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MSP430F67791A: SD24 About vref settings

j.yamamoto
Prodigy 40 points
Part Number: MSP430F67791A


Dear All

The MSP430F67791A datasheet has a description about Vref of SD24.
I have a question about Vref.
https://www.ti.com/jp/lit/ds/symlink/msp430f67791.pdf?ts=1686576069827&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fja-jp%252FMSP430F67791

　About the input voltage of VREF


P59 Table5-49 shows MAX1.5V when Vcc3V
　Is it possible to use 1.65V (Vcc/2) when Vcc=3.3V?

　About the VFS setting(P52)


If VREF is sourced externally, the analog input range should not exceed 80% of VFS+ or VFS-
For example, if VREF=1.5V, VFS+=1.2, VFS-=-1.2, so should the measurement range be 0.3 to 2.7V?
The AD input voltage range is 0 to 3.3V, so if the 0 to 3.3V range cannot be measured, I am considering adjusting it with an external circuit.

Regards,

  • 0
    TI__Mastermind 23105 points

    Hi Yamamoto

    >> P59 Table5-49 shows MAX1.5V when Vcc3V, Is it possible to use 1.65V (Vcc/2) when Vcc=3.3V?

    Refer to 29.2.3 Voltage Reference chapter of UG "The internal SD24_B reference provides one fixed voltage of approximately 1.2 V".

    So, I think VREF typical value should be 1.2V when Vcc is 3.3V.

    >> For example, if VREF=1.5V, VFS+=1.2, VFS-=-1.2, so should the measurement range be 0.3 to 2.7V?

    according to datasheet note(1) of table 5-39: 

    The full-scale range (FSR) is defined by VFS+ = +VREF/GAIN and VFS– = –VREF/GAIN: FSR = VFS+ – VFS– = 2 × VREF/GAIN.
    If VREF is sourced internally, the given VID ranges apply. MIN values are calculated based on a VREF of 1.125 V. TYP values are calculated based on a VREF of 1.16 V.

    I agree with you.

    Thanks!

  • 0 in reply to Xiaodong LI
    Prodigy 40 points

    Thank you for your answer.
    Isn't the description in Refer to 29.2.3 an internal voltage description?
    Table 5-49 seems to be the external reference voltage (SD24REFS = 0).
    It is listed as Typ1.2V Max1.5V at Vcc=3V.
    Can I use up to Max1.65V at Vcc=3.3V?
    Sorry if I am mistaken.

  • 0 in reply to j.yamamoto
    TI__Mastermind 23105 points

    Hi Yamamoto

    >> Isn't the description in Refer to 29.2.3 an internal voltage description?

    Yes. it is for the internal reference

    >> Can I use up to Max1.65V at Vcc=3.3V?

    VREF typical value should be 1.2V when Vcc is 3.3V

    >> Table 5-49 seems to be the external reference voltage (SD24REFS = 0).

    I think this is note(1) of table 5-39

    Thanks

  • 0 in reply to Xiaodong LI
    Prodigy 40 points

    Sorry, let me check again.
    I understood that when Vref is internal, Vref=1.2V when Vcc=3.3V.
    Then when Vref is external input, how many volts can be input when Vcc=3.3V?
    In Table 5-46, when SD24REFS=0, Vcc3V is Max1.5V.
    I don't see any mention of Vcc=3.3V, should I assume Max1.5V even at 3.3V?
    Sorry for repeating.

  • 0 in reply to j.yamamoto
    TI__Mastermind 23105 points

    Hi Yamamoto

    Thanks for your detailed question! I will check this question and reply to you later

    Thanks

  • 0 in reply to Xiaodong LI
    TI__Mastermind 23105 points

    Hi Yamamoto

    according to Table 5-46. SD24_B External Reference Input of device data sheet, 

    VREF(I) Input voltage is from 1.0V to 1.5V at the External Reference Input

    Thanks

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