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MSP430F6736A: Conversion formula for SD24 module

Part Number: MSP430F6736A

Hi,

I have a doubt regarding Conversion formula for SD24 module.

Voltage=Vref*Digital value/FS value

1.When using Internal Reference, Vref for SD24 module is ~1.2V (1.151).
   But VREF module does not generate 1.2 V it generates 1.5 V, 2.0 V, or 2.5 V
   So while converting the acquired signal to Voltage, Vref value used should be 1.2 or 1.5V ?

2.Full scale value of the Digital Filter is given by
    FS = 2^(3×log2(OSR)).
  When OSR =607,
    FS=223648543(0x0D54 9B1F), Which is a 28 bit value.Since this is a 24 bit ADC do we have to discard the last 4 bits.
    If we are not discarding the last 4 digits, Which value do we have use in the conversion formula 2^28 or FS value(0x0D54 9B1F).

Data is in twos-complement mode (SD24DFx = 01b) with right alignment (SD24ALGN = 0).

Thanks

  • HI,

    1. The VREF has a dedicated 1.2V reference for the SD24 module. You should use 1.2V for calculation. 

    2. I think the UG chapter 29.2.7.3 has discuss this situation. Please find it in UG page 770. 

    https://www.ti.com/lit/pdf/slau208

    Best regards,

    Cash Hao

  • Thanks for the reply.

    I have read the document and find the FS value using the equation.

    When Right alignment the digital filter's output value is directly mapped to the SD24_B conversion result registers, So I am assuming All 28 bits contain usefull information. while converting this digial value to its analog equivalent,which value t use  2^28 or FS value calculated.(Input signal is having a very high noise and I was unable to figure out whether the error in voltage is due to   noise or  calculation error

  • Hi,

    Are all 28 bits useful information? Do you need to consider the left aligned map in the chapter 29.2.7.3.

    Best regards,

    Cash Hao

  • Hi,

    If left alignment is selected as per Table 29.2 the LSB of the filter output is mapped to Bit 1 of SD24BMEMLx,in that case total bits will be 29 and we have to discard the last 1 bit. Actually my confusion is on the last 4 bits of the Filter Output .Number of bits of Filter output doesn't depend on the Alignment.My doubt is Whether to use  2^28 or FS in the conversion formula.

  • Hi,

    I think using the FS make sense to me. 

    Best regards,

    Cash Hao

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