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ADS7957: p.47 of ADS7957 datasheet

mizushima kei
Prodigy 10 points
Part Number: ADS7957
Other Parts Discussed in Thread: THS4031

Tool/software:

Hi,

In my current circuit, MXO and AINP are connected with 0Ω resister, and all analog signals connected with ADS7957 are added voltage follower circuit to separate.

I think that voltage follower circuit becomes unnecessary by using the method on p.47 of ADS7957 datasheet (MXO and AINP are connected with op amp).

Is my understanding correct? 

Could you tell me what is the appropriate op amp if the input SCLK is 2.3MHz and this method is used?

Thank you and Best Regards.

  • 0
    TI__Genius 14670 points

    Hello Mizushima-san,

    If all input signals are already buffered with an op-amp voltage follower circuit, then yes, the buffer between the MXO and AINP pin is not necessary. 

    With an SCLK of 2.3MHz, you have about 2.83us of acquisition time. The input signal for a 10-bit device needs ln(2^10) RC time constants to settle, so you should ensure the R and C values of your input filter meet this requirement. 

    The THS4031 recommended in the datasheet would be a decent place to start and simulating with. I also recommend watching our TI Precision Labs videos on SAR ADC drive for guidance on simulating the action of the SAR ADC sample and hold circuit.

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    Prodigy 10 points

    Hi Joel-san,

    Thank you so much for your reply.

    I would like to import analog signals from a system to ADS795x.

    In order not to affect the system, each analog signal is branched off through op-amp, but I think that MXO and AINP connected with op-amp could be used instead.

    Is my understanding correct?

    I would like to ask you the question again because the opinion was expressed in my team that the question was not clear.

    Thank you for your support.

  • 0 in reply to mizushima kei
    TI__Genius 14670 points

    Hello Mizushima-san,

    Yes, that is correct. Rather than having an amplifier for each channel, you could use one amplifier for all channels placed between the MXO and AINP pins. All I would recommend is an RC filter at the inputs to each of the CHx pins, to help maintain the input stable as it is sampled.

    Regards,
    Joel