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MSP430FR2355: ADC input MUX ON resistance and Tau equation

Ryo Akashi
Genius 9470 points
Part Number: MSP430FR2355

Dear experts,

I have question for Ri(Input MUX ON resistance) on datasheet.
In datasheet P51 Table 5-20, Ri= 2k ohm (max).
However, in Table 5-21, Ri= 4000 ohm for TEST CONDITION.
It means Ri will change depends on Vcc condition?
I'm not clear about this point...

Also, regarding with Tau equation in tSample, can we ignore Cexternal value?
In equation, Ci is only considered. 
I would appreciate if you would give me advice.

Thanks and best regards,

Ryo Akashi
  • 0
    TI__Mastermind 22415 points

    Hi Ryo Akashi

    Question: In datasheet P51 Table 5-20, Ri= 2k ohm (max). @8bit ADC and 2V DVcc. However, in Table 5-21, Ri= 4000 ohm for TEST CONDITION @ 12-bit and 2.4-3.6V.

    Feedback: I am checking with the expert of FR2355 DS

    Question: tSample= ln(2n+1) ×τ, wheren = ADCresolution,τ= (RI+ RS) × CI. In equation, Ci is only considered.

    Feedback: I think the formula should be  τ= (RI+ RS) × (CI+ Cexternal). I will check with the expert of FR2355 DS as well

  • 0 in reply to Xiaodong LI
    TI__Genius 9470 points

    Hello Xiaodong Li-san,

    Thank you for your reply.
    Ok, once you get update, please let me know.

    Thanks and best regards,
    Ryo Akashi
  • 0 in reply to Ryo Akashi
    TI__Mastermind 22415 points

    Hi Ryo Akashi

    Please check the updated feedback below:

    Question: In datasheet P51 Table 5-20, Ri= 2k ohm (max). @8bit ADC and 2V DVcc. However, in Table 5-21, Ri= 4000 ohm for TEST CONDITION @ 12-bit and 2.4-3.6V.

    Feedback:

    The note(1) is actually for the first column, not for whole ADC spec.

    The Ri in table 5-20 is actually just indicating the analog MUX on resistance, not calculating the ADC’s resistance itself. This name should be updated to RI,MUX. we have found this issue. we are updating this name step by step. Please check FR2311 DS. Ri.MUX is used on table 5-20

    The Ri in table 5-21 test condition is actually accurately reflecting the real input resistance.

    Question: tSample= ln(2n+1) ×τ, wheren = ADCresolution,τ= (RI+ RS) × CI. In equation, Ci is only considered.

    Feedback: we confirmed the formula should be  τ= (RI+ RS) × (CI+ Cexternal). we will update the DS.

  • 0 in reply to Xiaodong LI
    TI__Genius 9470 points

    Hello Xiaodong Li-san,

    Thank you for your feedback.

    Regarding with Ri in table 5-20, it should be Ri=Ri,MUX + Ri.Misc. (Ri,Misc is Input miscellaneous resistance as described in FR2311's DS)
    Ri in table 5-21 contains Ri.Misc, so the value is 4000 ohm (larger than Ri,MUX). In other word, Ri,Misc is about 2000ohm.
    Is my understanding correct?

    For tau, I understand tau should be (Ri+Rs)*(Ci+Cexternal).

    Let me ask addtional question.
    My customer want to calculate maximum ADC input current.
    It should be estemated by (applied voltage)/(Rs + Ri,MUX +Ri,Misc), I think.
    So, if we input 3.3V for the confition in table 5-21,
    ADC input current = (3.3V)/(1000ohm + 4000ohm) = 660uA.
    Is my understanding correct?

    I would appreciate if you would give me comment.

    Thanks and best regards,
    Ryo Akashi

  • 0 in reply to Ryo Akashi
    TI__Mastermind 22415 points

    Hi Akashi-san

    I think the below items are correct.

    • Ri=Ri,MUX + Ri.Misc., Ri,Misc is about 2000ohm.
    • For tau, I understand tau should be (Ri+Rs)*(Ci+Cexternal).
    • ADC input current = (3.3V)/(1000ohm + 4000ohm) = 660uA.

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