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LMT86-Q1: Recommendation on output RC

John Pan
Intellectual 990 points
Part Number: LMT86-Q1
Other Parts Discussed in Thread: LMT86,

Hi,

      I plan to use LMT86-Q1 temperature sensor to measure the temperature of the board and the output will be connected to a STM8AF micro-controller. An input capacitor of 0.1 uF is recommended in the LMT86 data sheet. I am wondering whether an external RC circuit is needed between the LMT86-Q1 and STM8AF. If needed, what is the recommendation of the R and C value? Could we just pick a typical C = 0.1 uF and R = 1k?

      Thanks,

      John

  • 0
    TI__Guru* 91086 points

    Dear John - 

    Thanks for posting. Table 4 on page 11 and section 9 are intended to show you what choices to make, based on the measurement or knowledge of your circuit. https://www.ti.com/lit/ds/symlink/lmt86-q1.pdf

    Page 85 here https://www.st.com/resource/en/datasheet/stm8af5268.pdf indicates the ADC cap load might well under the 1100pF CLoad limit mentioned on page 11 of the LMT86-Q1 datasheet. 

    You might be OK to leave off - for first protos, you may want to put placeholders for an RC, just in case, then use 0 ohm for Rseries and leave Cshunt DNI. 

  • 0 in reply to Josh Wyatt
    Intellectual 990 points

    Hi Josh,

          Thank you very much. Please confirm whether my understanding is correct. Based on the stm8af5268 datasheet, page 85, the sample time, ts is 0.75 us. Thus, the RC time constant should be less than the sample time. Typical 5 times RC time constant should be less than the sample time. Thus, the RC time constant should be less than 0.15 us, or 150 ns. The such short RC time constant requires a very small external capacitor or no capacitor for the microcontroller. Thus, the capacitive load for the LMT86 is much smaller than 1100 pF. Such small capacitive load requires no resistor in series for the LMT86.

          Thanks again and I look forward to hearing from you,

          John

  • 0 in reply to John Pan
    TI__Guru* 91086 points

    John - 

    I think you are reading the meaning of Figure 11 in the datasheet correctly, if the system is not noisy and needing filtering, and the connections are close, then most likely there is nowhere near 1100pF on the traces, and therefore will not need filtering.