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ADS1248 RTD 3WIRE

Mahmoud Mohammed
Intellectual 315 points
Other Parts Discussed in Thread: ADS1248

Dear all,

should  i use the ADS1248 in common mode input range only?

can i use RTD 3 wire without Rcomp that used to mid the range of CMI?

thanks

  • 0
    TI__Guru 55466 points

    Hello Mahmoud,

    Yes, you would want to ensure that the ADS1248 is used in the common-mode input range in order to ensure the PGA is in the linear region to obtain accurate measurements.

    You may configure the device to perform a 3-wire RTD measurement with RCOMP or without RCOMP.  Below is a link for application note SBAA180; which provides examples for measuring RTD's in two-wire RTD application, three-wire RTD application, and three-wire with hardware compensation (RCOMP present).   The RCOMP resistor is chosen to be the same value as the RTD at the middle of the temperature range in order to make the voltage at the input of the ADC swing equally in both the positve and negative direction.  The RBIAS resistor between REFP and REFN is used to set the common-mode voltage of the RTD and also produces the voltage reference in a ratiometric fashion.   This is also explained in detail on the application note.

    Please let me know if you have additional questions,

    Thank you and Kind Regards,

    Luis

    Please find the link for the application note below.

    http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=sbaa180

  • 0 in reply to Luis Chioye
    Intellectual 315 points

    Hi Luis,

    1-how can i get the output code from ADS1248 {what is the conversion formula?}?

    2-what is the optimal way to set the external Vref according to operation range?

    i have used pt100 3 wire to measure temp from 0 to 104C and set Rcomp to 120ohm which equal to PT100 Mid range of measurement, PGA to 128, IDAC to 1ma/channel and on board external REF to 3.00V BUT ADS1248 gives me unstable readings is there are any wrong with the above calculations?

    Many thanks

  • 0 in reply to Mahmoud Mohammed
    TI__Guru 55466 points

    Hi Mahmoud,

    The output code for the ADS1248 is given on Table 21, page 35. The output code is a function of the VREF voltage and PGA gain.  The ADS1248 conversion full -scale is +/-VREF/PGA; where PGA is the gain setting of the internal programmable gain amplifier.

    So for example you decided to use a reference of 2.048V and a PGA gain of 2, the convertion is a function of the differential voltage between the inputs:

    (AINP - AINN) >= (+VREF/PGA) =1.024V would correspond to 7FFFFFh

    (AINP - AINN) = (+VREF/PGA/(2^23-1)=  0.122uV would correspond to 000001h

    (AINP-AINN) = 0 would correspond to 000000h

    (AINP - AINN) = (-VREF/PGA/(2^23-1)=  -0.122uV would correspond to FFFFFFh

    (AINP-AINN) <= (-VREF/PGA)*{(2^23)/(2^23-1)}= ~ -1.024V = 800000h

    So in this case, with VREF=2.048 and  PGA=2, the ideal LSB size is 0.122uV.

    On your second question; the VREF can be set depending on the maximum/minimum scale voltage that you want to measure. The application note SBAA180 page 6 provides a very similar example of what you want to accomplish with a PT100 RTD.  The RCOMP is set to 110 Ohms, the reference voltage is set with a RBIAS resistor of 833.33 Ohms with a 3mA total IDAC current  (1.5mA+1.5mA); providing a ~2.5V reference.  Notice the maximum scale voltage input to the PGA is 2.5/128= 19.53mV.  So depending on your max/min input voltage swing required, which depends on the impedance variation of the RTD over temperature and the IDAC current setting; you may optimize the required Reference voltage. Notice that implementing the Reference with RBIAS creates a ratiometric measurement.

    If you like, you may post your schematic showing the RTD set up for review to see if we find anything that could be improved.

    Thank you and Regards,

    Luis

     

     

  • 0 in reply to Luis Chioye
    Intellectual 315 points

    Hi Luis,

    i want to ask about the effect of

    -Offset Calibration Register:OFC[2:0]

    -Full-Scale Calibration Register:FSC[2:0]

    on the output code from ADS1248 and what is the default values for it?

    can i load it with values that doesn't affect my output code format from ADS1248?

    what is the maximum differential input to the modulator?

    how can i calculat the VCMI and it's limits?

    can i use output data"in TWO's compliment format" directly without any manipulation or not?

    many thanks,

    Mahmoud