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Manual issue concerning ADC calibration

42Bastian
Expert 1430 points
Hi,
I think there is a bug in the manual.
According the manual, BRIDGE_EN == 0 and HILO == 0 gives:
/* (AD(REFHI)*R1 + AD(REFLO)*R2) / (R1 + R2) */
but (at least on the TMS570USB stick) it seems to be
/* (AD(REFLO)*R1 + AD(REFHI)*R2) / (R1 + R2) */

The other bit-combinations give correct values.
  • 0
    TI__Mastermind 49120 points

    Hello,

    The resistances R1 and R2 are typically 7K-ohm and 5K-ohm respectively. These values are selected such that two combinations of the BRIDGE_EN and HILO bits yield 7/12th and 5/12th of VREFHI as the reference voltage converted (with VREFLO = 0V). The {BRIDGE_EN = 0, HILO = 0} combination gives a conversion result of 0x952.

    This is a result of the reference voltage being VREFHI * 7/12.

    Similarly, you will get a reference voltage of VREFHI * 5/12 for the {BRIDGE_EN = 0, HILO = 1} combination.

    I have verified this on a USB stick here.

    Please confirm that there is no issue with the documentation.

    Regards,

    Sunil

  • 0 in reply to Sunil Oak
    Expert 1430 points
    Hi
    sorry, for the late answer.
    On the USB stick I get:

    BRIDGE_EN = 0, HILO = 0 => 2386 ( ~ 7/12)
    BRIDGE_EN = 0, HILO = 1 => 1704 ( ~ 5/12)
    BRIDGE_EN = 1, HILO = 0 => 0
    BRIDGE_EN = 1, HILO = 1 => 4094

    But the documentation states that BRIDGE_EN = 0, HILO = 0 shall give 5/12 (table 15-3).
    So, yes there seems to be a bug in the docu.
  • 0 in reply to 42Bastian
    TI__Expert 7650 points

     

    Hi Bastian,

    Since R1 is 7KOhm and R2 is 5KOhm, BRIDGE_EN = 0, HILO = 0, and AD_REFLO is 0V then I would interpret row 1 of the following table

    to result in 7K*AD_REFHI / (7K+5K) or 7/12*AD_REFHI rather than 5/12*AD_REFHI.  You don't agree?


  • 0 in reply to Brian Fortman
    Expert 1430 points
    Hi Brian,
    right, but Fig. 15-16 says R1 == 5k :-)
    So what is now correct ? The table or the figure.
  • 0 in reply to 42Bastian
    TI__Mastermind 49120 points

    Hi Bastian,

    The table is correct as evidenced by the actual conversion results. The figure will be corrected in the next update to the TRM. Thanks for identifying this issue.

    Regards,

    Sunil

  • 0 in reply to Sunil Oak
    Expert 1430 points
    Hi
    ok, I will fix my code.
    Maybe it is a good idea to have the values of R1/R2 also noted somewhere in the text and not only in the figure!
  • 0 in reply to 42Bastian
    TI__Expert 7650 points

    Bastian,

    You are correct.  We need to be more clear on this point.  We have this as a ticket to fix this in future TRM releases.   Sorry for your trouble!