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ADS1248 accuray calculation

Chandrashekhar Rane
Prodigy 90 points
Other Parts Discussed in Thread: ADS1248

I am using ADS1248 for my application. I am going to interface 2 nos. of 4/3 wire RTD and 2 nos. of 4-20mA sensor to ADS1248 IC.

RTD temperature range is -100 deg C to +300 deg C. IDAC = 1.5mA, Rbias = 1.65K, Vref is 2.475V.

4-20mA I/P = Vref=2.048V

 We have requirement of 0.05% accuracy. So my question is how to calculate accuracy of ADS1248? And for how much resolution we will achieve it?

  • 0
    TI__Guru* 97585 points
    Chandrashekhar,


    I think the accuracy that you're looking for is achievable, but you'll need to be very careful about your sources of error and it may require calibration. There are several contributors to the error.

    The accuracy of Rbias can be one of the largest contributors. You'll need a very accurate resistor since this is used as your reference. The gain, offset and INL error from the ADC are also contributors of error to the measurement.

    Finally, the IDAC mismatch for a 3 wire measurement is also a large contributor, but this may be reduced by chopping or swapping the excitation currents to average out the result.

    Go to the following link:

    www.ti.com/.../slau520a.pdf

    The error analysis is shown in this TI design. You can read through it and see the process of how the theoretical accuracy is derived and how the design is put together.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 90 points

    Thanks for the reply Joseph,

    I want to calculate accuracy in %. So how can I calculate?

    Because they have not mentioned formula to calculate accuracy in % in attached document.

    So pl. guide me for this.

  • 0 in reply to Chandrashekhar Rane
    Prodigy 90 points

    Can u please send me application note of ADS1248 for 4-20mA i/p measurement?

    It include with accuracy, resolution calculation.

  • 0 in reply to Chandrashekhar Rane
    TI__Guru* 97585 points
    Chandrashekhar,


    I'm sorry, but I don't know of any ADS1248 application notes for 4-20mA current-to-pressure measurements.

    For the previous question, to get the accuracy in %, I would convert the measurement to Ohms. Make the measurement in Ohms and compare the error with the full scale. At the temperature (at 850C), this would be the full scale at 390.48 Ohms. From that, you can get the % error.


    Joseph Wu
  • 0 in reply to Chandrashekhar Rane
    TI__Guru* 97585 points
    Chandrashekar,


    There is one other thing that you should be aware of. For the ADS1248, the measurement does not extend all way to the supply rails. Even in a gain of 1, the lowest input measurement is AVSS+0.1V (while the highest input measurement is AVDD-0.1V. This is because of limitations of the PGA on the front end of the ADC. For more information, look up the input common-mode range of the device. I mention this because this is often a low side measurement where the ADC negative input is connected to GND for a single supply.


    Joseph Wu