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LMH6401: Need to amplify and add DC offset to my output

Travis Berger
Prodigy 20 points
Part Number: LMH6401
Other Parts Discussed in Thread: THS3491, OPA695

Hi,

I am using a Tektronix AWG70001B arbitrary waveform generator. The output of the generator is 500 mV signal ended or 1 Vpp differential. I need to amplify this signal and add DC offset in order to drive a VCSEL. For the resulting signal I need a 1 GHz sine wave with 3 Vpp amplitude centered at 1.5V. I am looking for the best way to configure the LMH6401 with EVM to achieve this. Additionally if anyone has other suggestions that could be better I am open to hearing them. Thank you for the time and support. 

Best,
Travis

  • 0
    TI__Genius 15565 points

    Hello Travis,

    Is using a variable gain amplifier one of your requirements? What are the load conditions? A gain of 3 V/V isn't one of the gain settings but you can configure to a higher gain setting and add series resistors to each output accordingly so that you can attenuate to your desired gain. 

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Prodigy 20 points

    Hi Hasan,

    Thank you for the reply. No using a variable gain amp isn't one of my requirements. My main requirement is to deliver a signal with 0-3V amplitude which requires a gain of 3x and offset of 1.5V. The load of the VCSEL is dynamic with min = 20, typ = 30, and max = 55 Ohm. 

    Thank you,
    Travis

  • 0 in reply to Travis Berger
    TI__Genius 15565 points

    Hey Travis,

    After looking into this a little bit further, I don't think any of our FDAs can meet your requirements.

    The first issue is that all the FDAs that have the bandwidth requirements use a 5V supply and at most have 3V output range (LMH6401 meets this requirement). You may be able to get around this by increasing supply voltage slightly past 5V and use the max supply rating. However, with the need to set Vocm at 1.5V, the supplies would need to be unbalanced so that 1.5 is mid-supply.

    The second issue is the dynamic load. Even if you were able to get a larger output range on a part such as the LMH6401, the attenuation would vary when changing your load due to the internal 10-ohm resistors on this and similar parts. Outside of this, you're Vocm would be adjusted by the attenuation as well and would need to be compensated for.

    Best,

    Hasan Babiker

  • 0 in reply to Travis Berger
    Guru 48395 points

    Hey Travis, can you be AC coupled? >1GHz CFA with your 1.5V DC offset on the V+ input. Maybe the OPA695 and or LM6703, but the real problem is the slew rate, 3Vpp at 1GHz requires 9.4V/nsec. Only the THS3491 gets close to that, but not the 1GHz bandwidth 

  • 0
    Guru 48395 points

    You might ask the Tektronix folks what they might suggest.