• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 46 subscribers
  • Views 334 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

ADS131E08: Relation of input range vs. AVDD

ADS131E08: Output Code in case of Pseudo-Differential Input

Shota Mago
Expert 8120 points
Part Number: ADS131E08

Hi team, 

Let me ask you basic question in terms of output code of ADS131E08 in case of Pseudo-Differential Input.

When AVDD=VREFP=2.5V/AVSS=VREFN=-2.5V and VINN=0V, full scale range should be ±2.5V because of Pseudo-Differential Input.

In this case, ADS131E08 can generate output code as 400000(-2.5V)~3FFFFF(2.5V).

Because when differential input, full scale output code should be 800000(-5V)~7FFFFF(5V).

So in case of  Pseudo-Differential input, output code range is supposed to be half of differential input.

Is this understanding correct?

If so, in case of Pseudo-Differential input, 1LSB is NOT derived by (VREF / Gain) / 2^24 BUT (2 × VREF / Gain) / 2^24 even though full scale range of Pseudo-Differential is VREF.

Is this also correct?

I'm looking forward to hearing back from you.

Best regards,

Shota Mago

  • 0
    TI__Guru* 97695 points
    Shota-san,


    If the AVDD=VREFP=2.5V, and AVSS=VREFN=-2.5V, the full scale range of the ADC is ±5V not ±2.5V. The pseudo-differential input can only go to ±2.5V, and that's why the ADC output code goes from 400000 to 3FFFFF.

    Just because the input is limited to ±2.5V, it doesn't change the input range of the ADC. If the reference input is 5V (2.5V-(-2.5V)), the LSB is the same.

    If you look at table 8 on page 33 in the datasheet, the LSB is FS/2^23. In this table, the FS is the positive full scale. With this example, the positive full scale is +5V/gain, so the LSB is (+5V/gain)/2^23. This is the same as the entire full scale range LSB which would be ((+5V-(-5V))/gain)/2^24.


    Joseph Wu