• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 47 subscribers
  • Views 125 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

ADS9218: Common mode input range

Jerome Kaufmann
Prodigy 30 points
Part Number: ADS9218

Hello TI team,

We are planing to use the ADS9218 with a differential LC filter at its input, driven by a fully differential amplifier. In The datasheet under recommended opperating conditions it is stated, that the input common mode voltage should not exceed Vocm+-0.025V (i assume the test conditions should read (Ainxp+Ainxm)/2 not (Ainxp-Ainxm)/2), however since in our desing the differential amplifier is on the other side of the filter, the tolerances of the passive components (especially regarding ESR) will not allow for this much precision.

Hence my question is: what is expected to happen if the input common mode voltage is not within this specification but the diffferential mode input is still within the specified limit of +-3.2V and all inputs are within the single ended range of 0.7V..4.1V?

Thanks and best regards
Jerome Kaufmann

  • 0
    TI__Guru 59126 points

    Hi Jerome,

    Thank you for your post. We plan to update this specification in the data sheet before the device is fully released. Based on the characterization data we've seen, the limit can be increased to VOCM ±75 mV. Will this work for your application?

    And yes, the calculation is (AINxP + AINxM) / 2 - this will also be corrected in the final data sheet release.

    If the input common-mode range is grossly beyond the specified range, the analog core will actually shut down and the output will read all zeroes (I believe only for that channel). This will not, however, cause any damage to the device as long as the absolute input voltages are within the Absolute Maximum Ratings. 

    May I ask why an LC filter is needed at the ADC inputs? Could it be placed in front of the FDA instead? If it's preferred to share such details about the signal chain over email, I'd be happy to reach out to you offline.

    Best regards,

    Ryan

  • 0
    Prodigy 30 points

    Thnk you.

    I think we found a Solution regardless, that should allow for the common mode voltage to be in that range. But your answer definitely helped. The filter cannot be on the other side of the FDA due to limitations in the input impedance and we wanted to avoid having to use two amplifiers.

  • 0 in reply to Jerome Kaufmann
    TI__Guru 59126 points

    You're welcome, Jerome. Let us know if you have any other questions. - Ryan