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  • TI Thinks Resolved

OPA847: Noise figure for op amps

Expert 1080 points

Part Number: OPA847

Hello 

I want to use OPA847 as a gain block for a receiver between 50 to 500KHz.

I am using Keysight ADS with the workspace found on TI website, and simulating the noise figure using the spice model with ADS S parameter solver. How accurate are the results? Does anyone has a spreadsheet for calculation of noise figure from op amp noise parameters? 

Thanks 

  • Hi,

    I am moving your thread to our TI model support group. I am not sure how our devices will perform with the s-parameter analysis in ADS. We do model the complex and and output impedances, but I am not sure about how accurate the reverse isolation measurements will be. One of our modelling team engineers can explain in more detail.

    Regards,

    Jacob Freet 
    High Speed Amplifiers

  • Expert 1080 points

    In reply to Jacob%20Freet:

    Hello


    Any updates from the model support group?
  • Izik,

    There is an application note that gives equations for op amp noise figure.
    The title is Noise Analysis for High Speed Op Amps, and you can download it by searching on ti.com for the literature number: SBOA066.

    The app note gives an in-depth treatment of spot noise performance, and gives the detailed noise figure equations in section 7 - Computing Noise Figure.

    Please review he app note and if it doesn't answer your questions let me know by replying to this thread.

    Regards,
    John

  • Expert 1080 points

    In reply to John Miller - WEBENCH Design Center:

    Hello

    Please see attached spreadsheet, where I tried to follow example in section 8 in the application note. The numbers don't add up, please advise.

    In addition could you please explain if my ADS noise figure simulations are accurate. 

    Thanks

    op amp noise figure.xlsx

  • In reply to Izik:

    Izik,
    What is meant by "the numbers don't add up"?
    John
  • Expert 1080 points

    In reply to John Miller - WEBENCH Design Center:

    See the attached spreadsheet in my previous message, the when you try to calculate the equation 21 in the example you don't get the same result as in the app note
  • In reply to Izik:

    Izik,

    Please see he attached spreadsheet. The revs are shown in red font.
    It wasn't clear to me if you used an input termination resistor or not, so each case was added to the spreadsheet.
    The spreadsheet results are a little different than those in the app note, but they seem consistent with one-another.

    Please note the OPA847 has a minimum stable gain of 12 V/V, which is greater than your chosen gain.
    This may matter when you compare the calculated and simulated noise results.

    Regards,
    Johnop amp noise figure_revd.xlsx

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