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Highly Linear ADC Driver Amplifier - need suggestions

Discussion in 'Electronic Design' started by Darol Klawetter, May 14, 2013.

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  1. I'm currently researching the amplifier options for the output stage of a RF receiver I'm designing. This stage will have a single-ended output and will drive an ADC that has a SFDR of 100 dB. Of course, I need a highly linear amplifier to prevent washing out my ADC performance, but I don't want to use one with differential I/O, which is typically used to drive high-performance ADCs. I would prefer something simple, like a gain block that would reduce my parts count. The frequency range of my output is 2 MHz to 50 MHz, which is too low for most RF gain blocks' optimal frequency range. My goal is to drive 2.5 vpp, while preventing distortion that kills the SFDR of my ADC. A gain of 15 dB to 30 dB is acceptable. Any suggestions?


    Darol Klawetter
  2. Joerg

    Joerg Guest


    Then you can set the gain via a control voltage. These things are very
    good, I used them in ultrasound systems where signal integrity is very
    critical. Unfortunately they have become a bit expensive for my taste so
    I haven't used one in new designs in a while.
  3. Thanks for the suggestions. My ADC card (which is separate from my RF receiver card) already has a balun that is used to drive the diff input of the ADC from a single-ended SMA input. I want to drive this single ended input without having to use a diff output driver into another balun. I'll do it ifnecessary to get the performance I need, but I'm trying keep my parts count down on receiver's output stage.

    Joerg, the AD603 has 60 dBc harmonic distortion.
  4. Tauno Voipio

    Tauno Voipio Guest

    It seems to me that you're looking for a high dynamic-range HF
    front end amplifier. Whole volumes have been written about
    the subject. You should start with the ARRL Handbook and
    follow the leads (if you have not done so).

    I strongly suspect that you need to go to a discrete
    component design. The power levels needed to handle most
    of the HF range will be much for integrated designs.
  5. No, I'm just looking for an acceptable single-ended driver for my IF output stage.
  6. Hmmm...maybe so, but I haven't found one with distortion products that are lower than my ADC noise floor. Can I live with that? Probably, but I would prefer to use an amp that doesn't degrade my ADC performance.

    Darol Klawetter
  7. josephkk

    josephkk Guest

    Read the thread. Listen up, the high performance ADCs use differential
    for good reasons. Quit goofing around and go fully differential and dump
    the baluns. 100 dB is one heck of a range (about 26 bits), so whatever
    the converter cost figure on at least 20% of that for the input amplifier,
    maybe even over 300% as much. If it is differential input then give it
    what it wants and is designed for. Then use really good layout or you will
    waste all that capability.

  8. Robert Macy

    Robert Macy Guest

    26 bits?, why not more like 18 bits?
  9. josephkk

    josephkk Guest

    100 dB SFDR. Linear at well better than 1 part in 10^5 in voltage,
    including noise contribution. Then again 1 part in 10^6 is only 20 bits,
    but spur free dynamic range at some given sample rate needs some bits of
    excess to average out the inaccuracies. Maybe only 20 bits, but at well
    over 100 MS/s what ADCs do you know of that are really linear for that
    many bits. It usually takes 3 more bits minimum to control the
    quantitization noise to a worthwhile level.

  10. Robert Macy

    Robert Macy Guest

    thank you for the explanation, didn't pay attention to that "SFDR"
    ignored it entirely. at least 20+3 is less than 26

    It's just that I'm used to routinely using my 24 bit system out to 22
    bits and almost all of that damage comes form the supporting
    electronics, didn't know at MS/s would need so much 'extra' overhead.
  11. As I began researching my amp options, I did decide to go with a differential amplifier. I was hoping I could meet my goal using some single ended options. Anyway, I'll be using a diff amp that was designed to drive my ADC, which is a LTC2217 from Linear Tech. I'll be using the LTC6401-26 diff amp.

    About the 100 dB SFDR: For ADCs, this measurement is taken by capturing data at the max ADC clock rate and doing a frequency spectrum calculation (typically with a FFT). You will see that 100 dB SFDR can be attained with a 16-bit ADC, even if the ENOB is less than 16-bits.

    Darol Klawetter
  12. Nice HF performance. They do spec the DC performance but it's pretty
    ugly- 50uV/°C typical.

    Best regards,
    Spehro Pefhany
  13. josephkk

    josephkk Guest

    And that is the other tradeoff. Your ENOB is likely no more than 12 maybe
    13 bits with a 16 bit converter setup at max speed. Still sounds like a
    pretty cool design.

  14. josephkk

    josephkk Guest

    Actually i overreacted a bit. The situation is not really all that dire,
    you can trade off ENOB as well, and this is running at 100s of MHz. It
    starts getting difficult and fighting the components and their intended
    usage usually gets you much more trouble than it seems to save.

  15. Guest

    Suggestion, read the spec's on those parts you listed.

    If you're trying to get 100dB out of them @ 50MHz,
    with your state Vo, the spec's say otherwise.

    OIP of the amp and SFDR of the A/D won't cut it at 50MHz.

    But you are correct re the theoretical calculation of SFDR.

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