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XTR300 Frequency Dependence

Hind Kraytem
Prodigy 100 points

Other Parts Discussed in Thread: XTR300, XTR300EVM

Hi,

I'd like to use the XTR300 in Current output mode as a V-I converter. Ideally I'd like it to output a constant 1mA independent on the load and the frequency of the input voltage by using Vin=10V and Rset=100k. 

I'm trying to characterise the component's behaviour by using the XTR300EVM. Initially I had done load tests tests and am pretty pleased with the results ie there is a clear linear dependency between input voltage and output current and the curves superimpose with 10% error for loads of 1k-10k, this at 10Hz.

However, I'm having difficulty understanding the frequency response. I tried plotting the output current versus the frequency ranging from 1-1000Hz. Although ideally I'm expecting a constant 1mA output, this is only true up to 10Hz. Between 10Hz and 100Hz there is a prominent slope and the values tend to stabilise between 100Hz and 1000Hz at values depending on the load.

eg. for load of 10k the output current drops from 1mA to 0.8mA, for 5.6k the output current increases to 1.45mA and for a load of 2k it increases up to 3.9mA.

Any help on understanding the reason then stabilising the output is much appreciated.

Many thanks,

Hind

  • 0
    TI__Guru 64795 points

    Hello Hind,

    I have to ask a few questions before we can diagnose this circuit behavior further. 

    1.)  Do you have a complete schematic you can share?  This way I can verify power-supply voltages as well as the other passive components around the device.

    2.)  What is your supply voltage?  The issues you're having with the changing load impedance sound like compliance issues and doing some quick tests yield the following:

    - 10,000 * 0.8mA = 8V
    - 5600 * 1.45mA = 8.12V
    - 2000 * 3.9 = 7.8V.

    Is is possible that you're limiting the output current due to compliance issues to the positive rail?  The OPA output can only swing to within (V+) - 3V.  So if you're supplying the V+ terminal with around 11V then this is expected behavior.  The output current (Iout) multiplied by the load resistance (Rload) can not exceed the maximum output voltage of the OPA.  So:  Iout * Rload < (V+) - 3V.

    Please share the schematic and we'll get this worked out.

    Regards,
    Collin Wells
    Precision Linear Applications


     

  • 0 in reply to Collin Wells
    Prodigy 100 points

    Hi Collin,

    I've had another look at the circuit and figured out where I went wrong; I just wanted to share with any readers experiencing the same problem:

    I had originally looked at the transfer function for the XTR300 in Current Output mode: Iout=10Vin/Rset

    To obtain the desired output current, I had chosen a value of Rset as 100k but left all other passive components as in the datasheet.

    However, this is an increase of Rset by 50fold and to keep the frequency response normal I decreased Cc by 50fold (maintain time constant t=RC).

    Best!

    Hind

  • 0 in reply to Hind Kraytem
    TI__Guru 64795 points

    Hello Hind,

    Thanks for sharing your findings!