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TMS570LS2125: MibADC: The initial voltage of the Cmux and the Csamp.

Hideaki Nambu
Mastermind 9125 points
Other Parts Discussed in Thread: TMS570LS2125

Sirs,

On the page-120 of the TMS570LS2125 DS "SPNS164" we find the MibADC input equivalent circuit as Fig 5-1.

In the figure we find Cmux and Csamp.

Then, can I ask the initial voltage of the caps when they are connected to the ADC input pin? The replies would be ADREFHI, ADREFLO ,the center voltage, or no initialization,,,

     Q1. Cmux initial voltage.

     Q2. Csamp initial voltage.

 

  • 0
    TI__Expert 7650 points

    Hello Nambu san,

    Sorry for the delay responding.  I've forwarded your question to one of our experts.  will get back to you shortly.

    Regards,

    Brian

  • 0 in reply to Brian Fortman
    TI__Expert 7650 points

    If in "discharge" mode the initial voltage is ADREFLO. If not in "discharge" mode, there is no initialization as the initial voltage depends on the voltage of the previous input conversion.

    Potential Gotcha:  If insufficient sampling time is programmed based upon internal and external RC components, then a "ghosting" of the previous input voltage will appear on the current input conversion result because not all of the previous signal residue on Cmux and Csamp has been removed. Jim Childers wrote an App Note (SPNA118B) on this as it first appeared as crosstalk between input channels. The discharge feature was added to the ADC so that a "ghosting" error from a known ground reference can be dealt with and/or removed entirely easier than "ghosting" from an unknown external input.  (Thank You, Steve Howard.)

  • 0 in reply to Brian Fortman
    Mastermind 9125 points

     

    Brian,

    Your keyword "discharg" was very good for me to study the SPNA499. And everything was in the document. Thank you for your kind reply.

    I will read the SPNA118B also.