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MSP432 ADC14 Precision

Kenneth DeBower
Prodigy 50 points
Other Parts Discussed in Thread: CC3100, ENERGIA

I have questions about ADC14 precision:  Are these results typical?

Using the MSP432 with CC3100 booster, I am reading analog channels A6,A8,A9,A11,A13,A14,A10 and A15, which correspond to pin numbers 28,27,26,25,24,23,6,2.

I read these using the Autoscan mode, performing the reads 4 times per second.  The measured input voltage to all 8 channels is 1.644 volts, and I've setup external VREF+ measured at 3.288 volts.  This is a simple LDO voltage source and voltage divider with two 1000 ohm precision resistors). I've applied capacitor filters to Vss, Vcc, and input data.

I'm using Energia but have similar results with CCS.  Results are similar across five Launchpads.

I'm averaging the raw data over 40 reads.  All of the raw inputs are on the order of 8200-ish, so that's consistent.  Here is a typical set of results after collecting and averaging 40 reads:

Raw data means=              (8193.5,  8215.4,  8204.7,  8217.9,  8204.1,  8211.4,  8208.0,  8228.5)

Raw standard deviations= (   15.01,    25.80,     25.24,    26.99,    23.46,    25.51,    61.66,    26.33)

Calculated millivolts=              1645      1649       1647      1649      1647     1648       1647     1651

Is this level of precision typical?  The standard deviations are higher than I expected, and variation channel-to-channel is more than I expected.

Any experiences, observations, or suggestions?

Thanks to all

Ken

 

 

 

  • TI__Mastermind 25045 points
    Hi Kenneth,

    Let me look into this.

    Best regards,

    David
  • in reply to DavidL
    TI__Mastermind 25045 points
    Hi Kenneth,

    Those numbers seem to be correct but I'm getting difference results, could please share your CCS code.

    Thanks,

    David
  • in reply to DavidL
    Prodigy 50 points

    David,

    Thanks for the reply.  Sorry to be slow getting back to you, but only just got it figured out.

    After a lot of experimenting, I've made some discoveries.  The results posted Sept 29 were obtained in the Autoscan mode, with Energia.  That code has been
    cannibalized and is no longer available.

    Regardless, I finally noticed the first channel reading using Autoscan was generally pretty good and that subsequent channels had greater standard
    deviations and were less accurate, especially when a VOM is attached.  I now read the analog channels in the Single-Channel Single-Conversion mode and find these results to be much more reliable and reproducible.

    For example, I'm reading approximately 4 times per second, and averaging over 60 samples, measured voltage ~ 1648 mv:

    means=(8227.7   8223.1   8223.3   8230.2   8227.5   8226.3   8234.2   8224.0 )
    sdv's=(6.88   18.62   25.70   20.13   4.83   4.55   10.16   15.79 )
    volts=1649 1648 1648 1650 1649 1649 1651 1649

    Pin Numbers, respectively:  28,27,26,25,24,23,6,2

    I'm using the external voltage VREF+ measured at 3285 mv.  The voltage is divided between two 1k precesion resistors.  These results are pretty good.  This
    is without filter capacitors but I think averaging takes care of that.

    My main problems were inconsistencies and these took a while to find.  I have 7 LP's, but 4 of them have problems.

    On some of the bad LP's, the VREF+ voltage on the top side male pin differs from the VREF+ voltage measured on the under side female VREF+ pin, by 7 to 10
    mv.  These LP's are producing readings higher than actually measured.

    On several LP's, I find an occasional pin that pulls the voltage up or down, and then gives faulty readings.  These LP's run too hot to touch.

    So far, all results are independent of the presence of CC3100 Booster Pack.

    I know I've handled some of these LP's roughly, especially early on.  I believe I've fried some components with too much voltage, etc.  I'll be ordering
    more and keeping better records and quality checks.

    I'll be happy to supply more details and help any way I can.

    Many thanks.

    Best regards,

    Ken DeBower


  • in reply to Kenneth DeBower
    Guru 46710 points
    >This is without filter capacitors but I think averaging takes care of that.

    If you want consistent ADC measurements - always use lowpass antialiasing filters for ADC input(s), obviously having capacitors between ADC input and analog ground which will act also as charge storage and low impedance voltage source for ADC.

    Note that particular ADC have single sample&hold (virtual) capacitor having consequences as residual charge after conversion which will play bad jokes with your measurements if you do not properly care about correct signal source impedance versus sampling time relation. This could be your problem BTW.
  • Former Member
    Former Member in reply to Ilmars
    Kenneth,

    I noticed you mentioned having 4 bad LaunchPads that seemed to suffer from an overheating issue possibly? We are in the process of looking into a similar issue and would like to ask do you still have these LaunchPads or have you gotten rid of them? Also was it the MSP432 IC or the emulator that was getting hot?

    You mentioned that using the CC3100 and ADC14 code was the only thing you were doing before realizing the LaunchPad overheating, correct?
  • in reply to Former Member
    TI__Mastermind 25045 points
    I'm marking the thread closed due to inactivity, but you can always use "reject answer" or simply post back to re-open it if you need more help on this issue.

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