IDACs in high precision ADCs?

Discussion in 'Analog & Mixed-Signal Design' started by Abbas_BrainAlive, Sep 12, 2018.

  1. Abbas_BrainAlive

    Thread Starter Member

    Feb 21, 2018
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    Hello all!

    I have beeen working with high-precision ADCs for some time now, and one thing that I notice in almost all high precision ADcs frrom prominent manufacturers is that most of them have one or more integrated IDACs within the same single package. I do understand that they are to facilitate ratiometric measurement of physical quantities, and that ratiometric measurement has a lot of advantages. What I am not able to understand is why current sources play a monopoly in ratiometric measurement?

    Aren't voltage sources as good for excitation sources in ratiometric measurement, and that the high precision ADCs may offer simple DACs to act as the excitation sources?


    With best regards,
    Abbas.
     
    Last edited: Sep 12, 2018
  2. MrChips

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    Oct 2, 2009
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    What is your specification of "high-precision ADC"?
     
  3. RichardO

    Senior Member

    May 4, 2013
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    Can you supply some example part numbers -- or better yet data sheets -- of the ADC's you are talking about?
     
  4. Abbas_BrainAlive

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    Feb 21, 2018
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    For example, the latest one I am working with is the ADS1120.
     
  5. nsaspook

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    The IDAC is used with a RTD (low resistance) or resistance changing element. With the current(s) set the voltage across the RTD in series with this current is a direct factor of temperature and a precision reference resistor (for the ADC reference voltage), not the current source. The ADC measures this RTD voltage and the controller scales this voltage to a temperature. Using voltage sources here would make the absolute accuracy and temperature drift of the excitation voltage a factor.

    From the ADS1120 doc:
    For three-wire RTD applications, the matched current sources can be used to cancel errors caused by sensor lead resistance (see the 3-Wire RTD Measurement section for more details).
     
  6. Abbas_BrainAlive

    Thread Starter Member

    Feb 21, 2018
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    Thanks for your response, nsaspook.


    That's the same with a voltage source as well!


    Nope, not possible! We are talking about ratiometric measurement, here. source disturbances are eliminated in a ratiometric measurement, irrespective of the nature of the source.
     
    Last edited: Sep 25, 2018 at 3:46 AM
  7. Abbas_BrainAlive

    Thread Starter Member

    Feb 21, 2018
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    Moreover, can a ratiometric measurement be accomplished with a Wheatstone bridge?

    A common implementation is as follows:
    [​IMG]

    Now, there's a great problem with this interpretation, irrespective of the nature of excitation source (current or voltage). As the variable resistance changes, the overall effective resistances changes, which changes the reference voltage as well. And since the reference voltage itself is not stable, but is fluctuating with the load resistor, the ratiometric measurement can never be correct.

    Am I correct?
     
    Last edited: Sep 25, 2018 at 4:08 AM
  8. nsaspook

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    Look at the three-wire RTD circuit again. It's ratiometric with 'matched' sources. On-chip matched current sources are much easier to fabricate and it's usually better in these types of applications to generate a voltage reference from a current source.
    http://www.ti.com/lit/an/sboa046/sboa046.pdf
     
    Last edited: Sep 25, 2018 at 10:46 AM
  9. Abbas_BrainAlive

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    Feb 21, 2018
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    Thanks for your response, again, nsaspook.

    But those matched sources help in eliminating the errors due to RTD leads.

    Also, how can current sources be easy to fabricate, when a current source basically uses a curent stabilizing circuitry in conjunction with a voltage source?
     
  10. nsaspook

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