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ADS1220 offset calibration

A K
Intellectual 905 points
Other Parts Discussed in Thread: ADS1220

I have a T-type thermocouple connected to AIN0 and AIN1 and a PT100 connected to AIN2 and AIN3 for CJC. 

Ref: TIDA00168 Software example code from: https://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/492875

The ADS1220.c file function has two gain correction factors, one for RTD and other for TC that are used to get calibrated data. My understanding is that the TC offset is obtained by setting MUX[3..0] to 1110 : AINP and AINN shorted to (AVDD + AVSS) / 2.

What are the MUX settings to obtain the RTD correction factor?

Thanks!

  • 0
    TI__Guru** 113765 points

    Hi AdtK,

    I did not write the code so I can't speak to this directly.  Maybe someone familiar with the code will add some specifics.

    In general the process of calibration can be a two point calibration where you measure the known temperature at two different temperature points and determine the code to temperature slope based on the code values of the two points.  It is best to remove offset first and then calculate the gain slope.  The trick is making sure that the known temperature values used are far enough away from each other so that noise does not make a significant impact to skew the slope.

    As the offset and gain can drift over temperature, the effects can be minimized by removing the measured offset (which is what you mentioned by changing the MUX to the internal short) by making periodic measurements to remove any change in offset from that initially done at calibration.

    So the internal short will help reduce offset error by subtracting the result from further measurements.

    Best regards,

    Bob B

  • 0
    TI__Intellectual 1010 points

    Hi AdtK

    in the example code, we realize the offset calibration by chopping the inputs. Basically we take a TC measurement having AINP mux'ed to AIN0 and AINN mux'ed to AIN1. Then we take another TC measurement having AINP mux'ed to AIN1 and AINN mux'ed to AIN0. With this we do for every measurement an offset calibration.

    In case you want to do offset cal with your approach by muxing AINP / AINN to (AVDD+AVSS)/2, this would be valid for all inputs (AIN0, AIN1. AIN2, AIN3) because the connection of AINP and AINN to (AVDD+AVSS)/2 is after the MUX.

    You can see this in FIG38 of the ADS1220 datasheet.

    Regards

    Alex

  • 0 in reply to AlexW
    Intellectual 905 points
    Bob B, Thank you for the details!

    Alex,
    Thanks for explaining the offset calibration by chopping. If I implement the 2nd approach of shorting the MUX then the value obtained should be the same for RTD and TC as well then why the two different variables?