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Original question:

ADS7038-Q1: ADC Offset Calibration

ADS7038-Q1: ADS7038

Pascal Regache
Prodigy 120 points
Part Number: ADS7038-Q1
Other Parts Discussed in Thread: ADS7038

Tool/software:

Hello, I would appreciate further clarifications on the ADS7038 offset specfication.

1- There are no specification of offset performance without calibration. Is it possible to have an indication of the values (value and drift) ? Or it simply not recommanded at all ti 

2- The offset temperature drift unit is ppm/°C. Is it ppm of AVDD ? LSB ? Both seems weird: former case drift would depend on reference voltage, later case seems way to small (less than nV)

3- Once calibrated, can you confirm that the 1 ppm/°C applies as long as the device is powered ? Section 7.3.4 (8.3.4 for -Q1) says: "The variation in ADC offset error resulting from changes in temperature or AVDD can be calibrated..." and seems to imply that temperature tracking would require a regular calibration process.

Thanks,

Best regards

Pascal

  • 0
    TI__Genius 11550 points

    Hi Pascal,

    1. It is not recommended to operate the device without offset calibration. Performance of the device without running offset calibration is not characterized for. Is there any concern at all from your side on implementing the calibration routine?

    2. Yes, the ppm/°C spec is in reference to AVDD. LSB size and AVDD are related by a factor of 4096, per the resolution of the device. See below typical characteristics plots for offset error vs temperature and AVDD.

    3. I'm not exactly sure if I'm understanding the question exactly, so let me pose it as an example. I will also say since the offset error drift is typical only, it is not a guaranteed datasheet spec. Say the device starts at -40°C ambient, and is calibrated for offset here. If it is then brought up to 125°C, then the offset error has potentially increased/decreased by 165ppm of AVDD. You would then want to calibrate offset once again to compensate for the drift over temperature. In general, yes, if the ambient temperature is expected to change dynamically, it is expected to calibrate to compensate for the temperature drifts. 

    Feel free to clarify the above and let me know if you have any further questions.

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    Prodigy 120 points

    Hi Joël,

    Thanks for the answers. I do not have a problem with calibrating the device, just need to decide whether I shall make it a regular task of not.

    About the third point: to me, the datasheet sentence "The variation in ADC offset error resulting from changes in temperature or AVDD can be calibrated..."  might imply that the 1ppm/°C drift after calibration may not apply across the temperature change. In you example, I calibrate once at -40°C then temperature drifts to 125°C, I expect the 1ppm/°C to hold true. Is that OK ?

    A related point is: the calibrated offset is specified at +/-2 LSB min/max for the whole temperature range. So I might use this specification across the whole temperature range as well. Said differently, once calibrated, can I expect the offset to be +/- 2LSB across the whole temperature (no matter what the calibration point), or shall I add 1 ppm/°C * temperature change to the 2 LSB. In the later case, I might have to go for a regular calibration.

    Regards, Pascal 

  • +1 in reply to Pascal Regache
    TI__Genius 11550 points

    Hi Pascal,

    You are correct that the 1ppm/°C drift will not apply uniformly throughout the entire temperature range on a typical device. Drift might be worse or better in a given temperature range, but won't exceed 1ppm from one temperature to the next degree Celsius. A typical device should have better drift performance than the 1ppm/°C specified, so this is a "min/max" typical spec if you will. For example, from 0°C to 1°C, the device might drift by 0.8 ppm, but from 1°C to 2°C, it might drift by 0.3ppm, but in no case along the temp range is it expected to exceed 1ppm drift on a typical device. 

    Considering the "worst case" 1ppm/°C drift for how often you should calibrate the device will give you sufficient margin throughout the temp range.

    On the max/min calibration spec, this is considering calibration at every point along the temperature range. So to clarify this does not mean calibration is done at -40°C, and the max offset error seen is 2 LSB at 125°C. As you mentioned, you should consider the latter case, and implement more regular calibration according to the drift spec. Thanks for clarifying! 

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    Prodigy 120 points

    Joël, thanks for the details.

    Best regards 

    Pascal