AD 8375 output impedance

i found in datasheet that output impedance is 16K,but when i am trying  to simulate it in ADI SIM RF ver1.6,it shows input & output impedance is 150 Ohm only.which one is correct.

  • 0
    •  Analog Employees 
    on Dec 11, 2012 7:30 PM

    This is a structural issue with ADIsimRF.  Or to put it another way, the issue is how to fit the AD8375 into the ADIsimRF environment which expects each component to have real and relativelylow input and output impedances. The AD8375's output is a current source (hence the high output impedance). So its output voltage is formed by the output current and the load resistance. So with the output current driving a particular load resistance, we achieve all of the specs that are detailed in the datasheet (e.g. gain, IP3, Noise Figure, etc), even though the output stage does not look like the classically matched system that we expect in an RF signal chain. To fit the AD8375 and AD8376 into the ADIsimRF environment which recognizes and corrects for interstage mismatches,  we have to set the output impedance 150 ohms. If we set it to 16K ADIsimRF would apply a massive inter-stage mismatch RF Power loss.

  • Thanks enash for your reply, but if we want to fit AD8375 in our RF system which has load impedance 50 Ohm,so how matching has been done or it will not give  massive inter-stage mismatch RF Power loss assuming output impedance 150ohm.

  • 0
    •  Analog Employees 
    on Dec 14, 2012 11:31 PM

    Hello,

    The AD8375 has been optimized for an output load of 150 ohms.  As Eamon had mentioned the datasheet specs were obtained for this particular load impedance.  You can terminate the AD8375 with 50 ohms, however you will see degraded performance especially in IMD performance.  There are a few hardware options available to avoid this degradation in performance.   One option is to terminate the amplifier with a 3:1 impedance transformer.  This acheives the necessary impedance tranformation between the 150 output load of the amplifier and the 50 ohm source impedance of your system.  The other benefit to this option is that there is no power loss.  The second option is to place 12.5 ohms on each leg of the amplifier output so the series combination with the 50 ohms of your system will present a 150 ohm load to the output of the amp.  This will result in a gain of 10 log (150/50) = 4.77 dB. 

    Qui