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ADS1225: Self-calibration.

Kengo.Y
Guru 11245 points
Part Number: ADS1225

Hi team,

I'd like to know about Self-Calibration.

When my customer use self-calibration, they get a difference in the output data.
Could you check their waveforms?





And then, I have some questions about self-calibration.

1. Self-calibration does not need an input signal. So what does it compare to?
2. Is Self-calibration affected by changes in Vref?
3. Is Self-calibration a close to factory condition?
4. Is there a frequency recommendation?

Sincerely.
Kengo.

  • 0
    TI__Guru** 113765 points

    Hi Kengo,

    The self-calibration will first adjust the offset by applying a short to the input and determining a value to correct for offset.  The second part of the calibration is to apply a full-scale signal to the ADC inputs from the reference to adjust for any gain error.  Any noise on the power supply and especially the reference will affect the outcome.  Also, keep in mind that the noise of the conversion process will result in varying results to be within the level of the noise of the ADC.  In other words, high-resolution mode will have lower noise than high-speed mode resulting in a more accurate calibration.

    1. Self-calibration does not need an input signal. So what does it compare to? [Bob] This is done internally by applying a short on the inputs for offset calibration and connecting to Vref for gain calibration.
    2. Is Self-calibration affected by changes in Vref? [Bob] Yes, the Vref noise and drift will affect the results, especially the gain calibration.
    3. Is Self-calibration a close to factory condition? [Bob] I'm not quite sure what you mean here, but by procedure yes.  There is a dependency on the reference voltage used and whether the buffer is enabled.  As a full-scale input voltage will be applied I would disable the buffer (if enabled) when calibrating to avoid error due to the buffer. 
    4. Is there a frequency recommendation? [Bob] The self-calibration should be done following device power-up after the supplies have reached nominal operating voltage and the reference is fully settled.  Also, after any device reset.  Any calibration done following power-up and reset will depend on how much the temperature of the ADC and reference may drift over time.  There is really no guidance to be given as this will depend on the system to which the ADS1225 is deployed.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Guru 11245 points

    Hi Bob,

    Thank you so much for your kind reply.

    I understood about the calibration.

    Could you tell me about the timing of the calibration?
    To what extent does the offset change when the power remains on for a long time after calibration?

    My customer is considering the timing of the calibration.

    Sincerely.
    Kengo.

  • 0 in reply to Kengo.Y
    TI__Guru** 113765 points

    Hi Kengo,

    I'm not totally clear on what you are asking.  What do you mean by the timing of the calibration?  Are you asking how long the calibration takes to complete once initiated?  Or are you asking about how often you need to calibrate?

    The offset and gain will change with respect to operating temperature of the ADS1225.  This would be temperature drift as specified in the ADS1225 datasheet in the electrical characteristics table under System Performance.  The same would be true with respect to the gain error due to reference drift and would be an additional error.  To reduce the error you need to issue the self-calibration routine.

    As I mentioned in my earlier post, the need to issue additional calibration(s) following power-up will depend on the temperature stability of the system.  That is something I cannot answer.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Guru 11245 points

    Hi Bob,

    Thank you so much for your comment.
    I wanted to ask you about how often I need to calibrate.

    I understood I need calibration to reduce the error.

    For example, Is the reference temperature 40℃ when calibrated at 40℃ in the ambient environment?
    And, Is the error reset when the ADC power is turned on and off?

    Sincerely.
    Kengo.

  • 0 in reply to Kengo.Y
    TI__Guru** 113765 points

    Hi Kengo,

    I understand your question, but it is very difficult to answer how often you need to calibrate as this is going to be system dependent based on the operating conditions.  If we describe this in human terms it would be like asking me if you should wear a coat?  How can I accurately answer this question?  Are you inside or outside?  Is it hot or cold?  Is it windy or calm? And the list of possible questions regarding your situation goes on and on.  The same is true with the system and the need for calibration will be dependent on the system and the operating environment.

    The calibration values obtained in the self-calibration are stored in volatile memory.  This means if power to the ADC is turned off, the calibration values will be lost.  So after initial power-up when the supplies and reference are stable, you should issue the self-calibration.

    The gain calibration is dependent on the reference voltage source.  If there is significant drift of the reference over operating temperature, this would be a reason for issuing another self-calibration.  But again, I cannot tell you how often you should issue the calibration.

    Best regards,

    Bob B