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ADS1248 with Potentiometer

ka88
Intellectual 275 points

Other Parts Discussed in Thread: ADS1248, ADS1220

Hello,

What would be the best way to connect a simple analog resistive potentiometer to be read by the ADS1248?

Nominal Resistance: 5k ohms

3 - pins: V+, Wiper,V-

AVDD: 3.3V

Assuming excitation would be provided by the IDAC current sources.

  • 0
    TI__Mastermind 25105 points

    Hi Kevin,

    a possible implementation would look very similar to a 2-wire or 3-wire RTD measurement.
    I attached an according proposal using one of our newest 24-bit ADC's, the ADS1220. This device might be easier to use than the ADS1248 and should suffice your needs if you only need to measure the potentiometer. But you could certainly implement the same circuit with ADS1248 as well.

    You might want to use an IDAC setting of 100uA, Gain = 2 and an RREF which is slightly larger than 10k. But you can play around with those values to get the best performance out of your system.

    Regards,

    ADS1220 - Poti Example.pdf
  • 0 in reply to Joachim Wuerker
    Intellectual 275 points

    Joachim,

    Thank you for the reply.  I have a couple questions about the your implementation:

    1) You show the POT as floating on one end, is that purposeful?

    2) Is it necessary to use 3 of the AIN inputs?  On the ADS1248 datasheet (page 57) it shows IDAC excitation on the same pins being used for the differential measurement.  However, in your diagram the IDAC comes from its own dedicated pin.

    3) Just curious why the ADS1248 datasheet does not recommend an input CM/DIFF filter at the reference inputs, like that shown for the ADS1220?  

    Thank you!

  • 0 in reply to ka88
    TI__Mastermind 25105 points

    Hi Kevin,


    you are welcome.

    1) To be honest I didn't know where to connect the 3rd terminal of the POT to. I think you should be able to short it to the tap if you don't want one end of the POT floating.

    2) Your system will work in general also when routing the IDAC through the pin that you use as the input for the measurement (IDAC connected to AIN1 in the example shown). However as we recommend customers to implement some kind of RC filter on the inputs, the IDAC current would have to flow through the filter resistor RF1 and thus create a voltage drop which would show up as an additional offset error.
    In case you would use a very low drift resistor for RF1 you could potentially calibrate this error out.
    Another concern is that the voltage drop across RREF, POT and RF1 might exceed the compliance voltage of the IDAC.

    3) We recommend the same filter implementations for ADS1248 as well. We usually just don't show it for the sake of simplicity. But we will be more detailed in all our new datasheets.

    You might actually find the following app note useful to configure the filters correctly:
    http://www.ti.com/lit/pdf/sbaa201

    Regards,