3.4.1 and following):
http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/CD00211314.pdf
So it's good you have put capacitor.
See also:
https://developer.mbed.org/questions/6317/AnalogIn-problem-pics-included-strange-v/