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ADS7128: about ADC Transfer Function issue

Lawrence He
Genius 3105 points
Part Number: ADS7128

Hi, Support Team

We saw the ADC7128 datasheet of 8.3.3. as ADC Transfer Function

Equation 1 is 1 LSB= Vref / 2^N

if AVDD=1.66v and 3.2v and we calculator

LSB=1.66v/4096=0.00040527

LSB=3.2v/4096=0.00078125

and we follow Table 8-1 :(AVDD/2)+1LSB to (AVDD/2)+2LSB= 801(IDEAL OUTPUT CODE)

so AVDD 1.66v and 3.2v are the same IDEAL OUTPUT CODE is 0x801?

if any suggestion, Please advise me.

Thanks,

Best regards,

Lawrence

  • 0
    TI__Guru**** 180751 points

    Hi Lawrence,

    For the ADS7128, AVdd is the reference voltage.  Your calculations are correct with the exception the minimum AVdd voltage is 2.35 (not 1.66).  For any AVdd in the range of 2.35 to 5.5V, the LSB size will change according to equation 1.  The output code will always follow Table 1 - FS+ will be 0xFFF, FS- will be 0x000.

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    Hi Tom: 

    thanks for your reply, I think my question is whether the AVDD is 3.3V fixed and whether the input voltage from AINx is according to the table1 to get the output code

    for example : 

    the green block is (AVDD / 2) + 1 LSB to (AVDD / 2) + 2 LSB and the AVDD is 3.3, the LSB is 3.3v/2^12 = 0.0008056640625, so the input voltage range is 

    1.6508056640625v to 1.651611328125v.

    for this function, I have two question

    1. in this range the output code is 0x801 right?

    2. What is the output code if the input voltage from the pin AINx is between 1.66 and 3.2991943359375? 

    if any suggestion, Please advise me.

    Thanks,

    Best regards,

    eason

  • 0 in reply to eason chang
    TI__Guru**** 180751 points

    802 to FFF, the LSB size does not change with the analog input.  It is solely dependent on the analog supply voltage.