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XTR300: Output expression For +-10V Output, XTR300

Ignacio Diz Castro
Prodigy 225 points
Part Number: XTR300
Other Parts Discussed in Thread: XTR111

Dear Sirs

Our Incoming XTR300 Based Design Will Output +-10V Or +-24mA (Depending on M2 State)

Design Its Based On tiduah1 www.ti.com/.../tiduah1.pdf

For Current Output, Simulation Works Fine, and I understand the formula

But For Voltage Output, the red marked formula seems to be wrong

Ignoring the red marked formula and usng the blue marked formula, simulation is not precise at all, also when solving For VDAC=2.5, blue equation fails  (VDAC-VREF=0)

What is the correct output expression ? I mean: Vout=........

I´ve tried to look at datasheet but datasheet equation expects ROS and RSET, i have only 1 of them

NOTE: For IOUT, i placed 250R to observe the output current

  • 0
    Prodigy 225 points

    Probably Vout Expression is:

    Vout= ( (VDAC-2.5) / 1020 ) x (RG/2)
    Vout= ( (VDAC-2.5) / 1020 ) x (4125)

    PD: Similar to the Currrent Expression

    Iout=  ( (VDAC-2.5) / 1020 ) x 10

  • 0 in reply to Ignacio Diz Castro
    TI__Mastermind 23050 points

    Hi Ignacio,

    You are right, the red equation (or equation 5 in the TI design) is not correct and R65 was left out. The correct expression for Vout would be (Rgain/2) *(Vdac - Vref)/Rset. Unfortunately there are some problems with the XTR300 model when operating in voltage out mode. It does not always produce accurate answers but if you follow the datasheet guidelines then your design will function as intended. I apologize for the inconvenience, I am hoping to get this model updated in the near future to resolve these issues.

    Solving the blue equation presumes you are selecting R65 to set the desired full scale output voltage. It wouldn't make sense to set this point at Vdac = Vref because this implies a net 0V input, and the output will be 0V regardless of the value of R65.

  • 0 in reply to Zak Kaye
    Prodigy 225 points

    Dear Sir

    You equationVout=  (Rgain/2) *(Vdac - Vref)/Rset its the same as i purposed

    Gives for vdac 4.96 :  (4125*2.46) / 1020 : 9.94, Fine!

    Please tell me what can i do with the schematic for achieving precise results for bipolar voltaje output?

    What is the problem, when bipolar out only?

    I´ve selected XTR300 for having an all in one solution and i dont want to design a new stage only for Voltage out...

    Kind regards

  • 0 in reply to Ignacio Diz Castro
    TI__Mastermind 23050 points

    Hi Ignacio,

    I'm not quite sure what you are asking. The simulation is not going to reflect the true accuracy of the XTR300 in voltage output mode. The true output should be closer to that predicted by the equations than what is shown in sim. The dominant error sources in this design are going to be your gain setting resistors (which you can adjust by using higher precision resistors), the gain error of the internal op-amps, and the offset voltages of the internal op-amps. If you want an estimate of the error you will see in voltage and current output modes, then you can use the attached spreadsheet to approximate this.

    TIPD155_TUE Calc.xlsx

  • 0 in reply to Zak Kaye
    Prodigy 225 points

    Ok so i can perform real tests ( low scale, zero, and full scale) -10, 0, +10

    Measure results

    And apply a FW correction constant?

    Kind Regards

  • 0 in reply to Ignacio Diz Castro
    TI__Mastermind 23050 points
    Hi Ignacio,

    Exactly, you can identify your error sources and calibrate your DAC to minimize these as much as possible.
  • 0 in reply to Zak Kaye
    Prodigy 225 points
    XTR111 can be also adjustable (between current and voltage output)

    Does the XTR111 include such unexpected VO problems

    Probably I can redesign for final product with xtr111 (Also much more cheaper)

    Kind Regards
  • 0 in reply to Ignacio Diz Castro
    TI__Mastermind 23050 points
    Ignacio,

    The XTR111 does not provide an integrated voltage out function, but you can produce a voltage output as shown in the datasheet. When doing so, I would recommend including a buffer amplifier between the SET pin and the output to improve performance and prevent the SET pin from possibly being damaged. You could also run an amplifier in gain if that is more desirable. The precision will largely depend on the amplifier you select.
  • 0 in reply to Zak Kaye
    Prodigy 225 points

    Its necessary to redesign the boards for XTR111 ?

    Kan I keep Using the XTR300?

    What´s your suggestion

    :(

  • 0 in reply to Ignacio Diz Castro
    TI__Mastermind 23050 points
    Ignacio,

    Yes, the XTR111 and XTR300 are completely different devices and require very different design considerations. If you need both voltage and current outputs I would recommend sticking with the XTR300.
  • 0 in reply to Zak Kaye
    TI__Genius 11135 points
    Hi Ignacio

    We haven't heard back from you so we assume this resolved your issue. If not, post below, or start another thread if this one has timed-out.

    Thanks
    Dennis