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MSP430F6779: Errata SD3 workaround

da
Expert 2780 points

Part Number: MSP430F6779

Hello,

As the workaround of SD3 which is the errata of SD24_B, is there any problem with the following idea?


When "-VFS" is input to SD24_B, if it is set to 2's complement, it will not be the value described in the manual.
However, its output value may be treated as a minimum value.

Or

It is an illegal value and can not be used at all.
(In that case, is it a fixed value?)

Refer
e2e.ti.com/.../646603

Regards,
Da

  • 0
    TI__Mastermind 49692 points

    Hello,

    What gain are you using? If it's set to 1, then you may have to modify your external passives (e.g. burden resistor) to reduce the max amplitude of the input signal. If you're using a gain higher than 1, you could reduce your gain to ensure that the amplitude of your input signal never exceeds +/-FS.

    If you're reaching -FS during testing only and your input signal will never reach -FS, then I wouldn't worry about this issue.

    Regards,

    James

    MSP Customer Applications

  • 0 in reply to James Evans
    Expert 2780 points

    Thank you for your reply.

    What was supposed to be assumed is when it exceeds the input range.
    That means that the input range must not exceed "-VFS".

    Regards,
    Da

  • 0 in reply to da
    TI__Mastermind 49692 points

    Hello,

    Yes, you're correct. The input range must not exceed -VFS. If I've answered your question, please click the green "This Resolved My Issue" button.

    Regards,

    James

    MSP Customer Applications

  • 0 in reply to James Evans
    Expert 2780 points

    We have a request from the customer and ask you a question.

    Is there a way to avoid this phenomenon using software other than Gain setting and 2's complement?

    In the customer's environment, it is not possible to limit the input range by the gain control.
    In this state, they want to know how to deal with "- VFS" input.

    In the range confirmed by the customer's environment, it becomes 0xF7FFFFFF at the input of "- VFS".
    Which individuals will be 0xF7FFFFFF?

    When "0xF7FFFFFF" is output as AD conversion data, please tell me how to deal with software.

    Regards,
    Da

  • 0 in reply to da
    TI__Mastermind 49692 points
    Hello,

    If they are not able to change the Gain, then they'll need to change the external circuitry to ensure that +/-VFS is never reached. For example, in electric meter applications, a burden resistor is used with a CT to set the max differential input voltage to the SD channel. In this case, you would just reduce the resistance of the burden resistor to ensure +/-VFS was never reached. I understand that this application is most likely different, but I wanted to give an example. If this is not possible, other alternatives would be to address this in software such as detect when you're nearly at -VFS and ignore, estimate, or interpolate those conversions. If your input signal is a differential sinusoidal, then perhaps you could focus on the positive half of the sinusoid and ignore the negative half.

    Regards,

    James

    MSP Customer Applications

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