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XTR111 clarification of datasheet

BrandonAzbell
Guru 64560 points
Other Parts Discussed in Thread: XTR111

I have the following questions related to the XTR111 datasheet.  The intent of clarifying these questions is to document the expected accuracy of a system, which uses the XTR111 as a part of that system.  I have found other E2E threads that discuss linearity errors, in particular this one.  Your assistance is appreciated in addressing the questions below.

 

I need some help defining electrical characteristics in the XTR111 datasheet. The datasheet lists nonlinearity error (defined as Iout/Iset) as 0.02% of span, and then something called ‘span error’ (also defined as Iout/Iset, although with a different footnote).

 

First question : I would like to know what the definition of span error and nonlinearity errors are. It is not clear to me. My best guess is that span error is similar to a gain error, but I am not sure. Defining both nonlinearity and gain error for a digital device like a DAC makes sense. But I am not sure I understand it for an analog device like the XTR111. It seems like a gain error would be enough to specify the accuracy. So in essence, I do not think that I understand these terms as used for theXTR111 and would like clarification.

 

Second question : are the offset, nonlinearity, and span error in the XTR111 independent of each other? If they are independent, I could take the square root of the sum of squares to calculate the total error. If not, then I have to add them.

 

Third question : the 2011 datasheet, on page 13, states that the !EF flag is pulled up to 5V. I do not understand where the internal 5V comes from, I see nothing else in the datasheet that generates 5V. Is it pulled up to some other voltage like Vsp or Vregf, or is it really 5V?

  • 0
    TI__Guru 64145 points

    Hello Brandon,

    The .XLS spreadsheet that you found in the E2E post you linked can be used to calculate the expected performance of the XTR111.  I've attached it at the bottom of this message for reference.

    Span error is synonymous with gain error and defines the deviation in the measured transfer function slope from the ideal transfer function slope.

     

    Non-linearity error for the XTR111 is the same as the INL spec for the DAC and defines the deviation at any point of the transfer function from the measured average transfer function slope. If the offset and gain errors are removed through calibration the non-linearity error will be the only remaining error source.  You are correct and most linear devices like op-amps do not define a non-linearity error.  The topology used inside the XTR111 to provide the high dc accuracy results in the non-linearity errors reported in the datasheet.

    Yes, the error specs are independent and can be RSS'ed together as you've suggested and as shown in the attached .XLS file.

    The /EF flag has an open-drain output driver and should be connected to the supply of the digital circuitry it's connected to through an appropriate pull-up resistor. The /EF flag has an internal pull-up to +5V and can be left unconnected if it is not used. 

     

    6746.XTR111_Error_Calcs_Final_4-20mA.xlsx

  • 0 in reply to Collin Wells
    TI__Guru 64560 points

    Collin, thanks for the reply.  I do have an additional clarifying question before I send the responses to the customer.  Thank you.

    Collin Wells said:

    The /EF flag has an open-drain output driver and should be connected to the supply of the digital circuitry it's connected to through an appropriate pull-up resistor. The /EF flag has an internal pull-up to +5V and can be left unconnected if it is not used. 

    If the /EF flag output does have an internal, albeit weak, pull-up to +5V, this leads me to a few questions.

    1) Where is the +5V generated?  Is it used for anything else in the device?

    2) If the output is externally connected to the supply of the digital circuity, isn't there a possibility of a continuous current path if the external supply is not +5V?

    3) Just to be clear, the supply you are referencing is pin 1 VSP, correct?

  • 0 in reply to BrandonAzbell
    TI__Guru 64145 points

    Hi Brandon,

    1.)  The +5V is generated internally and used for the digital error flag and output disable circuitry internal to the XTR111.  It is independent of the programmable VREG output. 

    2.) Yes, this is mentioned in the datasheet on page 16 in the "Output Error Flag and Disable Input" and the external voltage that is used for the pull-up voltage will need to supply the 2uA to the internal pull-up. 

    3.)  The /EF pull-up could be connected to the +24V VSP supply voltage without damaging the XTR111.  The pull-up would need to be sized such that the sinking current is less than 2mA as suggested in the electrical table on page 4.  However, if the /EF flag was connected to the interrupt input of a microcontroller then it would need to be pulled-up to the supply voltage of the microcontroller (+2.7, +3.3, +5V, etc.) or it would cause an over-voltage condition on the microcontroller digital input pin.