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TMS320F2800137: there is a hardware mechanism to ensure the accuracy of continuous multiple channel switching for sampling.

Shaoxing Ke
Expert 3540 points
Part Number: TMS320F2800137
Other Parts Discussed in Thread: ADS7826

Hi Experts,

I am here asking for my customer.

Customer are using round robin SOC Priority, configure 6 Channels on one ADC module. And they concerned that SOC0 and SOC1 adjacent sampling channels, when SOC0 done and SOC1 start, does the charge on the Ch/7.5PF capacitor completely released? Or it still means that the Ch charge still exists, but there is a hardware mechanism to ensure the accuracy of continuous multiple channel switching for sampling. Thanks.

  • 0
    TI__Expert 3540 points

    Hi Experts,

    Before SOC1 start,  does the charge on the Ch/7.5PF capacitor completely released w/SOC0?

  • 0 in reply to Shaoxing Ke
    TI__Expert 3540 points

    Hi Experts,

    I want to double confirm with you, the Type-5 ADC in F2800137/157 are the same as the Type-3 ADC circuit. Right?

  • 0
    TI__Expert 3540 points

    Hi Experts,

    We want to confirm with you, the Type-5 ADC IP is the same as ADS7826? Thanks.

  • 0 in reply to Shaoxing Ke
    TI__Mastermind 33725 points

    Hi Shaoxing,

    Just to be on the same page, where are you referring the ADC types from?  ADS7826 is not a C2000 device so it will for sure not use the C2000 ADC SARs.  As for the sampling capacitor inside the SAR, the switching is will only happen when the trigger is ready from the SOC configuration and remains on (charging) for the duration specified by ACQPS + 1 (or SH time).  It will automatically open after the quration of SH time and then the SAR will convert for the duration of tLAT in the timing digaram.  The capacitor will remain open for the duration of tLAT.  It will only close nd charge if a trigger is received.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Expert 3540 points

    Hi Joseph,

    1. Here I want to confirm with you, when SOC0 done and SOC1 start, does the charge on the Ch/7.5PF capacitor completely released? So the  duration of tLAT is enough for Ch/7.5PF capacitor completely released here? Right?

  • 0 in reply to Shaoxing Ke
    TI__Mastermind 33725 points

    Hi Shaoxing,

    After tLAT, the charge on the SH cap would be discharged inside the SAR module when conversion completes.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Expert 3540 points

    Hi Joseph,

    After tLAT, the charge on the SH cap would be discharged inside the SAR module when conversion completes,- want to confirm with you, does the SH cap discharged completely when the next SOC starts? Thanks.

  • +1 in reply to Shaoxing Ke
    TI__Mastermind 33725 points

    Hi Shaoxing,

    After looking closely at the SAR implementation on the F280013x device, i realized that the sampling capacitor does not actually discharge after tLAT.  To enable discharge of the sampling capacitor, users can either sample on intrnal VREFLO channels (A13 for ADC-A ore C13 for ADC-C).  If ACQPS (or SH time) is sufficient, then sampling capacitor will charge (equalize) to the same level as the analog input signal even if the sampling capacitor is not fully discharged on the succeeding sample/conversion cycle.  F280013x does not have a feature to automatically discharge the sampling capacitor but this feature is added in our next generation of devices. 

    In summary, to mitigate the issue of sampling capacitor not fully discharged:

    1. ensure that settling time is more than sufficient for the sampling capacitor signal to equalize or settle to the value of the analog signal being sampled OR
    2.  add a conversion cycle for VREFLO channel (A13 or C13) between conversions to ensure that the sampling capacitor charges to VREFLO level.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Expert 3540 points

    Hi Joseph,

    Very thanks for your answer here.

  • 0 in reply to Shaoxing Ke
    TI__Mastermind 33725 points

    Hi Shaoxing,

    Closing this post.  Let's continue discussions over email.

    Thanks,

    Joseph