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problem with ads1248 calibration

Stéphanie Huon
Prodigy 10 points
Other Parts Discussed in Thread: ADS1248

Hello,

I'm developing a medical product which has to measure  the temperature with accuracy. We use a PT100 sensor.

The pt100 is connected between AIN0 (positive input channel and AIN1(negative input channel). We use only these two input channels.

Without calibration I have 0.1074V between two terminals of my sensor and I should have 0.1083V.

With the calibration I have a variable voltage of 6V to 8V, with FSC0=255,FSC1=255,FSC2=254 and OFC0=0,OFC1=0,OFC2=2.

To calibrate my ADS1248, I done following steps:

 

For offset calibration:

-I put 0V on AIN0 and I send the command "SYSOCAL" to my spi.

-to check my calibration is OK, I wait DRDY=0 and i send a character with RS232.

 

For Full-scale calibration:

-I put 5V on AIN0 and I send the command "SYSOCAL" to my spi.

-to check my calibration is OK, I wait DRDY=0 and I-I put 0V on AIN0 and I send the command "SYSOCAL" to my spi.

-to check my calibration is OK, I wait DRDY=0 and i send a character with RS232. send a character with RS232.

 

I don't understand why I don't get what I want.

 

I need your help...thank you for your answer.

 

Stéphanie.

  • 0
    TI__Guru* 97755 points

    In general, the ADS1248 has the FSR values programmed into the part so that the gain error is very small (typically 0.005% gain error). In most cases, I don't think the calibration will get that much more accurate of a result.

    To start, you are trying to read a 0.1083V PT100 RTD, and you are getting 0.1074V.

    What is the code that you read out from the ADC? and what is the value of the reference ?(I need you to measure the reference value directly). All of the measurements should be as accurate as possible with whatever meter you have access to (5 digits is good, 6 digits would be better).

    Voltage readback = ((ADC code)/(2^23))*(voltage reference value)

    If this comes back correctly, then we can go back to the calibration and check through that.

    Joseph Wu