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ADS1148: Calibration

SimonF
Expert 3350 points
Part Number: ADS1148

I have another question about the ADS1148:

The ADC offers different calibration options.

Regarding the offset there are the system- and the self-offset calibration.

Regarding the gain there is only the system gain calibration.

 

Do I understand correctly that we have to create an external input short circuit or full scale for the system calibration itself?

If we do not want to provide any further external electronics, can we only calibrate the offset via the self-offset calibration?

But would it be possible to force the input to fullscale via the MUXCAL setting 010?

 

Offset and Gain calibration coefficient registers are only available once and not for each channel?

So we would have to perform a calibration for each channel, store the data in the µC and then write the channel related calibration data into the OFC and FSC registers after each switch of the multiplexer?

Best regards

Simon

  • 0
    TI__Guru 55062 points

    Hi Simon,

    Before reading my answer below, I recommend you read this post of Chris'. This is very comprehensive and well written: https://e2e.ti.com/support/data-converters/f/73/p/707084/2605680?tisearch=e2e-sitesearch&keymatch=self%2520vs%2520system%2520calibration#2605680

    The ADS1148 supports system gain calibration, system offset calibration, and self offset calibration.

    The system offset calibration would require the differential inputs of the system be shorted together.

    The self offset would require the differential inputs of the ADS1148 be shorted together. 

    The system gain calibration requires a very accurate voltage source. Ideally it would be a full-scale magnitude, but it is possible to scale the resulting calibration based on what magnitude of voltage source is used. i.e. if you perform a gain calibration with a voltage that is exactly half the full-scale range, the device will store a calibration value that results in scaling the input by a factor of 2. Then multiply the gain calibration result by 0.5 and re-write the result to the calibration register. 

    1. yes. short circuit for system offset and full-scale for system gain.

    2. it depends on what electronics are currently in place. If the system input cannot be shorted in a differential configuration, yes. 

    3. Correct. 010: Gain calibration. The analog inputs are connected to the voltage reference.

    4. Correct, there is only one set of offset and gain calibration coefficient registers.

    5. The approach you outlined is an option, but it really depends on just how accurate they would like the measurement to be. For the utmost accuracy, a calibration can be performed after power up, temperature change, change of gain, or even change of channel. Keep in mind that this is a 16bit device so calibration does not have as much as impact as calibration with 24 or 32 bit devices. Performing the self offset calibration should be sufficient.