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OPA855: High Speed Amplifier for 750MHz Differential to Single Ended conversion

Netanel Desta
Prodigy 40 points
Part Number: OPA855
Other Parts Discussed in Thread: THS4302, THS4304, LMH3401

Hi, My name is Netanel, i'm making a project as a student in Israel, Haifa.

I'm looking for an OpAmp which is suitable for differential to single ended conversion. the conversion is intended to be made on differential data signal which came out of a high speed voltage out DAC.

the signal frequency is 750MHz. the Amplifier OPA855 may suitable but the data sheet lacks information about linearity and harmonic distortion. Do you have such information?

In addition, my Bandwidth requirement is demanding- the signal is a data signal so the amplifier's frequency response need to be as flat as possible until 1GHz. I found another amplifiers which may comply like: THS4302, THS4304.. Are there any other amplifiers?

thanks for the help

  • 0
    Guru 140200 points
    Hi Netanel,

    the OPA855 is decompensated and needs a minimum gain of factor 7.

    Can you tell more about your signal? Is it only a sine? Or does it contain harmonics? And what gain do you need?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 40 points

    Hi Kai,

    the PCB is a RF transmitter and the signal is an IF baseband data signal which comes out as a differential signal from a DAC. the required BW is 1.5GHz so I need to process a 750MHz signal. the signal is not a pure sine but contains more harmonics. the differential signal is 0.5V peak to peak ( and therefore the output voltage is 1Vpp ). 

    I only need a unity Gain. to be more accurate, unity Gain in the inverting node and G= +2 in the non- inverting node. Is there any suitable Op Amp? thanks

  • 0 in reply to Netanel Desta
    TI__Genius 17136 points
    Hi Netanel and welcome to E2E,

    We don't have any additional information to release for the OPA855 at this time.
    The THS4304 is our best amplifier for diff to single conversion at 750MHz. What DAC and what type of antenna are you using?

    Best regards,

    Sean
  • 0 in reply to Sean Cashin
    Prodigy 40 points

    Hey sean,

    the DAC is: voltage out, 2 GSamples/sec. there antenna is irelevant- the output of the amplifier will be conectted to IQ Modulator.

    it seems that THS4304 has high distortions. Is there any other amplifier with low distortion and high enough BW for 0.5Vpp input and 1Vpp output?

    thanks, Netanel

  • 0 in reply to Netanel Desta
    TI__Genius 17136 points
    Another possibility is to use a fully differential amplifier and use a balun at the output. In that case you could use the LMH3401.
  • 0 in reply to Sean Cashin
    Prodigy 40 points

    Thanks for the advise.

    I want also to examine other ways. I've noticed the THS4302 Fixed Gain OpAmp. It seems to have good linearity and nice frequency response performance. It has a 14dB fixed gain. If I will contact a serial resistor to Rg    ( the internal resistor which is connected to the Inverting Node ) to reduce the Gain to G= +2 ( so I will contact a 150 ohm resistor ), would it effect the amplifier performance in terms of BW and linearity? In what way? Will it reduce the BW or Increase it? Will it reduce\increase the amplifiers linearity?

    Plus, is there a way that I can use this amplifier as an differential to single ended converter?

    I want to clarify that my transmitter BW is 1.5 GHz which means that my baseband signal's frequency varies from DC to 750MHz. so the amplifier should be able to work appropriately with 750 MHz signal. 

    Thanks, Netanel

  • 0 in reply to Netanel Desta
    TI__Genius 17136 points
    Hi Netanel,

    You can't modify the gain in this way, it will cause the amplifier to lose phase margin and oscillate.

    You can use a single ended amplifier for a 2:1 conversion or an FDA for a 1:2 conversion.

    Can you help me understand why your transmitter BW is different from your baseband signal's frequency?

    Best regards,

    Sean