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high speed differential input to single output amplifier

Other Parts Discussed in Thread: OPA2836, THS3202, VCA821, LMH5401, LMH3401

Any suggestions for >1GHz differential inputs to single ended output amplifier?

  • Hello Jeff,
    The VCA821 is our highest BW monolithic Differential to Single-ended amplifier. Unfortunately it only has around 400MHz of BW. For 1 GHz BW, I think you will need to construct your own difference amplifier or diff-to-single ended amplfier using multiple opamps and resistor components. For the opamp itself you can use the THS3202. As far as the design of the circuit goes, you please refer to Figure 57 of the OPA2836 datasheet. Alternately, if you want high input impedance the instrumentation amplifier in Figure 59 is a good example.
    www.ti.com/.../opa2836.pdf-Samir
  • Hello Jeff,

    Could you use an FDA either the LMH3401 or LMH5401 followed by an RF balun? Is the system requirement to be DC-coupled?

    Currently, we don't offer > 1GHz BW differential to single-ended amplifier but using an RF balun at the output of an FDA could be a solution if DC performance is not required. You would have to compensate then for the insertion loss of the balun in the PA which would be close to 2.5dB, which might be acceptable. Also, it might be good to know what system performance like distortion numbers you are targeting for the differential to single-ended amplifier? 

    Best Regards,

    Rohit

  • It can be ac coupled. The bandwidth is from 10K to 1G Hz.. I also read it is okay to ground one end of differential amplifier, except losing some benefits of being differential. Is it correct?   Jeff

  • Hello Jeff,
    It is not a good idea to ground the unused output, it can be connected to ground through a resistor, preferably around 200 Ohms.
    The LMH5401 would work for this application.
    See my blog post for more information:
    e2e.ti.com/.../differential-to-single-ended-what-happens-when-you-use-only-one-differential-amplifier-output
    Regards,Loren
  • if the output amplifier + is connected to 50 ohm should the - side connected to 50 ohm as well? Does it have balance problem if the -side is connected to 200 ohm?
  • Hello Jeff,
    I would recommend to use a balanced load on both the output amplifier + and - pins even though you would use only one output. I believe Loren was mentioning to use a 200ohm on each of the output pins to ground, as 200ohm on the used pin would mean a lighter load at the output.
    Best Regards,Rohit
  • Hello Jeff,

     

    I have done tests with both outputs balanced (same equivalent load) and with the unused output completely floating (still has feedback resistor connected).  There was no difference in the performance.

    We do not recommend to leave an unused output completely floating, though, so using 200 or 400 Ohms to ground would be a good idea.  There is no need to match them exactly. 

    This recommendation is for single ended outputs only.  When using the amplifier in differential configuration it is very important to balance the two outputs.

    Regards,

    Loren