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Buffer Op for ADS1248

Takumi Suzuki1
Mastermind 8610 points
Other Parts Discussed in Thread: ADS1248

Hi,

I woule like to confirm about buffer Opamps for ADS1248.

Do we need buffer OPamps to use ADS1248 and to prevent input data error occurring? (influence of crosstalk by MPX)

Could you please advise me which case we should use buffer OPamps for ADS1248?

Thanks and Best Regards,

  • 0
    TI__Guru* 97595 points
    Takumi-san,


    Are you asking if buffer opamps are needed to prevent channel-to-channel crosstalk in the mux?

    I do not believe that buffer opamps are necessary. The ADS1248 has an input PGA which acts to increase the input impedance to the ADC. For crosstalk, I'm not sure that a buffer would help.

    Attempts to measure crosstalk of both AC and DC signals between channels have been done before. However, it is small, in the level of the noise of the measurement and not seen as a problem.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8610 points
    HI Joseph-san,

    Thank you very much for your quick response!
    I understood.

    Do you know "Memory effect" that means the error by "stray capacitance" of MUX?
    I have heard the effect recently.

    Let me try to explain as below,
    After one ch is ADC, the MPX is changed to next ch.
    Then there is a little voltage charged on stray capacitance of MPX.
    The value has infuluenced the input signal of next ch...
    To prevent the influence of the stray capacitance, we should use buffer OPAmp.

    What do you think of this?

    I have the document and let me send it to you if you would like.
  • 0 in reply to Takumi Suzuki1
    TI__Guru* 97595 points
    Takumi-san,


    Why don't you send me a link to the document? I'll look it over, but I don't think it's a problem.

    The multiplexer is low in stray capacitance. Charge injection from the capacitance in the switches in the multiplexer does have some minor affect in the measurement, but this will often show up as the integral non-linearity (or INL) in the measurement. However, this is something we design for and simulate the effect of. As I mentioned in the previous post, we've made measurements for crosstalk, and it looks extremely small.

    Again, why don't you send me the link and I'll read it over.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8610 points
    Joseph-san,

    Thank you for your reply.

    I'm sorry I cannot find original material, I know just Japanese version...perhaps this material is Japan original...
    www.tij.co.jp/.../jaja231.pdf
    About "Memory effect", please check page 10-11.
    Though the sentense is Japanese, I think you can understand the grafical data (Fig13, 14).

    I got another Japanese material which seems to be easier to understand.
    As I do not know if I can disclose it to E2E, I asked you to let me send it to you.
    Please understand my situation.

    Regards,
  • 0 in reply to Takumi Suzuki1
    TI__Guru* 97595 points
    Takumi-san,


    I'll need to take some time to translate the document.

    However, I would point out that the ADS1248 is a delta-sigma type ADC, which generally has longer sampling times, and uses many samples to get a single data. This way, the settling is less of a problem than in the SAR type ADCs.

    I've sent you a request to friend you through the E2E community. If you want to send the other material through a message, you can. However, let me look at pdf first.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8610 points
    Joseph-san,

    Thank you for your kindness!
    Let me friend with you.

    Sorry, Though I would like to send you the material, I do not know how to send.
    How should I do via E2E?