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distortion in current-mode opamps

Discussion in 'Electronic Design' started by John Larkin, Sep 28, 2007.

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  1. John Larkin

    John Larkin Guest

    Jim Williams' advice for achieving very low distortion in opamps is
    "always invert." The idea here is that even if an opamp may have
    screaming bandwidth and good open-loop linearity, it may well have
    rotten common-mode performance, especially at high frequencies. And if
    neither input pin has any substantial voltage on it, the various
    nonlinear parasitic capacitances won't be pumped.

    And the best opamps for fast apps are current-mode parts. They have
    the speeds and slew rates to get really low (below -60 dB) THDs at
    frequencies upwards of 30 MHz or so. But *every* datasheet we've seen
    on current-mode amps shows THD specs/curves only in non-inverting
    mode, almost always with a gain of +2.

    We wonder why.

    John
     
  2. Yep, I saw that from JW as well.

    Just been looking at some TI headphone amps, to drive 32 ohm stuff,
    and it's spec'd to 100MHz+.
    That is stupid, the wider you open the window, the more shit you let
    in.

    Good for marketing......


    Martin
     
  3. Tom Bruhns

    Tom Bruhns Guest

    Normally I find Jim Williams' advice to be pretty good, but this one I
    gotta disagree with. You need to evaluate each op amp carefully to
    determine what actually works best in a particular application. "One
    size most certainly does NOT fit all."

    Even at low frequencies, we've used some tricks to get rid of
    distortion from nonlinear capacitive effects, so we can use non-
    inverting (and thus high impedance) inputs. Some of the tricks we've
    used I've seen built into some especially good low distortion
    monolithic op amps.

    As a counter-example of a current-feedback amp that shows a graph, at
    least, of inverting configuration distortion, check out the OPA695.

    I can also give you examples of voltage-feedback amps with very low
    distortion at tens of MHz...

    Cheers,
    Tom
     
  4. Phil Hobbs

    Phil Hobbs Guest

    Williams' law is "Always invert, except when you can't." Besides
    distortion, this also saves startup problems caused by exceeding
    somebody's CM voltage range on power-up. (That one bit me in the days
    of my youth, when I wasn't used to op amps whose output swing was wider
    than their CM range.)

    Cheers,

    Phil Hobbs
     
  5. Fred Bloggs

    Fred Bloggs Guest


    The non-inverting gain of x2 would be apply to driving a controlled low
    impedance transmission line with series termination, as in video and
    baseband A/D applications. It would then make sense to publish a graph
    pertaining to the most common application. Don't they specify an output
    amplitude with that?
    And I'm not sure that the non-linear capacitance effect is avoided
    entirely by the inverting configuration. The most effective technique is
    to keep internal signal swings small.
     
  6. Although it is generally true statement, I'd rather say "it depends".
    The inverting configuration advantage is because of no CM swing in the input
    stage. It may or may not be important depending on the topology.
    The optimal feedback resistance in CFB opamps is given for every particular
    opamp; you can't choose it arbitrarily. Usually this resistance is from
    several hundred Ohm to several KOhm. This is a serious limitation for the
    CFB in the inverting configuration; the non-inverting mode is more flexible.

    Vladimir Vassilevsky
    DSP and Mixed Signal Consultant
    www.abvolt.com
     
  7. John Larkin

    John Larkin Guest

    Tricks? Tell if you can.
    Thanks, that's interesting. THD is actually a bit less in non-inv
    mode. Since you get an extra free gain-of-one in that mode, that may
    account for a bit of the difference.

    John
     
  8. I assume you're referring to cascode input stages, etc.
    Anyway, Tom, we're all ears, or should I say, all eyes.
     
  9. Tom Bruhns

    Tom Bruhns Guest

    :) Well, I can't give away all our secrets, but one that I think
    Analog Devices has actually mentioned in ap notes (Walt Jung...) is
    that in a lot of amplifiers, the main parasitic capacitance is to the
    negative-rail substrate. Thus, you can bootstrap the negative supply
    to follow along with the input terminals and significantly lower
    distortion.

    Some of the lowest distortion discrete audio circuits I know of were
    made by a friend from the distant past. He offered me a couple rules
    of thumb: don't let the voltage change across the junctions of the
    input device, and don't let the current through the input device
    change. Although you must, of course, let them change _some_, you can
    reduce the changes a whole lot: run the input not only cascode, but
    bootstrapped cascode, where the control element (base; gate) of the
    upper device is driven to follow the input voltage. Operate the
    cascode into a current source load, and buffer its output with a
    follower (or other circuit whose input current change is small
    compared with the standing current magnitude). Note that the OPA627
    input stage does these things...in addition, it uses a dielectrically
    isolated process. It's capable of really low distortion operating as
    a follower even when the source impedance is high. It has been a
    while since I played with the test circuits, but as I recall, I was
    able to get audio-range harmonics well below -100dBc with a 100k ohm
    source resistance; that is not easy to do with other monolithic op
    amps that I have tried.

    But that doesn't help necessarily with high frequency stuff. My
    earlier comment about the need to try amps in your application comes
    from personal experience about four years ago. We had been using a
    hybrid op amp that went obsolete. I spent quite a long time and tried
    LOTS of different amps that all looked good on paper, in several
    different topologies, to come up with a two-op-amp composite that
    would do as good as the old hybrid in that particular application
    (requiring very low distortion through about 40MHz). Interestingly,
    the output stage amplifier did not do nearly as well when operating at
    a gain of 2, as it did at unity gain, but at unity gain, it was
    devilish to make stable. And the input stage, operating at pretty
    high voltage gain, had very low distortion, but only when its output
    was very lightly loaded (thus the buffer output stage).

    I'm sure the op amp manufacturers are well aware of the fact that
    certain amps shine when the conditions are just right. The OPA847
    data sheet shows a couple circuits that have been carefully optimized,
    and boy are they good. And that's a voltage-feedback amp.

    Also, if you let the manufacturer include the feedback resistors in
    the package with the amplifier, you'll generally get better
    performance. Have a look at the LTC6400-20. It is particularly easy
    to bypass, in my experience. TI also has some high frequency amps
    with built-in resistors that set the gain. AD has packages that bring
    the feedback in through a second pin, close to the inputs.

    Beware how you place bypass caps! Realize that if the output stage is
    not class A, there is a lot of harmonic current in the power supply
    leads, and if the signal path shares even a very tiny resistance in
    common with the current through the bypass caps, the distortion may be
    much higher than a more optimum bypass topology.

    OK...enough for now. ;-)

    Cheers,
    Tom
     
  10. Winfield

    Winfield Guest

    [ snip various secrets ]
    More, more!
     
  11. John Larkin

    John Larkin Guest

    Nice part, but a little more slew rate would be nice.
    Yup; a lot of opamps have nearly 0 dB of PSRR as the frequency goes
    up. If you bead+cap filter their supplies, to avoid crosstalk from
    other gadgets, you open up another bag-o-worms.

    What we have is a differential-current-output DAC that has to be
    converted to a single-ended signal; a discrete LC lowpass filter; and
    an output amplifier. The target is 20 volts p-p out (10 p-p into 50
    ohms), 30 MHz bandwidth, low distortion (goal -60 dB, under
    unspecified circumstances), flat to 30 MHz (for certain values of
    "flat") and good DC accuracy. The interesting things we've noted...

    There's no obviously-best way to convert the DAC outputs to
    single-ended. The DAC datasheets usually just use transformers. The
    new gen of ADC-driver difference amps don't swing much if you use just
    one of their outputs.

    LC filter design has always been a PITA, and still is.

    There aren't many opamps that will swing this far at this bandwidth,
    much less into a hefty load. At higher output currents and big swings,
    all sorts of things go to hell, especially the GBW of the output
    transistors.

    The output stage may wind up being a closed-loop composite, which
    gives me bad dreams at these sorts of speeds.

    If you look at a variety of name-brand ARBS and RF generators, output
    swings like this are rare, whereas even at low signal levels THD
    numbers can be shocking. ARBS typically run -30 or -40 dB THD, and we
    have one late-model RF signal generator that runs about 1.5%
    distortion!

    Thanks for the tips.

    After this is wrapped up, we may try it at 100 MHz.


    John
     
  12. john jardine

    john jardine Guest

    [...]
    [...]

    The old Wavetek 166 function generator, runs 50MHz, 15Vpp, into 50ohm.
    They use 19 of those transistor thingies in a conventional two stage
    arrangement. Their #178 'waveform synth' does 50MHz at 10Vpp into 50ohms and
    benefits from an unusual output amp' design.
    I have their #145 and # 164, 30Vpp at 20MHz and 20Vpp at 30Mhz and know the
    bare-bone output amp's run flat and at less than 1% distortion but would
    have thought 0.1%! was decidedly uneconomic, given the reducing choice of
    R.F. power transistors.
     
  13. John Larkin

    John Larkin Guest

    We need to do this with ICs, because we want 8 ARB channels (with
    AM/FM/PM/PWM/Noise/Summing on a 6U VME card. It looks possible, but
    optimizing the distortion looks to be a labor-intensive procedure.

    The other problem is measurement: our spiffy new spectrum analyzer has
    harmonic distortion spikes about 65 dB down, so we'll probably have to
    feed it through highpass or notch filters or something. I *knew* there
    was a reason I've spent my life avoiding RF designs.


    John
     
  14. Fred Bloggs

    Fred Bloggs Guest

    MCL had a high frequency hybrid that would easily meet that as OIP3/2 at
    your 24dBm, it used two amps differentially to eliminated the 3rd order
    stuff very well and it was a power amp. Problem: the transformer
    coupling high passed in at 5MHz or something. The little board was $40
    at the time I used them.
     
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