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MSP430FR (FRAM) 2x1x Family - ADC Input Impedance specification

Chris Yorkey
Genius 15855 points
Other Parts Discussed in Thread: MSP430FR2110

Team,

On behalf of our customer:

Just want to confirm an assumption here for one of our customers:

The question is:  What is the ADC Input Impedance on this device?

Based on several other posts on this forum, such as this one:

e2e.ti.com/.../232929

...it appears we can use the Figure 22-4 and the Analog Input Equivalent Circuit information even for this FRAM family, since they both (F2x and FR2xxx) use the same basic ADC10 peripheral.  Correct?

And if so, what do we typically offer as a general answer to this question for our customer base?

Thank you,
CY

  • 0
    Guru 316380 points

    It looks as if the FR2xx ADC is different (figure 20-7 in the FR4xx/2xx User's Guide); it refers to the datasheet, which (for, e.g., the MSP430FR2110) mentions in table 5-19 that RI = 36 kΩ, but this is not even a typical value but just a test condition.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 49692 points

    Hi Chris,

    Hope you are doing well and staying warm!

    Clemens made an excellent observation here. In Table 5-19 in the MSP430FR211x datasheet, the values for the source resistance (RS) and input resistance (RI) should be reversed in my opinion. In Table 5-18, the max RI is clearly defined as 2kOhms, which is much less than 36kOhms. I'll submit a request to change this accordingly.

    In Section 1 in the Migrating From the MSP430F2xx and MSP430G2xx Families to the MSP430FR4xx/FR2xx Family User's Guide, there is a small NOTE declaring that the term "F2xx" indicates the MSP430F2xx/G2xx families and the term "FR4xx" indicates the MSP430FR4xx/FR2xx families. Therefore, the significant differences between the ADC10 modules of the MSP430F2xx and MSP430FR2xx families can be found in Section 8.3.1.

    Ignoring the parasitic capacitance values in Figure 20-7 in the MSP430FR4xx/FR2xx Family User's Guide, you can use the tsample equations in Section 22.2.5.1 in the MSP430x2xx Family User's Guide for determining the minimum sampling time. If your customer uses the maximum RI value, this will be worst case, regardless if the RI for that specific device is less.

    Does this make sense?

    Regards,

    James

    MSP Customer Applications

  • 0 in reply to James Evans
    TI__Genius 15855 points
    Hi James,

    Really appreciate your detailed reply. Yes, this does make sense. I would add that we believe the 2K MUX impedance may be correct (Table 5.19), as this would be in series with the ADC resistance and would likely have little effect if the ADC input resistance is very high. Figure 20.7 shows Rs for the external circuitry (source), but nothing to the right of the Mux resistance Ri or in the rest of the User Guide from what we can tell. This implies very high but just doesn’t state exactly how high.

    I appreciate your help!

    -Chris

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