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ADS1248: Calculating tolerance and accuracy over time and temperature

Leonid.Y.Ayzenshtat
Prodigy 40 points

Part Number: ADS1248
Other Parts Discussed in Thread: ADS1247, ADS124S08

Hi,

I am trying to figure out if I can meet my design requirement using the ADS1248, as this part was chosen by previous engineer.

I need to know how to calculate the tolerance and accuracy of the part. Which terms can be root sum squared?

Also I need to know which of the error terms can be calibrated out. 

After calibration how much drift can occur?

Thanks;

Leo

  • 0
    TI__Guru** 100225 points
    Leonid,


    First, pull up one of the example TI Designs for the ADS1247 (the ADS1248 would be similar). It's titled 3-Wire RTD Measurement System Reference Design, -200°C to 850°C. You can find it at the following link:

    www.ti.com/lit/ug/slau520a/slau520a.pdf

    It has a write up of some of the error terms in making measurements with this device. Note that this design is written up without the error terms from the drift. With drift there will be additional errors in both the offset and gain error. Also, the drift of the reference will be seen as a gain error.

    In the example design, the errors are all root sum squared. I think you could do this with all the error components where you're using the typical value as the standard deviation of that given error.

    After calibration, the drift for offset and gain error are shown in the datasheet. Using Figures 9 to 12 and then Figures 17 to 20, you can see how the offset gain drift with temperature. As an example using Figure 17 with PGA=32, the drift is about -3.5ppm/C. If you expect to operate from -40 to 85C, then you might expect that the drift would be about 230ppm (0.023% of additional gain error) over that range. I would take these curves a typical and consider them to be the drift of a single standard deviation.

    If you're up to looking at other devices, we have the ADS124S08, which would be considered the next generation of the ADS1248. This device has a wider range of operation for temperature, from -50C to 125C, and upgraded specifications.

    www.ti.com/.../ADS124S08


    Joseph Wu