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ADS1118: Please tell me about ADS1118 internal reference long term stability.

Ilja Vlaskin
Prodigy 50 points
Part Number: ADS1118
Other Parts Discussed in Thread: ADS1115

Hello. I not found in datasheet information about reference voltage long term stability. Please tell me about this.

  • 0
    TI__Guru* 92655 points
    Ilja,


    There is no datasheet specification for the ADS1118 internal reference long term stability. Specifications about the internal reference covered in the specification on gain error.

    Because an ADC compares the input voltage against a reference, any error in the reference is seen as a gain error. As an example, the maximum gain error for the ADS1118 is 0.15%. This is the equivalent of having a no ADC gain error, but a reference error of 0.15%.

    In the ADS1118, the internal reference is not brought out to a pin and is always internal. We only can observe the reference as a function of the ADC measurement. This is why we specify gain error and gain error drift instead of specifying the reference.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 50 points

    Yes, yes!!! I understand all this!!! But I dont sure that long-term stability included in these gain error values. Datasheet dont mentioned about long-term stability directly. On the contrary (from datasheet, I marked out suspicious word):

    9.3.4 Voltage Reference

    The ADS1118 has an integrated voltage reference. An external reference cannot be used with this device. Errors

    associated with the initial voltage reference accuracy and the reference drift with temperature are included in the

    gain error and gain drift specifications in the Electrical Characteristics.

    I dont sure that long-term stability error included to initial voltage reference (and gain) error. Generally, long-term stability described separately.

    Even if long-term stability error is included in the gain error, is necessary known long-term stability error value separately (because this is uncalibrated gain error, and initial gain error may be set to zero with calibration).

  • 0 in reply to Ilja Vlaskin
    TI__Guru* 92655 points
    Ilja,


    The long term stability is included in the gain error specification. We use the term initial voltage reference accuracy as the accuracy you get when you turn on the device, while the drift includes any changes from that point.

    I can look to see if we have any data to show the long term stability of the gain error. However, I'm not sure I've ever seen any.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points

    Ilja,


    I wasn't able to find any specific data for the ADS1118, however I did find some for the ADS1115. The ADS1115 is basically the same ADC, but with an I2C interface instead of the SPI interface that the ADS1118 has.

    This data shows the change of gain error through a HTOL test. This should mimic a long term aging of the device. This data has a sample size of 116 devices.

    Based on the plot, the change in gain error is very small compared to the specification of 0.15%


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 50 points

    Thanks for reference to AD1115. This is suitable for my. And this information about long term stability need to be included in AD1118 datasheet :-)