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Op-Amp with ref voltage

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by drkidd22, Nov 9, 2017.

  1. drkidd22

    drkidd22 Member

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    So I have this op-amp circuit I can't get my head around.
    It has a reference of 10V on the non-inverting input and an input voltage of 9.98V.
    Wouldn't my output then be (Vin-Vref) * R2/R1. Which would be an output of 6.06V? But I get 16.06V.
    Capture.PNG
     

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  2. Grossel

    Grossel Member

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    16.06 is correct. Just think about - you have a lower voltage at negative output - so therefore the output voltage HAS TO BE higher than Vref.
     
  3. schmitt trigger

    schmitt trigger Active Member

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    Perform the next two simulations;
    1 make both supplies exactly 10 volts

    2 make both supplies exactly 9.5 volts.

    In both instances measure Vout
     
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  4. dave

    Dave New Member

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  5. drkidd22

    drkidd22 Member

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    So my equation turns into [(Vin-Vref) * R2/R1] + Vref?
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Because Vin is an inverting input it's actually [-(Vin-Vref) * R2/R1] + Vref
     
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  7. drkidd22

    drkidd22 Member

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    So I've built 2 circuit similar to the original. The only difference is that one is built with an OP279 and the second circuit is built with a OPA2340.
    Now with the same imput voltage the output is significantly different. Could this be due to the input offset voltage?
    I get a wider swing with the Op279 than the OPA2340.
     
  8. schmitt trigger

    schmitt trigger Active Member

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    "Significantly different" meaning exactly what, 1 volt? 100 millivolt? 10 millivolt?
     
  9. drkidd22

    drkidd22 Member

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    More than half volt.
    See simulation, I see similar results on bench test. Capture.PNG
     
  10. schmitt trigger

    schmitt trigger Active Member

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    Have you compared, on their individual datasheets, what is the Voh for each device with a load of well over 10 mA?
     
  11. drkidd22

    drkidd22 Member

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    The OP279 has Voh of +4.8V and Vol of 75mV. I can't find the Voh or Vol on the datasheet of the OPA2340.
    The OP279 does have a supply range of 4.5V to 12V while the OPA2340 is 2.5V to 5.5V.
     
  12. drkidd22

    drkidd22 Member

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    Ok I fixed an error on the simulation. But still you can see a difference of about 250mV. Capture.PNG
     
  13. schmitt trigger

    schmitt trigger Active Member

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    Its there, but shown in not explicit way, on the chart: Figure 15. Output Voltage Swing vs Output Current

    Do something: significantly reduce the load current, at least by a factor of 10X, and measure/simulate again.
     

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