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ADS1247: What's the expected difference between measures of the same value with several gains?

Youry Yakimov
Prodigy 45 points
Part Number: ADS1247
Other Parts Discussed in Thread: ADS124S06

I can't find answer to my question in the datasheet. For example, I measure 0.1 VDC with gain 1 and gain 16. What's the maximum error between two values I can expect? What about all gains from 1 to 128 (of course, the VDC at input will be lower)

ADC power supply is +3.3V at AVDD and -1.24 at AVSS. Data rate is 20 SPS. I do selfcal before measurement. I also user an internal reference voltage source.

As I understand the 'gain error' at the datasheet says about gain linearity not the difference in measurements between several gains?

For example, I gave 0.1 VDC linear source powered from 18650 Li-po (so, no noise, no ripples) and I have next results:

PGA   Measured Value, mV

1        0.99957

2        0.99961

4        0.99928

8        0.99994

16       0.99899

As we see at the best gain (more gain,more scale is used) we have 'strange' result. So, why the error between first for (1, 2, 4, 8) gains is small, and it is so big between the fifth (16) gain and first four gains?

I'm not interested in measurement of absolute value, only in ratio between gains...

  • 0
    TI__Guru 53666 points

    Hi Youry,

    How is the battery connected to your system? Is the (-) lead connected to AVSS or ground (0V)? Or is the battery voltage floating between two inputs? It might help to see a diagram of your system.

    Are these measurements repeatable? In other words, if you measure the input 20 times for each gain, are you getting exactly the same result each time? Or does it vary? If you perform this test, please post the results.

    What about other data rates? Are the results the same?

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 45 points

    Hi Bryan! Nice to speak again))

    Ok, I'm going to simplify the question: 1) what parameter 'Gain Error' in the datasheet mean? 2) And why it's specified at DR from 40 SPS up to 160 SPS?

    3) What I'm goinп to see in this 'Gain Error' at DR=10 SPS, for example?

    But my main question: what does this parameter mean?

  • 0 in reply to Youry Yakimov
    TI__Guru 53666 points

    Hi Youry,

    Gain error is a deviation from the ideal ADC transfer function. A gain error changes the slope of the transfer function in either a positive or negative direction, and is usually specified as a percentage assuming the gain error is linear. So if you apply a 1V signal where the positive full-scale (+FS) voltage is 5V, and the gain error is +1%, you might actually get a digital code that corresponds to 1.01V. If you then put in 2V, you would get 2.02V, and so on. You can check out our Precision Labs training content for more information: https://training.ti.com/ti-precision-labs-adcs. Specifically, review module 2.2 and modules 5.x

    The gain error is only specified at the given data rates for the ADS1247 because that's what our characterization team decided to do. Unfortunately I do not have a much better answer than that. I would guess this has something to do with the way the modulator clock works for the different data rates (see Table 9 in the datasheet). They also likely assumed that these data rates would be most commonly-selected by the user, which is why they were targeted for characterization.

    If you want to know the gain error at a specific data rate, I would suggest characterizing it using the Precision Labs content as a guide. Basically, apply a close-to-full-scale signal e.g. 95% of FS, take several readings, average the results, and then store the difference between the actual and ideal measurements as the gain coefficient (this is also how you would calibrate out the gain error. Make sure to remove the ADC offset first, or this will interfere with the gain error calculation. Also, use a precision source to generate the +FS signal - any error in this output will impact the gain error calculation as well.

    I should also point out that we have a next generation version of this device available, which is the ADS124S06. Compared to the ADS1247, the ADS124S06 has more channels, faster data rates, comes in a smaller package, is lower power, offers better DC and noise performance, more features, and is slightly lower cost. If you are not able to switch your ADC at this point in your design, it might be worth considering the ADS124S06 for a future design.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 45 points

    Hi Bryan! Sorry for a long answer. We make a calibration of every gain (1, 2, ..., 128). After this we get a good result! Thank you very much!

  • 0 in reply to Youry Yakimov
    TI__Guru 53666 points

    Great to hear, Youry, I am glad you are getting good performance out of your board.

    Let us know if you need anything else. If you have additional challenges that are unrelated to this topic, please feel free to start a new thread and we will support you there.

    -Bryan