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ADS1232: Can not get Offset Calibration working

Brad Davison
Prodigy 60 points
Part Number: ADS1232

Hello, I have been trying to do offset Calibration with the ADS1232, I have searched these  forums about the 1232 and it looks like I am doing everything correctly.

I have a load cell connected to the 1232 with gain of 128 / internal Osc / 10SPS. I am reading data from the chip and sending it out the Micro-controllers UART so I can display the data on a terminal program and the data will vary as I load up the load-cell so am happy with the SPI data between the 1232.  I am sending 25 clock pulses to get data out and to place the DRDY/Dout high after the 25th pulse. 

I am getting the ADC value from my loadcell via the UART of 156952 ( fluctuates by 80) (Full scale count is 8388604)

Then after every 50 samples (test code) I send 26 clock pulses to put ADS1232 into offset Calibration, which I can tell is working as the Dout line does not go low again for around 800mS

which is written in the data sheets as how long the Calibration should take.

But when I look at the data again via the UART, the ADC count has not changed (Still around 156952) I am expecting the value to drop down to or close to zero which it is not doing.

I rewired the PDwn line to the Micro to do a powerup Sequence I saw in these forums which made on difference. 

Help me please.

  • 0
    TI__Guru** 113765 points

    Hi Brad,

    Welcome to the E2E forum! The self-offset calibration only removes the offset of the ADS1232 itself and will not adjust for any system offset.  Generally with load cells you would issue the self-offset calibration on device power up.  You would then do at least a 2 point calibration of the load cell as it will have some inherent offset.  This is accomplished with a no load condition (capture the number of counts) which will now be your scale offset.  Then you would place a known weight (or use a calibrator) to place the load cell near a full-scale condition.  You would use this calibration to determine a gain slope and essentially determine your outcome by using the simple formula of y = mx +b, where m is the slope just determined and b is the initial offset.

    There are formulas shown in the ADS1232REF user's guide that more clearly show the calculation in Section 2.1.1 on page 8.

    http://www.ti.com/lit/ug/sbau120b/sbau120b.pdf

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 60 points
    Thanks Bob, I misunderstood what the offset Cal did in the datasheets so thanks for clearing that up.