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ADS1248 wrong measurment values.

Linards Laugalis
Prodigy 20 points
Other Parts Discussed in Thread: ADS1248

Hello, I'm using ADS1248 to measure PT100B (RTD) temperature sesors (-50 ... 550°C), but I'm getting wrong ADC values. I'm using 3 wire measurment, without hardware compensation resistor. I'm using resistance box to check various ADC measurments and compare to multimeter reading. (Blue line in graph). I tested measurment with PGA=4 and PGA=1, results were the same.

ADS1248_adc_test.xlsx

 

My values:

Resolution 24bit
AVSS 0
AVDD 3.3VDC
Sampling Rate 5sps
PGA GAIN 4
RREF 1k
IDAC 750uA

  • 0
    TI__Guru* 97375 points
    Linards,


    Normally, when there's a problem with some sort of gain error, I generally have someone measure both the input and the reference to make sure ADC is putting out reasonable values. This assumes that the full-scale calibration register hasn't been changed or calibrated to a wrong value.

    However, for your case I think the problem is a bit different. What I suspect is that have the reference turned on AND you have the ADC using the internal reference as the ADC reference. In that condition, your reference is really 2.048. instead of the 1.5V that you are expecting. That is why your measurements are coming in at about 75% of the expected value.

    What you want to do is to turn on the internal reference but not have it used as the ADC reference. This is selected in the MUX1 configuration register. You want to make sure that REFSELT is set to 01 (or 00 depending on how you have it connected). You don't want it to be set to 10 or 11 where the internal reference is selected to be the ADC reference.

    Check the setting in the register and let me know if this is the problem. If it isn't, we can look over your configuration and circuit and see if there's something else to look at.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Thank You very much, this solved the problem.

  • 0 in reply to Linards Laugalis
    TI__Guru* 97375 points
    Linards,


    Thanks for posting back. I was pretty sure this was the problem. Your result was very close to what I expected for that specific setting.

    If you have any other questions. Feel free to post back.


    Joseph Wu