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ADS124S08: Need help to Calibrate Offset and Gain error using system calibration method

Nilav Choksi66
Intellectual 500 points
Part Number: ADS124S08


Hi TI team,

We have developed custom 12 channel SG and RTD support board using two ADS124S08 ADCs. We are in the testing stage and receiving nearby values with some tolerance with the dummy load when connect at the input differential channel of the ADCs.

Now we want to calibrate the ADCs using system offset and Gain calibration method. First,  we tried for the user system offset calibration method but can not receive correct values by connecting 0V(difference between INp-INn)

The strain gauge circuit is made up using whistone bridge circuit as per attached image and directly connected with ADC differential pair and AVDD is 3.3V.

So, Can you please guide for the offset and Gain calibration?

Note: We already read the datasheet and follow the steps as per section 9.3.14 and configuring OFCAL and FSCAL registers.

Waiting for your response here.

Thank you,

Nilav Choksi.

  • 0
    TI__Guru** 113765 points

    Hi Nilav,

    To use a system offset calibration you must apply the input short within the input range of the ADC.  The input range will greatly depend on if the PGA is disabled or enabled, and if gain is also applied.  Also, the calibration should be completed with the data rate and configuration (PGA settings) you intend to use.  You will see a very large difference in the offset code from a gain of 1 as compared to a gain of 128.  If you compare to how the internal self-offset calibration is done, the internal short is applied at (AVDD-AVSS)/2  which is mid-analog supply.  This eliminates any issues with the input range.

    Also consider that as you have the potential of many sensors connected, the system offset for one sensor may be different from another.  Usually in this type of situation using the system calibration is not beneficial.  Instead I would suggest that you use the SFOCAL self-offset calibration for removing internal offset of the ADC and also use the factory calibrated gain settings for the PGA.  Where the system calibration is useful is if you have an external mux and buffer circuit where you wish to remove the offset/gain error of the buffer.  In any case, if you plan to issue the system calibration both the shorted input for offset calibration and the full-scale input for gain calibration must be within the ADC input range.

    I would assume that both the strain gauge and RTD measurements require gain and PGA enabled.  This would require that the inputs cannot be ground referenced as this will be outside of the input range.  For example, the RTD circuit shows the AINN input connected to ground as you have a 0 Ohm resistor.  This measurement will be outside of the input range if the PGA is enabled.  You would need to bias the input above ground.  Depending on the output of the strain gauge you may need to do something similar with that circuit as well.

    I also noticed a few things with the schematic that I wish to mention. There is a required cap between AVDD and AVSS of 330nF and I see you are only showing 100nF.  Also, it is recommended not to include any inductance on the supply inputs including ferrites.  The added inductance can choke required very short current demands on startup and when powering various internal circuitry.  This is especially true on the analog supply when turning on the internal reference, IDACs, etc..

    I recommend correcting the circuit to make sure the input does not violate the input range, then recheck your measurements again with just doing the self-offset calibration and see how close you are to your measurement.  For the RTD measurements the reference resistor should be a precision low-drift resistor.

    You may find the following useful:

    A Basic Guide to RTD Measurements

    ADS124S08 Design Calculator

    Best regards,

    Bob B