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ADC offset error calibration

shuto kim

Hi,

We are evaluating ADC functionality.(We use nrf52832 on a device with sdk 16.)

The evaluation was performed under the conditions with and without offset calibration (temperature 6 conditions), but the error was reduced under the conditions without offset calibration.

Offset calibration was performed only once when the power was turned on, and the power was turned on again at each temperature.

This result suggests that (1) it is not offset calibrated, or (2) it is offset calibrated, but an error is added.

About (1)
Please tell me how to calibrate the correct offset.
For example, is there a specification for the timing of offset calibration? How many seconds should be applied after supplying power to the microcomputer, or how many seconds should be applied.

About (2)
The error with offset calibration was stated as ± 2LSB, but what is the error without offset calibration? Also, is it possible that the overall error including the gain error will be larger if offset calibration is applied?

Best regards,

shuto

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  • 0

    Hi,

    There is indeed instances where the offset calibration might worsen results versus not using the calibration. This is often in instances where the SAADC is going to operate over a large temperature range. There is mainly two reasons for this:

    1.  We know that the offset changes with temperature, so it might actually be very close to a tripping point between two DAC values.

    2. 2) We know that there is noise in the system (inherent thermal noise) which introduces sufficient randomness to flip the calibration values back and forth.

    The error with offset calibration was stated as ± 2LSB, but what is the error without offset calibration? Also, is it possible that the overall error including the gain error will be larger if offset calibration is applied?

    It will not affect the gain error. If you see that the samples are better without offset calibration, then you should not use it. 

    regards

    Jared 

  • 0 in reply to Jared

    Thank you for your reply.

    If offset calibration is not performed, is the offset error range fixed? Is there a specification such as ± 2.0%?

  • 0 in reply to shuto kim

    Hi,

    No it's not fixed as it's dependent on external errors such as temperature.

    regards

    Jared

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