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ADS124S08: Offset Calibration at PGA Gain change

Toshiya Asai
Prodigy 10 points

Part Number: ADS124S08

Dear Support Team,

I would like to perform the following measurements using the ADS124S08:
Reset -> SFOCAL -> Execute a measurement loop while switching input channels.
Within the measurement loop:
(1) Perform one single-shot read for each channel and immediately switch to the next channel.
(2) Switch the PGA gain when performing the single-shot read. However, I would like to avoid performing SFOCAL here as it would slow things down.
Therefore, as a solution,
(1) At initialization, or at certain time intervals, execute SFOCAL for each gain to be used all at once and record the OFCAL0/1/2 register values.
(2) In the measurement loop, when switching gains, restore the OFCAL0/1/2 values corresponding to that gain and perform a single-shot read.
Is it possible to perform measurements without spending time on SFOCAL during the measurement loop using this process?
Please let me know if you have any prolblems.

Thanks,

Toshiya

  • 0
    TI__Mastermind 49565 points

    Hi Toshiya,

    Welcome to TI E2E forum.

    SFOCAL is a command for self offset calibration, so the offset error is almost the same under the same gain setting on the different channels of the ADC because all input channels use the same PGA and modulator, so you can use the saved OFCAL0/1/2 values for the self offset calibration on other channels, however you have to write the saved OFCAL0/1/2 value into those calibration register before perform a single-shot read or you can use the saved OFCAL0/1/2 value to do an offset calibration in your firmware.

    The SYOCAL command is different because the system offset error might be different on individual channels in your circuit, the system offset error on an individual channel includes the error of the front-end circuit.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 10 points

    Hi Dale,

    Thanks for your reply,

    Just to be sure, let me confirm again.
    As indicated in the title, the Gain is also switched when switching channels.
    Since the offset shifts when switching Gain, I thought restoring OFCAL0/1/2 would solve the problem.
    (Example)
    Select Ch1 → Restore OFCAL for Gain_x4 → single-shot read
    ↓
    Select Ch2 → Restore OFCAL for Gain_x32 → single-shot read
    ↓
    Select Ch3 → Restore OFCAL for Gain_x16 → single-shot read
    ↓
    Are there any drawbacks to frequently switching the gain?
    Also, as you suggested, correcting it in the firmware using OFCAL is also a good method.
    Could you tell me the formula for that correction?

    Regards,

    Toshiya

  • +1 in reply to Toshiya Asai
    TI__Mastermind 49565 points

    Hi Toshiya,

    The calibration can be done by sending calibration commands to the ADC, or by direct user calibration. In the user calibration you are trying to do, you have to test the offset error and calculate the correction value per gain in advance, then you can write the correction values to the calibration registers according to the gain after you switch the channels.

    For offset calibration, please watch the following video of TI Precision Labs - ADC series: 

    Understanding and calibrating the offset and gain for ADC systems

    BR,

    Dale