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Regarding ADS1248 internal Reference

Mangesh Jadhav
Prodigy 100 points

Other Parts Discussed in Thread: ADS1248

Hi.

     This is Mangesh working as Design Engineer in RENU Electronics. I am working on Universal Analog Module Which consists

     of measurement of Voltage, mA, mV, RTD, TC. But I have some Problems while Reading RTD.

     To read RTD we should have IDAC cuurent sources on. Therefore Internal Reference is on to provide 1 mA current.

     But It is Observed that Internal Reference is not on and no current is provided.

     After that I connected that Internal Reference to REF0  and  then some voltage like 1.628v is observed(it should be 2.048v) and current

     is also provided to RTD through IDAC current sources.But both Reference and Current were fluctuating.

 

    So my question is that it is necessary  to connect Internal reference to REF0 to make it on. Can I have a Internal reference

    independently on to give the current so that I can use REF0 as Reference to ADC ?

   System voltages are:

                                    1) AVDD =  +2.5v

                                    2) AVSS =  -2.5v

                                    3) DVDD =  2.5v.

 

      Please reply as soon as possible.

Thanks and Regards,

 Mangesh

 Design Engineer.

 

 

 

  • 0
    TI__Guru** 113765 points

    Mangesh,

    To use the IDAC current you must turn on the internal reference, but you do not need to use it for the ADC reference.  In this way the internal reference is used to establish the accuracy of the current.

    If you still are having issues, please post a schematic so that we can help troubleshoot your system.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 100 points

    Thanks Bob.

         Does it mean that I should use Internal reference for  current source only and not for ADC reference ? Because I have taken some readings

    with Internal Reference as ADC reference which gives more fluctuations in ADC count.

    Please let me Know.

     

    Thanks and Regards

    Mangesh

  • 0 in reply to Mangesh Jadhav
    TI__Guru** 113765 points

    Mangesh,

    I realize that the references can be a little confusing.  With the ADS1248 there are three possible reference options for the ADC.  There are two external reference inputs and the internal reference.  The internal reference is fixed at 2.048V.  There may be a need where you want to use an external reference with a different voltage because you may want a different full-scale range.  Those three references are for the ADC, but for the IDACs to function there is only one possible reference and that is the internal reference.

    So in what instance would you want to use an external reference? You may want to use an external reference in a ratio-metric system when using RTDs.  In this case you run a constant current through the RTD and into a RBias resistor.  The constant current induces a voltage across the RBias resistor, which is then used as the reference input at one of the external reference inputs.  To use the constant current source you must turn on the internal reference to establish the current, but you don't need to use the internal reference for the reference to the ADC.  You could if you wanted to, but in this RTD case you would be better served to use the external reference  established by the RBias resistor.  RBias should be a precision resistor.  And you will need to use the value of voltage dropped across the resistor for calculation of the voltage measured by the ADC.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 100 points

    Thanks Bob.

                  I have done  some changes as per your suggestions and its working fine.

     

     

    Thanks and Regards,

    Mangesh Jadhav

    Design Engineer.