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ADS131E08S reference accuracy discrepancy

Bruce Lott
Prodigy 220 points
Other Parts Discussed in Thread: ADS131E08S, ADS131E08

On page 7 of the ADS131E08S specsheet, the internal reference accuracy is listed as +/-0.2%, but the 4V nominal reference min/max limits are 3.88/4.12V, which amounts to +/-3%. Both are spec'd at 25C.  Which is correct, or am I missing something?

For comparison, on the ADS131E08 specsheet, the 2.4V min/max limits are 2.394/2.406V, which are much closer at +/-0.25% (accuracy also spec'd at +/-0.2%).

  • 0
    TI__Mastermind 43915 points

    Hi Bruce

    Thanks for pointing this out. The initial accuracy is typically +/-0.2% but we have hard limits of 3.88V to 4.12V in our final test program. This is a much broader ranger than the ADS131E08 but we are hoping that a calibration is run at room temp.

    Thanks

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 220 points

    Tony,

    Thanks for your reply. I need some clarification, so here's some context: I want to apply the ADS131E08S for power system V/I metering with a +/-0.1% max. AFE chip error (excluding external component tolerances) over a -40 to +85 C ambient temp range, after calibrating at 25 C. I don't need "fast powerup", so plan to use the ADS131E08 filtering recommendations on the VCAP1 and VREFP/VREFN pins; I will also be using clean +/-2.5V LDO supplies and good design practices for noise control. With sufficient synchronous oversampling and gain=1, it appears that the AFE chip accuracy will be primarily limited by the reference. The appeal of the "S" part is its low-drift internal reference, which I would set for 4 V. (c.f. the ADS131E08 reference drift of 20ppm/C alone equals +/-0.13% over -40 to +85 C.)

    So bottom line, can I count on the "S" part for the accuracy I'm trying to achieve? If that's too broad of a question, how is the reference's +/-0.2% typical accuracy reconciled with its +/-3% spec'd voltage limits (see below)?

    Since the typical accuracy, which implies a range, is more than an order of magnitude tighter than the Vref min/max range, the former seems pretty meaningless. You called it "initial accuracy". Do I need to worry about some sort of non-temperature-related drift or aging?

    Is there something about the design of the "S" part's reference that would make it less accurate (but with better drift) than the standard part?

    Finally, there is no max. temp drift spec, only a typical 8ppm/C.  Looking at the Vref vs. Temp graph on p12, I see a few curves that average 20, 28, 40 ppm/C over -40 to +85, for a "max" error of +/-0.25% from 25 C.  Only a few of the flattest curves seem to meet the 8ppm/C "typical".  Do you have any information as to the max drift?


    Thanks,
    Bruce

  • 0 in reply to Bruce Lott
    TI__Guru 66346 points

    Hello Bruce,

    While Tony is out of the office, I will try to offer some insight to your questions:

    1. Reference voltage accuracy. I spoke with the designers on this and there is actually a typo in the Electrical Characteristics table for the ADS131E08S. The min/max limits should be +/-0.3% from the nominal value of 4V, closer to 3.088V min and 4.012V max. We will get this changed as soon as possible. This is very similar to how we specify the 'E08 as well, where the typical accuracy is +/-0.2%, but the min and max limits are just a little wider at +/-0.25%. The reason behind this has to do with our screening accuracy - we repeat our test program for multiple devices across multiple testing sites and come up with a min/max limit based on the data we see from those results. The +/-0.2% typical accuracy is what we are actually screening for, so you can have faith in that spec.

    2. Reference drift. When calculating the reference drift, you must remember to divide the change in Vref by the nominal value. For example, in the dark teal curve (for which I believe you calculated 40pmm/C), the reference drift is equal to (4V-3.995V) / 4V / (85C - -40C) * 1e6 = 10ppm/C.

    I hope this clarifies your concerns. Please let me know if you have any further questions.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 220 points
    Ryan,

    Thanks for your reply. The closer voltage accuracy is comforting, and makes more sense. As for my Vref drift calculation error, I guess that's what I get for working on this in the wee hours. Those numbers were actually uV/C, which you correctly point out needed to be divided by the nominal Vref (in volts) to get ppm/C. So that yields 10ppm/C for the 40uV/C curve, with most curves under the 8ppm/C typical spec. This translates to a typical drift of 0.05% over 65 delta C, which is fine for my application.

    I'm very happy about this. Do you have any idea as to the likely max drift (which is not spec'd)?
  • 0 in reply to Bruce Lott
    TI__Mastermind 38745 points
    Hey Bruce,

    Unfortunately we cannot provide that information publicly. If you would like, I can contact you via email and we can discuss further.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 70 points

    Yes, Brian, please contact me by email on this last point (max drift).  Thanks.