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ADS8675: How to read ADC's conversion result

Noriyuki Takahashi
Genius 9245 points
Part Number: ADS8675

Hi team,

I want to confirm how to read ADC's conversion result.

1. The datasheet mentions like below at 7.5.3 Output Data Word. Does this mean that, even if NOP command is issued, frame(F + 1) contains ADC's conversion result?  

"For all other combinations, the output data word for frame (F+1) contains the latest 14-bit conversion result."

2. ADC's conversion result is MSB first or LSB first?

D[31:18] is Conversion result, but I am not sure conversion result is [13:0](MSB first) or [0:13](LSB first). 

Regards,

Noriyuki Takahashi

  • 0
    TI__Mastermind 35520 points

    Noriyuki-san,

    1. Correct.  See the timing for CPHA=0 as an example below, the procedure is

    • The rising edge of CONVST/CS signal will initiate a data conversion.
    • When the NOP command is issued and the CONVST/CS signal is forced to low, your controller will send out multiple clocks to the ADC. The ADC will shift out the the data converted in previous step to SDO. Your controller can read them in the same frame.
    •  The CONVST/CS signal is pulled up to high when the data transfer is done. The rising edge of CONVST/CS signal will initiate another data conversion.

    2.  MSB first, see the M on the SDO in the timing below also in the Figure 4/73/74 of DS.

    Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Genius 9245 points

    Dale,

    Thanks!

    Here is an additional question.

    In table 6 on the datasheet, D[5:4] is for Parity bits. Is it OK that D[5] is for ADC output and D[4] is for the entire output data frame? 

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Genius 9245 points

    Dale,

    One more question.

    In the description of PAR_EN, we can see below explanation. Does "the entire output data frame" mean D[31:0] including parity bits D[5:4] and D[3:0] or only D[31:6] in case of Table 6 in the datasheet? Then, how about in case of Table 7?

    "The output data frame parity bit reflects an even parity signature for the entire output data frame, including the ADC output bits and any internal flags or register settings"

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Mastermind 35520 points

    Noriyuki,

    Q1: No. When the output data word only has ADC output data, the parity bits reflect an even parity for the ADC output bits only.

    When the output data word includes the ADC output data and other internal flags or register settings, the parity bits reflect an even parity for the entire output data frame.

    Q2: It includes entire bits D[31:0].

    Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Genius 9245 points

    Hi Dale,

    I am still not sure how to calculate the parity bits.

    Please see the attached. I made examples for parity bits calculation. 

    Could you review it? Could you also tell the definition of parity bits for ADC outputs and output data frame?

    examples for parity bits.pptx

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Mastermind 35520 points

    Hi Noriyuki,

    I will get it back to you soon.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    TI__Mastermind 35520 points

    Hi Noriyuki,

    To be clear:

    • First ADC parity bit:   this bit will only consider the ADC output bits for parity calculation.
    • Second frame parity bit:   this bit will consider all bits in the output frame for parity calculation including ADC output bits and any internal flags or register settings also the ADC parity bit as well.

    To answer your questions:

    1.  ADC output parity - In this example, ADC output parity should be 1, right? 

         Answer:  correct.

         The output data frame parity - How should this parity bit be calculated? Parity bits[5:4] should be included in this calculation? Bits[3:0] is also included?

         Answer:  See the clarification for the bit calculation above. This second frame parity bit is 0 based on the clarification. Bit[3:0] are all 0s, these bits do not affect the parity calculation.

    2. ADC output parity - In this example, ADC output parity should be 0, right?

        Answer:  correct.

    The output data frame parity - How should this parity bit be calculated? Parity bits[5:4] should be included in this calculation? Bits[3:0] is also included?

        Answer:  See the clarification for the bit calculation above. This second frame parity bit is 0. Bit[3:0] are all 0s, these bits do not affect the parity calculation.

    I hope these help you understand.

    Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Genius 9245 points

    Dale,

    Please let me make sure one more thing just in case.

    Second frame parity bit, in case of the example I attached in the previous post,  D[4] doesn't include D[4] for parity bit calculation, right?

    I mean, D[31:5] & D[3:0] are included in the calculation, right?

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Mastermind 35520 points

    Hi Noriyuki,

    The parity bits are calculated for even parity, and  the parity bits self are used for parity check to meet even parity.  Also, Bit[3:0] are all 0s, these bits do not affect the parity calculation.

    Regards,

    Dale