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ADS1248_RTD

Shweta
Prodigy 115 points
Other Parts Discussed in Thread: ADS1248, ADCPRO

 

         We have interfaced ADS1248 to RTD interfacing. But I have stuck up in calculation of ADC value theoretically. Can anybody please tell me the equation to calculate the ADC value theoretically?  Because according to data sheet the equation is

                

      Vin   =     ADC value x LSB

                  ------------------

                        PGA

   But it is not verified with the obtained results with ADCPro set up.

Observation table for different data rate and input voltages versus different PGA settings will be available on following link.

http://rapidshare.com/files/273800823/ADC_value_calculation_for_Different_PGA.doc.html

 
With regards,
  Shweta
 

       

  • 0
    TI__Guru* 98785 points

    Shweta,

    The reason that you're getting some bad results is because the input is violating the input common-mode range of the ADC. In this case, AINN is below the input common-mode range as shown on page 3 (with the differential input at 0, the common-mode input should be at least .1V.

    I've recreated your circuit with some generic resistors. However, your 10ohm resistor from B to ground was changed to 1kohm resistor to raise the input voltage. I'm using AIN2 and AIN3 as my two inputs. The resistors are 1% and I have not tried to measure them directly, but this should solve your problem. With this circuit, I was able to get the following readings at 20SPS:

    G = 1,  Avg.Code = 203096
    G = 2,  Avg.Code = 406179
    G = 4,  Avg.Code = 812379
    G = 8,  Avg.Code = 1624784
    G = 16,  Avg.Code = 3249694
    G = 32,  Avg.Code = 6499838
    G = 64,  Avg.Code = 8388607
    G = 128,  Avg.Code = 8388607


    For gains of 64 and 128, the 50mV signal is larger that the input range of the part, so the part rails at (2^23-1).


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 115 points

     

    Thnks for your reply. It really works.  Your suggestion helped me to solve my problem. Thanks

     

    Regards,

     Shweta