AD7124-8 : Internal and System calibration with external reference

Hi AdityanDinesh 

1. After performing internal gain and offset calibration, are the ADC conversion results available in the data register adjusted with the offset and gain calibration registers? Or do I need to apply any correction in my firmware?

   - You do not need to apply any correction on the firmware, it is already updated on the data register. 

2. Is it valid to perform internal calibration while external reference is selected in the configuration register ?, or Do we need to perform internal calibration with internal reference configured ?

   - Internal full scale calibration should work just fine on using whether internal reference or external reference.

3. For system zero-scale and full-scale calibration, what should be applied to the AIN2/AIN3 analog inputs in this configuration?

   - System calibrations expect the system zero-scale (offset) and system full-scale (gain) voltages to be applied to the ADC pins before initiating the calibration modes.

4. It is mentioned that full-scale input should be provided while performing system full-scale calibration. As we are using IOUT as external reference, do we need to consider this while feeding voltage for system calibration?

   - Yes, the reference voltage through RREF should be considered for system calibrations

Thanks and regards,

Rod

Hi RAdante ,

Thanks for the quick response.

I have a few additional questions regarding the system zero-scale and full-scale calibration steps.

From your explanation, system calibration requires applying the system’s actual zero-scale (offset) and full-scale (gain) voltages to the ADC inputs before triggering the calibration modes.

1. System Zero-Scale Calibration:
   Am I correct in assuming that for zero-scale calibration, I should short AIN2 and AIN3 to GND so that the ADC sees 0 V at the differential input?

2. System Full-Scale Calibration:
   For full-scale calibration, since the PT1000 is normally driven by the excitation current, I assume I need to disconnect the RTD and apply a known resistor value instead.
   However, because my reference is generated ratiometrically using the excitation current and Rref, how should I correctly apply the full-scale voltage?

   • The REFOUT from the eval board measures around 2.495 V, not exactly 2.5 V.

   • If I apply an external voltage source for the full-scale calibration, will this introduce errors or conflict with the ratiometric setup?

Hi AdityanDinesh 

Am I correct in assuming that for zero-scale calibration, I should short AIN2 and AIN3 to GND so that the ADC sees 0 V at the differential input?

• Yes, you are correct

However, because my reference is generated ratiometrically using the excitation current and Rref, how should I correctly apply the full-scale voltage?

• Ideally, if you want the most accurate system calibrations, actual system measurement values should be used. Applying external voltage on your system for calibration purposes may introduce additional errors on your overall system. 

Thanks and regards,

Rod

Hi AdityanDinesh 

Am I correct in assuming that for zero-scale calibration, I should short AIN2 and AIN3 to GND so that the ADC sees 0 V at the differential input?

• Yes, you are correct

However, because my reference is generated ratiometrically using the excitation current and Rref, how should I correctly apply the full-scale voltage?

• Ideally, if you want the most accurate system calibrations, actual system measurement values should be used. Applying external voltage on your system for calibration purposes may introduce additional errors on your overall system. 

Thanks and regards,

Rod