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ADS1231 Output Code interpretation

Manuel
Prodigy 130 points
Other Parts Discussed in Thread: ADS1231

Hello

I have wired a load cell to the ADS1231 following the figure 24 of the datasheet.
AVDD = Vrefp = 3.6 volts
GND = Vrefn = 0 volts
The load cell has a FS output of 2mV/V. So FS output of the load cell is 7.2mV at 3.6V supply.

I calculated the expected AD output at load cell FS using two different ways, both leading to the same result:

a) Table 3 of the datasheet leads me to the following calculation:
Maximum AD ouput: 0.5 * 3.6V / 128 = 14.0625mV = 7FFFFF = 8388607
AD output at Load Cell FS: 7.2mV = 4294966

b) The output is allways relative to VREF.
At VREF = 5 volts the output of 7FFFFF is equal to 19.53125mV.
At VREF = 3 volts the output of 7FFFFF is equal to 11.71875mV.
At VREF = 2 volts the output of 7FFFFF is equal to 7.81250mV.
At VREF = 1 volts the output of 7FFFFF is equal to 3.90625mV.
So an output of 7FFFFF is equal to 3.90625mV/V, regardless of VREF. (Of course AINP-AINN must not exceed the mV listed above.)
To get the AD output at FS of load cell I calculate now:
AD maximum output 7FFFFF divided by 3.90625mV/V multiplied * LoadCell FS outpt
Or: 8388607 / 3.90625mV/V * 2mV/V = 4294966

When I connect the load cell to the ADS1231 and put (more or less) full load on the load cell, then the output is about 2162453 (Hex 20FF15). Which is only (more or less) half of the value I've expected.

So my question is if I made a mistake in my calculations or if there must be an error somewhere else?

  • 0
    TI__Guru** 113765 points

    Hi Manuel,

    Welcome to the forum!  Just like many ways that you can break down an equation and recalculate by substitution, that is what you are basically doing in your second method.

    I would first double check and make sure that you have the load cell connected properly.  If it is truly connected properly, I would then measure AINP to AGND and AINN to AGND to make sure that the input is within the correct common-mode (which should be about 1/2 of AVDD).  Third I would double check to make sure that the load I placed on the load cell is accurate with respect to the full-scale output.  This kind of relates to point 2, where you measure the difference in the load cell output to see if it is about 7.2mV.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 130 points

    Hi Bob

    Thanks for your advice. I checked all those (analogue) sources of error. But everything was correct.

    It finally turned out that the guy who wrote the program for the microprocessor messed up some bits (or even only one single bit). That made that the output of the microprocessor (which are the values I receive on the rs232) was only half the value of what the output of the ADS was.

    So the problem is solved now.

    Best regards

    Manuel

  • 0 in reply to Manuel
    TI__Guru** 113765 points

    Hi Manuel,

    Thanks for the update.  I'm glad that you were able to find the problem.

    Best regards,

    Bob B