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LMT70: 2 sensors on one board

Werner Vorster
Prodigy 60 points
Part Number: LMT70

Hi

We have two LMT70 sensors running off one ADC. One of the sensors works correctly, but the other one measures a much lower temp (16C) and, every now and then, jumps up to 200C +. This has now happened with 2 boards. We need help figuring out why please. I attach the schematic for you.

  • 0
    TI__Guru**** 244400 points
    Hello Werner

    Can you check the output from the failing LMT70A when the excursions happen? I would like to check if it is an LMT70A issue or some behavior at the ADC which is causing the same?

    To do so, connect scope probes to the output of U2 and U14 and another to a GPIO from the MCU which makes a L->H transition when the excursion happens so that we can check the state of the output from U2 and U14 at the time of sampling?
  • 0
    TI__Expert 7845 points

    Werner,

    Please check out pg. 15 of the datasheet.   Your RC load is out of spec.  If you want to use 10nF load, you will need a minimum series resistance of 3kΩ.  This is for stability reasons (phase margin for the output stage).  Here's article on stability you might find interesting to read:

    EDN_2008_11_27_opamp_stability.pdf

    -Kelvin