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Original question:

THS4302: High speed DAC buffer amplifier selection

LMH5401: Using a FDA as a difference amplifier

Jonathan Rabe
Prodigy 10 points
Part Number: LMH5401
Other Parts Discussed in Thread: THS4302,

Hi,

we are looking for an amplifier to use as as a differential to single-ended stage in a DAC signal processing chain. The amplifier must be high bandwidth (2 GHz minimum), DC coupled, and compensated such that it is stable at gains less than 5 V/V. 

Initially we were considering the THS4302, but we decided this would be unsuitable in a difference amplifier configuration due to the problem of matching the internal feedback resistors. We also considered the OPA819, but this falls short of our 2 GHz bandwidth requirement. We are struggling to find unity gain compensated amplifiers that meet our requirements.

We came across the LMH5401 fully differential amplifier, which meets all our requirements by a decent margin and would be ideal for us if it were not an FDA, which brings me to my main question: Can we use an FDA such as the LMH5401 in a difference amplifier configuration? I've personally never seen this done before, which makes me think there must be some caveat that I've missed. The diagram below illustrates the configuration I'm considering (the actual resistor values and rails are yet to be optimized, I'm more concerned about the circuit topology at this point);

I've configured it as a difference amplifier by omitting the feedback on the non-inverting input and feeding the negative output to the next single ended stage, while ignoring the positive output (but still terminating it). From my simulations this configuration seems to perform reasonably well, with good CMRR. The performance doesn't seem to degrade much from the conventional differential amplifier configuration.

Any advice or wisdom on this would be much appreciated.

Thanks,

Jon

  • 0
    TI__Expert 3320 points

    Hi Jon,

        Yes you can use one output leg of LMH5401 for this application. May I know why the feedback resistors you have chosen above are assymetrical?

    Regards,

    Hruday

  • 0 in reply to Hruday Kumar Mudireddy
    Prodigy 10 points

    Hi Hruday,

    thanks for getting back to me on this. I asked this question with a limited understanding of how FDAs actually work. I initially thought I could set up the FDA in the same way as a conventional (single ended) difference amplifier, but after doing some research (this app note helped me) it is clear to my why both feedback paths must be balanced.

    So my question is, assuming that both feedback arms are balanced, will the LMH5401 work in this configuration as a differential to single-ended amplifier with good performance?

    If the feedback is balanced, the positive output should (ideally) be proportional to the difference of the two inputs, so the single ended signal measured at each output should have a good degree of common mode rejection. Is that correct? If so, is that true over the range of supported common mode input bias voltages specified in the datasheet? Or would you expect it to degrade as a function of common mode input bias voltage?

    Thanks for your help,

    Jon.