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LMH6401: Differential to Single ended output DC-3GHz

Shawn Hineline
Intellectual 360 points
Part Number: LMH6401
Other Parts Discussed in Thread: THS3215, THS4304, THS4141

To Whom It May Concern,

I would like to convert the differential output of the LMH6401 to a single ended 50 ohm output. Just using a single output from the part results in a high noise figure (since common mode noise is not rejected) and lower IP2. Using a balun doesn't appear to work due to the DC requirement. I haven't found a single ended output op-amp with OIP3's nearly as good as the LMH6401 at 3 GHz. Do you have any recommendations for the differential to single ended conversion?

Thanks,

Shawn

  • 0
    Guru 140200 points

    Hi Shawn,

    what about a center-tapped balun? This can help to overcome DC issues.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 360 points

    Hi Kai,

    I don't believe that solves the issue. I have to send baseband data through the system and the output must be single ended. The baseband data could be at a very low data rate. It also has to pass a 3GHz RF signal for other applications.

    Thanks,

    Shawn

  • 0 in reply to Shawn Hineline
    TI__Mastermind 22250 points

    Hello Shawn,

       How about a broadband RF/microwave balun such as the Marki baluns, model number BAL-001?

    Thanks,
    Sima

  • 0
    Intellectual 360 points

    Hi Sima,

    I need to pass DC. The low frequency end of that part is 200KHz.. I did look into using a MiniRF RFXF9504 balun which will allow you to pass DC, you can acheive flat gain but not a constant impedance or constant impedance without flat gain. Achieving both doesn't appear to be possible. The THS3215 performs differential to single ended conversion but doesn't have enough bandwidth.

    Thanks,

    Shawn

  • 0 in reply to Shawn Hineline
    TI__Mastermind 22250 points

    Hello Shawn,

       Yes that is correct, the lower end that we can find is 200kHz while also maintaining the signal and differential distortion. For another option which will be the THS4304 but this device is lower bandwidth part at 3GHz and lower distortion performance compared to the LMH6401.

       Or you could just use the single-ended output of the LMH6401, but will have to compensate for the gain loss as well as this will increase even order harmonics of the signal. As an example of the effects of avoiding using differential output of the amplifier: "Lab testing of the THS4141 at 1 MHz shows that the second harmonic at the output is reduced by approximately 6 dB when measured differentially as compared to measuring either output single-ended. The third harmonic is unchanged between a differential and single-ended measurement"

      What device will the LMH6401 be driving/interfacing with? 

    Thanks,

    Sima 

  • 0
    Intellectual 360 points

    Hi Sima,

    One application with have the LMH6401 driving the baseband ports of a vector signal generator. The other issue with taking the output single ended is that it degrades the noise performance since the common mode noise is no longer rejected. Do you know if there is a significant noise figure difference between the LMH6401 and THS4304 when the output is taken single ended?

    Thanks,

    Shawn

  • +1
    Intellectual 360 points

    Hi Sima,

    Please disregard my last question. The THS4304 is not suitable for my application.

    Thanks,

    Shawn