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XTR111-2EVM: 1uA accuracy with modifications in the circuit

Krishna Kumar1
Prodigy 250 points
Part Number: XTR111-2EVM
Other Parts Discussed in Thread: XTR111, TIPD102

Hi,

I am designing a circuit for my application which is as described below

I/p: 0 to 10V

I/P step voltage: 10mV

O/P current: 0 to 1024uA

Compliance voltage: 12V

If I changed the Rset to 97.65k I would get max Iout of 1024uA

Approximately for 10mV input, I would get 1uA output.

So will this circuit work for this accuracy from 0 to 1024uA? basically Will the transistor and FET in the output side respond to such low currents?

Thanks 

Krishna

If not then what is the modification I need to do for 1uA output accuracy. Also is there a simulation circuit for this?

  • 0
    TI__Genius 10850 points

    Hi Krishna,

    The XTR111 is optimized and designed for 0-20mA applications; and its errors are only specified in the datasheet for the 0-25mA range. An estimate of the total accuracy can be calculated as follows for the 0-25mA full-scale range.

    Using the square root of the sum of squares and the values in the electrical characteristics table of the datasheet: Total Estimated Error= sqrt((offset_error)^2 + (gain_error)^2 + (linearity_error)^2)

    So a rough or approximate total accuracy error is approximately ±3.82uA accuracy typical, about ±26uA maximum for the standard 0-25mA range. Using a smaller current range of 0-1mA will get them closer to the desired 1uA accuracy; however, please note the % linearity, % offset, % gain errors  are only guaranteed for the 0-25mA full-scale, and may be different for the 0-1mA current range.  

    You mentioned another important issue and that is the external FET transistors such as the BSP170P which can add errors, for example the zero gate voltage drain current can be as high as 1uA at room temp, and up to 100uA at 125°C; and this FET's errors can change with device and temperature.  These errors will add up and not allow you to guarantee the 1uA accuracy.  

    -Tamara Alani

  • 0 in reply to Tamara M Alani
    Prodigy 250 points

    Hi Tamara,

    Thank you for the reply. In the XTR111 datasheet the internal block diagram plus the external P channel FET resembles the circuit in the document TIPD102- High side V to I converter.

    I have done the testing of the circuit in TIPD102 and posted the results in another question in the same forum. 

    I am afraid that this XTR111 also would give me similar results.

    Please let me know if my circuit comparison is correct and also result wise would be the same or not?

    - Krishna

  • 0 in reply to Krishna Kumar1
    TI__Genius 10850 points

    Hi Krishna,

    XTR11 integrates the circuit shown in the TIPD102 design, but you will need to do an accuracy analysis of your circuit with the XTR111 + FET.

    -Tamara

  • 0 in reply to Tamara M Alani
    TI__Genius 10850 points

    Hello Krishna,

    We hope your issue is resolved. I am going to close this thread now. Feel free to reply to re-open if you have any questions moving forward.

    -Tamara