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REF102: Op-Amp selection for constant current source

Dennis McGlumphy
Expert 1390 points
Part Number: REF102
Other Parts Discussed in Thread: REF200, OPA277, OPA602, OPA192, OPA189, OPA211, TINA-TI

Dear Sirs

I am trying to design a very accurate, very temperature stable current source.  I have not found a dedicated current reference that is suitable so I will use a voltage reference to build one.  In application bulletin sbva001, the author uses an OPA111 op-amp.  In the REF102 data sheet, the same circuit uses an OPA277 op-amp.  In addition, the REF200 data sheet recommends OPA128 or OPA602 op-amps be used to extend the current range of the REF200.  All of these op-amps are low bias current, FET front end op-amps.  In these applications, what is the more important specification of the op-amp?  Is it input bias current, input bias current offset, input voltage offset, low noise or something else?  Please advise me how to choose the best op-amp for this application.  Thank you for your time.

  • 0
    TI__Guru* 93105 points

    Hello Dennis,

    The point of buffering a precision reference is to affect the reference voltage as little as possible by the op and load attached to the reference output. Therefore, any op amp buffer should introduce minimal errors such as voltage offset, voltage offset drift, noise, etc. so all of these errors should be minimized. And the buffer should provide a low output impedance suitable for driving the load without having any effecting on the input reference

    Each of the op amps listed, the OPA111 (JFET input), OPA277 (bipolar input) and OPA128 (JFET input) are all legacy, high-performance op amps optimized such that the performance of certain parameters are optimized. You would have to study the datasheets to see where each one excels in minimizing voltage offset, input bias current, noise etc.

    Since the REF102 and REF200 are designed for higher voltage operation such as 15 V, it is best to team them up with what we refer to as high-voltage op amps. Even thought he legacy op amps mentioned are available, their performance is being superseded by more modern devices that provide very low voltage offset, voltage offset drift, voltage noise, current noise, etc. minimizing the added errors. Here are two examples of high performance op amps suitable for the buffer application. First, the OPA192:

    http://www.ti.com/lit/ds/symlink/opa192.pdf

    In particular see the datasheet Figure 69, Precision Reference Buffer.

    And another, the OPA189:

    http://www.ti.com/lit/ds/symlink/opa189.pdf

    See Figure 44, Noise Performance of the OPAx189 and OPA211 in Unity-Gain Buffer Configuration.

    I hope this helps answer you questions.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Expert 1390 points
    Dear Mr. Kuehl
    Thank you for the advise. Is there a way to test the effects of the op-amps' tolerances in the TINA-TI SPICE application?
  • 0 in reply to Dennis McGlumphy
    TI__Guru* 93105 points
    Hello Dennis,

    TI's Precision Amplifiers produced from now to about 10 years ago have highly accurate models that provide room temperature, typical parameter values as listed in the datasheet. Therefore, if you wish to simulate the errors that the op amp contributes, the TINA/PSpice models will allow you to include the voltage offset, voltage noise, current noise, etc. effects in the circuit. The REF102 and REF200 are legacy devices and if they have models they probably haven't been updated in some time, and may not be as sophisticated as the modern op amp models. The OPA192 and OPA189 models are very modern and accurate.

    We rely heavily on our TINA/PSpice models in our everyday work so you should be able to apply them with confidence.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Expert 1390 points
    Dear Mr. Kuehl
    Thank you again. This resolves my questions.