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Input shorted ADS1298

Al Luis
Prodigy 225 points

Hello!

I´m doing noise measurements with input shorted in all channels. According the datasheet, when Input are shorted, they have a AVDD-AVSS/2 voltage value.

Nevertheless, I get the following  ofset DC values:

G=1       -4.2768e-04
G=2           9.0398e-05
G=3            -1.5764e-04
G=4            -2.5289e-04
G=6              -1.6860e-04
G=8               -1.3284e-04
G=12                 2.3995e-04

How can it be possible if  AVDD is 3.11 and AVSS=0???



Thank you!

  • 0
    TI__Mastermind 38745 points
    Hey Al,

    The converter measures the voltage differentially. This means that the output voltage will be (AINP-AINN)*PGA. Even though the respective inputs are positive with respect to the board ground, AINP may be a smaller voltage than AINN which will produce a negative output.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 225 points
    The problem I had is that I was waiting to obtain (AVDD+AVSS)/2, but the values that I have obtained are too small.

    So.. I won´t get the 1,55*G voltage value.
    (1,55 from 3,11 V (AVDD)/2), I will get the difference, because it is a differential measurement... I mean, I will have approximately 1,55V in PgaP and approximately 1,55V in PgaN. Consequently I´m getting the difference between this values.
    Is that right?

    Thank you very much Brian!
  • 0 in reply to Al Luis
    TI__Mastermind 38745 points
    Hey Al,

    That is correct. To get a 1.5 V output code, you'd need the difference between the respective PGA output voltages to be 1.5 V.

    Regards,
    Brian Pisani