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Limited IF output interface for Logarithmic Amplifier AD8309 - cont.

Dear Suppoters,

    As per your recommendation, We have attached our finalized application schematic of AD8309 Log amplifier Log , Limited IF output circuit for 30MHz input to single ended 50Ohm Limited If output requirement.

Kindly verify and give your comments to proceed us further to Layout.


Whether we need to AC-couple the Limited IF output signal to Transformer input?



  • Hi Wayne,

    It look pretty good.  My only comment is on the limiter output, why do you have the 499 Ohm load resistors there?  You can take the open collector limiter outputs directly (DC coupled) to the balun.  This will reduce your BOM bit.

    Hope this helps,


  • Hi Joel,

         What is the output signal level in dBm at the Single ended transformer output for the attached schematic configuration?

    What will be the change in the limited IF output signal based on the 499Ohms resistor, The 499Ohms resistor was placed based on the Limited IF output calculation below formula,

    VLIM= VS– 400 mV × ( RLOAD/RLIM)

    What could be the required resistor values for RLOAD and RLIM for this single ended configurations?



    Sugumar K

  • Hi Sugumar K,

    The RLIM controls the the total current in the limiter output; you have 402 Ohms so total current is 1mA.  Between the 1:1 TC1-1T balun and the 499 Ohm resistors, the equivalent differential impedance seen by the limiter outputs is about 47 Ohms differential, so RLOAD should be half of that, 23.5 Ohms, So VLIM would be just under 5V in your configuration: 4.97V.  You will need more current (lower RLIM) and/or a bigger RLOAD to get more of a swing.

    I do not think dBm is a real useful metric when talking about the limiter output.

    I think you should decide what Vlim level you want and then work from there to get the RLIM and RLOAD values. 

    What are you using the limiter output for?