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Need clarification in MSP430F6779 SD24 B

Raviteja Veeragandam
Prodigy 80 points

As per datasheet it is said that the output of SD filter is left shifted by 'n' bits and filter output mapped to 

'(n-1)' bit of SD24BMEML.

If you shift by 'n' it should be mapped to nth bit of SD24BMEML. But why (n-1) bit.

And also i have another doubt regarding FS(Full scale) reading.

As per datasheet FS = 2^(3×log2(OSR)). For OSR = 1024 and right aligned, FS reading is

4000 0000h(1073741824) (hope it is for offset binary).

In 2's complement mode what is the range of FS?

If it is +1073741823 to -1073741824; How?

In two's complement it should divided by 2 with respect to offset binary.

Please do some needful help.

  • 0
    TI__Expert 5465 points

    Hello Raviteja,

    In reading your post and reading into the UG explanation and example with an OSR = 256, I agree and do think that the LSB should be mapped onto the n'th bit as you have stated. I will contact the team who wrote the UG/DS and ask them for an explanation of the n v. n-1 situation. If they confirm this suspicion as well, I will push them to make the change and I will update this thread accordingly.

    On the "FS reading" subject, I believe what the DS/UG states is correct, with OSR = 1024, the FS = 2^30 and the SD24_B will give data bits up to 4000 0000h (or more likely up to 3FFF FFFFh). Additionally, when in offset binary mode, this will allow for an additional bit to be present (for the sign of the signal). I highly suggest you refer to Table 29-3 which shows an example of the data formatting for an OSR = 256. You can then do the same exercise with an OSR = 1024.

    Not all data bits that the SD24_B supplies can be trusted as being accurate. For AC signals, you can use the SINAD quoted in the DS to calculate the ENOB of the SD24_B. For DC signals, you can take the standard deviation of a set of accumulated data (with differential input shorted) and apply the equation N - log2(std.dev.) = ENOB, where N is the number of data bits that the converter supplies. ENOB is a standard metric for determining the overall accuracy of a data converter.

    Hopefully some of this information is of help. If you have more questions, please let me know.

  • 0 in reply to Tyler Witt
    Prodigy 80 points

    Hi Tyler,

    Thank you for the detail description. Ur ans helps me a lot.

    And also update this thread if any feedback from the team who wrote the UG.

    Thanks & Regards

    Raviteja V

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