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Unity gain buffer amplifier to lower impedance

Discussion in 'Electronic Design' started by abhijit, Mar 31, 2005.

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  1. abhijit

    abhijit Guest

    Greetings!

    I have a medium frequency analog signal coming from
    a DAC (AD557), but the DAC's output impedance is
    too high for my purpose.

    So I am trying to design a medium frequency unity gain
    buffer amplifier with flat bandwidth from 10Hz to 2MHz,
    but I have the following restrictions:

    - single 5V supply (split power supply cannot be used)

    - input voltage (coming from the DAC) is in the range
    0 - 2.6V (it really goes down to 0V)

    - so buffer must be able to handle (without distortion)
    input voltages really down to 0 volts

    - input impedance should be > 50k, preferably fet-input
    so that it can be ac-coupled with a 0.1uF cap

    - output impedance must be less than 50 ohms

    - must have very low distortion

    - preferably no large caps

    - op-amp is OK provided it is +5V single-supply,
    low distortion, and has common-mode input
    really going down to negative-rail (ground)

    I have designed the circuit below, which gives
    a THD of about 0.3% with a 50 ohm load impedance.
    Also it may not be very good driving capacitive
    loads such as cables. So I am looking for something
    with lower distortion and lower output impedance
    which can drive capacitive loads with the above
    restrictions.

    Anyone with more ideas?

    Abhijit Dasgupta

    ============================
    Spice file (Berkeley spice3)
    ============================

    Buffer with ac-coupled input to njfet, bipolar output
    *
    * Improving distortion with a current sink feeding
    * the emitter of q1
    *
    * With rload = 1.5k, THD is minimal (< 0.001%) for
    * 0-3V p-p input, but THD increases under load,
    * to almost 0.3% when rload = 50 ohms
    *
    * 3 +--------+-------o vcc +5V
    * | |
    * |-+ j1 |
    * 1 cin 2 | J310 |
    * vin o---||---+--->|-+ |
    * 0.1uF | | 4 |/ q1
    * < +------| 2N2222A
    * rgate < | |\.e
    * 4.7Meg < < |
    * | < rsrc +---o<-----+ vout
    * | < 47K | 5 |
    * | | | |
    * | +--------+ |
    * | | |
    * 3 | rb 6 |/ q2 |
    * vcc -----------/\/\/\-+---| 2N2222A |
    * | 2.2K | |\.e |
    * | _|_ | |
    * | d1 \./ + 7 < external
    * | 1N914 -+- | < rload
    * | | < < 50 ohms
    * | +8 < re |
    * | _|_ < 56 ohms |
    * | d2 \./ | |
    * | 1N914 -+- | |
    * | | | |
    * 0 +---------+-----+---o<-----+ GND
    *
    vcc 3 0 dc 5
    vin 1 0 dc 0 ac sin(1.5 1.5 1000)
    cin 1 2 0.1u
    rgate 2 0 4.7Meg
    j1 3 2 4 J310
    rsrc 4 5 47k
    q1 3 4 5 Q2N2222A
    q2 5 6 7 Q2N2222A
    rb 3 6 2.2k
    re 7 0 56
    d1 6 8 D1N914
    d2 8 0 D1N914
    * rload 5 0 1.5k
    rload 5 0 50
    *
    *
    ..model D1N914 D(Is=0.1p Rs=16 CJO=2p Tt=12n Bv=100)
    *
    ..model J310 NJF(Beta=3.384m Rd=1 Rs=1 Lambda=17m
    + Vto=-3.409 Is=193.9f Cgd=6.2p Pb=1 Fc=.5 Cgs=6.2p
    + Kf=46.34E-18 Af=1)
    * National pid=92 case=TO92
    *
    ..model Q2N2222A NPN(Is=14.34f Xti=3 Eg=1.11 Vaf=74.03
    + Bf=255.9 Ne=1.307 Ise=14.34f Ikf=.2847 Xtb=1.5
    + Br=6.092 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=7.306p Mjc=.3416
    + Vjc=.75 Fc=.5 Cje=22.01p Mje=.377 Vje=.75 Tr=46.91n
    + Tf=411.1p Itf=.6 Vtf=1.7 Xtf=3 Rb=10)
    * National pid=19 case=TO18
    *
    ..control
    tran 1u 2m 0 1u
    plot v(1), v(5)
    fourier 1000 v(5)
    ..endc
    *
    ..end
    *
     
  2. I would worry about Vgs variation eating into the
    headroom at the high end and adding distortion
    due to q2 near-saturation at the low end. The
    possibly reactive load would also be a concern.
    The AD8615 from Analog Devices looks like it
    would deliver the performance you want for little
    more than $1. With a 4-resistor network to offset
    the output a little bit, you could stay within its low-
    distoration output swing range and stay DC coupled
    to avoid accidents of input spectrum from swallowing
    up some of the dynamic range.
    Here is an overview and (futher) link to datasheet:
    http://www.analog.com/en/prod/0,,759_786_AD8616,00.html

    Is your concern about cable due to intervening cable
    still having a 50 Ohm load at the end? In that case, I
    don't think you have to worry about it as long as you
    use nominally 50 Ohm cable.
     
  3. rickman

    rickman Guest

    I would not bother to use discrete parts when opamp circuits are so
    simple. Since you don't need DC coupling, the common mode range to GND
    is not really an issue. You just AC couple the circuit and bias to the
    middle of the VDD rail.

    The circuit you need is just a voltage follower with an AC coupled
    front end. It really couldn't be much simpler.

    +--------+
    | |
    | |\ |
    +--|-\ |
    C1 | >--+-----o Vout
    Vin o---||--+---|+/
    | |/
    |
    \
    R1 /
    \
    /
    |
    R2 |
    Vdd<--\/\/--+----+----+
    | |
    \ | C2
    R3 / ---
    \ ---
    / |
    | |
    V V
    R1 > 100 * R2
    R2 = R3
    R1*C1 < 1 Hz
    (R2||R3)*C2 <~ 10 Hz ( optional if your VDD is very stable)

    R1 is used to provide a high input impedance while C1 blocks the DC
    component. The produce of R1 and C1 form the high pass filter with a
    corner some 10x below the 10 Hz freq of interest to get a flat
    passband. R1 must also be some 100x greater than the parallel
    combination of R2 and R3 to provide good isolation between the bias
    voltage and the input voltage.

    R2, R3 and C2 form a voltage source and can be replaced by a regulator
    if you want improved stability and noise (or don't like the cap C2) of
    the bias voltage (which appears at the output like the circuit you
    provided). C2 is used to remove Vdd noise since it will appear at the
    output at -3dB gain if not filtered.

    To get good linearity at the upper end of the freq range, use an op amp
    with ~100x unity gain bandwidth or about 200 MHz give or take. Also
    pick an op amp that meets your distortion requirements and you are
    done.
     
  4. Fred Bloggs

    Fred Bloggs Guest

    Not even close- not enough slew rate for 2MHz at 2.6Vpp, not enough
    drive power for 50 ohm load without overheating, cannot handle cable
    capacitance without severe bandwidth reduction compensation. Guess this
    is yet another misleading Brasfield post- and how did you resist telling
    us another "the time I put someone down" story...
     
  5. Close, and spot-on for all the OP has actually said.
    The OP stated a bandwidth requirement: "a medium frequency unity
    gain buffer amplifier with flat bandwidth from 10Hz to 2MHz".
    This does not mean "full power bandwidth", as you well know.
    The OP may have intended that, granted, but I see no need or
    reason to assume the OP did not mean precisely what he stated.
    Wrong. If the output is biased at 1.5 V to handle the OP's
    stated 0 - 2.6 V input, and powered from +5 V per the OP's
    stated requirement, the amplifier never has to dissipate more
    than 125 mW to drive the grounded 50 Ohm load shown
    by the OP. For the package with highest thermal resistance,
    junction to ambient, of 210 oC/W, this yields a temperature
    rise of less than 27 oC. With the part rated for a junction
    temperature of 150 oC, this would limit the ambient to only
    123 oC. Since the OP has stated no unusual requirement
    with respect to ambient temperature, (or any requirement,
    for that matter), your claim is without foundation.
    The question is still open whether the circuit will have to drive
    an improperly terminated cable or not. Until that is known,
    your criticism on this point is premature. Even if there will be
    a cable with mismatched termination, its length has not yet been
    stated, so again your issue is premature and possibly meritless.

    [Irrelevant interpersonal crap cut.]

    Why not take your best shot and show what a man you are
    by designing a circuit for the OP which will meet or exceed
    both his stated requirements and the ones he has not yet got
    around to stating? You can be Mr. Electronics Hero Man.
    And if your 'solution' ends up too expensive or takes too
    much board space, just chalk it up to faulty appreciation.
     
  6. Fred Bloggs

    Fred Bloggs Guest

    Not close at all- you don't know what you're doing.
    Yes it does, stupid. And even at signal levels less than full power, you
    don't have enough slew rate to avoid large distortion. Only an idiot
    would choose that amplifier- and that is you.
    You're always a damned clueless jackass when it comes to reading
    comprehension.
    It will overheat- all you know is brain dead static specs.
    Thanks for revealing that you don't sh_t about transmission lines- you
    think that damned cable looks like its characteristic impedance at
    frequencies in the 10Hz-2MHz range?...Hahahahaaaaaaaaaa... damned
    jackass and pretentious idiot.
    Why do that when I can be entertained by your hilarious display of
    ignorance- it looks like you can't do a damned thing right- you screw up
    every single project you undertake. BTW- where are your super-duper
    measurements of the zenered eb-junction transistor?
     
  7. Derf transform applied.

    As close as before, with nothing shown otherwise
    beyond vague handwaving regarding distortion.

    I note in passing that the full power bandwidth is
    approximately 1.4 MHz, typical.
    What you fail to comprehend is that within the slew rate
    limit, little distortion occurs and what does occur is still
    inside the feedback loop, hence reduced by the loop gain.
    Hmmm. Not reading more into the OP's spec than he stated
    is a reading comprehension problem. Maybe it is nothing more
    than giving the OP credit for knowing what he means to say.
    To "know" otherwise requires delusion or mind reading skills.
    For the load shown by the OP, the static load is very close
    to the dynamic load, (which is lighter, in fact, than the worst
    case static load I mentioned above).

    I note that a simple analysis based upon the datasheet and
    a conservative dissipation estimate shows plenty of margin
    with respect to heating. From you, we have nothing but
    "It will overheat [blah blah] static specs." From that, I
    conclude that either you cannot understand my above
    analysis or you have somehow divined a dynamic load
    severe enough to eat up another 50+ oC of temperature
    margin absent any evidence of such a load.

    Do you understand that extra current sourced by that
    op-amp to drive a reactive load during one direction
    of output swing will be sunk by the 50 Ohm load on
    the opposite output swing, up to about 30 mA? Can
    you see this leaves the op-amp dissipation unchanged?
    Of course, when you divine whatever load most suits
    your preening proclivity, such details would not matter.
    [derf]

    Amusing that you have to put words into my mouth to
    get your jollies. The question on the table is "What
    impedances are we actually dealing with?" I have
    made no effort to assume any answer as silly as you
    would like to pretend here. You are delusional.

    [snip]
    [derf]
    Asked and answered in the appropriate thread.
     
  8. Fred Bloggs

    Fred Bloggs Guest

    Nah- I don't call failures in bandwidth, slew rate, and drive power
    "handwaving"...maybe to you, but then you are a pseudo-intellectual who
    does little more than talk.
    Oh- do you note that in "passing", pseudo-intellectual...
    Really, pseudo-intellectual? And how much gain do you have left at the
    higher frequencies necessary for this so-called feedback pre-distortion
    of the signal? You're wrong as usual and it will be a cold day in hell
    before a little "block diagram pussy" like you tells us anything.
    Nah- it's called "understanding"- something you have damned little of-
    you need everything spelled out in black and white because you are a
    true moron pseudo-intellectual.
    You call a simple scalar multiplication "analysis"? What a pompous and
    pseudo-intellectual swine you are- a total pretentious and unemployable
    fraud....damned joke.
    Nah- you got caught on that one, pseudo-intellectual, and it is obvious
    you still don't get it....
    Nah- you have produced no measurement results, pseudo-intellectual and
    fraud? Just can't do much beyond shoot your pseudo-intellectual mouth
    off, can you fake?

    Starting to get the message that you aren't worth much to Newsgroup,
    unimpressive, pretentious, pseudointellectual?
     
  9. Ken Smith

    Ken Smith Guest

    This is not always true. The loop gain reduces the amplitude of a
    harmonic if that harmonic happens to land at a frequency well below the
    gain cross over point. If the phase margin is less than 90 degrees, there
    is a band where the circuit's gain increases the amplitude of the
    harmonic.

    For small amounts of distortion, you can simply consider the distortion as
    a signal injected at the output of the dirstorting section. The usual:

    G/(1 + GH)

    form can be used to find the effect of the circuit gain on each component
    of the distortion. The G part is the gain from the injected distortion to
    the output point and the H part is the gain from the measurement point
    back to the point of injection.
     
  10. I believe you have slightly overstated your case here,
    although I agree that, where response peaking occurs,
    the harmonic terms falling there are subject to it.

    That effect, as well as keeping the loop gain high enough
    to do some good at the upper end of the expected input
    band, are reasons to use an op-amp somewhat faster
    than needed strictly to get the desired bandwidth.
    I pretty much agree with that, as far as it goes. (And I expect
    that it predicts the peaking effect also, for small distortion.)

    (Slightly contrary to my above assertion), there can be some
    distortion appearing at the input stage of an op-amp, under
    large input error conditions such as would arise if the slew
    limit were broached or closely approached. This distortion
    is effectively outside of the feedback loop. Fortunately, for
    MOS input stages, (such as the AD8615 has), this distortion
    is relatively small. At or near 1.4 MHz, for 1.3 Vp inputs,
    the input error is in the neighborhood of 100 mVp, which I
    expect is well within the active input range of the stage.

    If the OP elects to consider that op-amp, he would be well
    advised to use the model provided by Analog Devices for
    simulation of the distortion performance. That model appears
    to use enough transistors, in the input and other stages, to
    have a good chance of creating much of the distortion to
    be expected from real parts. Caveat Emptor, of course.
     
  11. Fred Bloggs

    Fred Bloggs Guest

    But you don't have that here- you are down to a gain of 12 at the high
    end of your input spectrum...
    By input error you mean the difference between signal and feedback
    required to drive the output. Your gain error is very bad then- pushing
    10%- which seems strange in consideration of all your previous yapping
    about "accuracy". So I guess your rule is that an issue is raised only
    if it is a non-issue, and real issues are relegated to non-issues- isn't
    that typical of a pretentious, ignorant, stupid, dodging and evasive
    pseudo-intellectual like yourself.
    SPICE only returns harmonic distortion and this may not be so useful at
    2MHz. You are to be congratulated for reducing his DAC accuracy to 3+
    something bits- quit the precisionist you are- damned joke and
    pseudointellectual from hell really.
     
  12. Ken Smith

    Ken Smith Guest

    In what way do you think that I've overstated the case? You agree that
    those harmonics that land near the gain cross over may be peaked, so
    where's the disagreement? Did I miss something?

    But thats not right if you expect to keep the distortion low using the
    loop gain. It is the gain at the harmonic that matters not the gain at
    the signal frequency. Merely making the gain "some what faster than
    needed" simply isn't going to cut it unless you don't care about harmonics
    above the signal band or you have a following low pass filter.

    The OP is driving a cable that drives a load. No mension of a low pass
    was made nor could be expected in that case unless the coax. is fairly
    long.
    Why do you say "expect"? Is there some condition in which it does not
    predict peaking for the phase margins I suggested?

    Actually you don't have to get all that close to the slew rate limit
    before the input stage starts to show some distortion. The OP is speaking
    of distortions of 0.3% or so at 10MHz.
    I would not make that bet based on the data sheet. They don't give you
    nearly enough information to know what the linear range of the actual
    input circuit is. Although the leakage numbers imply that the first
    device you hit will be a MOSFET, it does not rule out a bipolar connected
    to that MOSFET. Without knowing the actual input topology or having a
    specification, you can't say for sure.

    I don't think the AD8615 will work very well for the OP's application:

    (1) It has a typical close loop output impedance of 3 Ohms at 1MHz. It
    could easily have more than the OP's required 50 Ohms impedance at 10MHz.
    The typical curve show huge peaking of Zout at about 30MHz. Since no
    min/max numbers are given you can't trust it.

    (2) The THD+N is only shown up to 20KHz, therefor we know that it goes bad
    at about 21KHz.

    (3) It rings badly with a 200pF load.

    I agree that the OP should proceed with caution.
     
  13. First, let's assume the usual case of a single dominant pole
    and some collection of higher frequency poles in the open
    loop response that contribute to reduction of phase margin
    from the 90 degrees contributed by the dominant pole.

    Peaking cannot occur when the loop gain is low enough
    to keep the closed loop poles on the real axis. There is
    a significant range of loop gain were the phase margin is
    less than 90 degrees but the closed loop poles remain
    real. There is some more range where those poles are
    complex but no sigificant peaking occurs.

    My agreement is with the general intent of your statement.
    I simply disagree with your criterion for where the effect
    you mention begins to occur.
    You got me for being vague. I did not mean "ever so slightly
    faster". I meant "enough faster to have enough loop gain to
    push down and not amplify distortion effects expectable with
    the signals actually used". We have no disagreement here.
    I don't know how a low pass got into this.
    "Expect" is plain English. I intended no trick meaning(s).

    As for "the phase margins [you] suggested", I suggest you
    take a look at a simple case where the phase margin is 78.7
    degrees. Suppose G = A / (s * (s + Pe)) and H = 1.
    If this G is to provide that phase margin, then at s = j Pe / 5,
    (since atan(11.3) ~= 1/5), the magnitude of G has to be 1.
    So A must be | s^2 + s*Pe | for s = j Pe / 5 .
    A = | - Pe^2/25 + j Pe^2/5 |
    A = Pe^2/5 * sqrt(1/5^2 + 1)
    A ~= Pe^2 / 5
    Using Acl = 1 / (1/G + H), and substituting, get
    Acl = 1 / ( (s/A) * (s + Pe) + 1)
    1/Acl = (1/A) * s^2 + (Pe/A) * s + 1
    The roots or poles are at -Pe/2 +/- sqrt((Pe/A)^2 - 4/A)*2*A
    Note that for A = Pe^2/5, the offset pair from -Pe/2 becomes
    sqrt( ((5/Pe)^2 - 4*5/Pe^2 ) / (2/(Pe^2/5))
    = Pe * sqrt(25 - 20) / 10
    So, for that phase margin, the poles are still real and no
    peaking is possible.

    You can work the same problem with more excess poles
    and the dominant pole not at 0 and complicate the math
    but it will not change the result. Phase margin has to go
    well below 90 degrees to obtain a peaked response.
    I agree that "some distortion" shows up at levels well
    below the slew rate limit, at least if that limit is imposed
    by a soft-limiting characteristic, (such as an input pair).
    And for some definition of "some", that much distortion
    will occur at any significant fraction of the output swing.
    My reading of his post does not reveal the frequencies
    at which his various distortion figures applied and I do
    not see 10 MHz mentioned at all. Have you run his
    simulation to get those figures?
    From the datasheet alone, I agree. From looking at
    their SPICE model, I doubt anything that strange has
    been built inside the part.
    From what little the OP has stated of his requirement, I
    have to agree that remains possible. But I also think the
    AD8615 could do what he needs, and that, when those
    needs are more fully specified, they could be consistent
    with what his first post indicated.
    I just don't see where you are getting this requirement.
    The highest frequency he mentioned was 2 MHz, and
    that was only vaguely specified. (Consider that Bloggs
    is able to insist it means "full power bandwidth" while I
    claim it could mean only small signal frequency response.)
    I do not see the relevance of the 30 MHz value. That
    would correspond to the 15th harmonic of the highest
    frequency mentioned by the OP. The same chart shows
    that the open loop output impedance is about 45 Ohms
    in the frequency range mentioned in the OP's post.

    As for not trusting it, that quantity is rarely specified as
    a maximum, so under your philosophy, the vast majority
    of op-amps are unavailable for trustable work.
    Not at all obvious, except maybe to cynics. Would you
    not agree that such a cutoff in the chart may reflect either
    the presumed interest of those considering the part for
    audio applications, or the limits of some instrument built
    for that market? Surely you do not believe that some
    strange circuitry is built into a 20 MHz GBW part that
    really sends it South near 21 KHz.
    If he is driving a non-50 Ohm cable between his amplifier
    and his 50 Ohm load, or more than a few feet of cable
    with a load resistance much higher than 50 Ohms, then
    that ringing could be a problem. We still have seen no
    answer to my question regarding the cable situation. I
    dare say the issue is getting attention here far in excess
    of the OP's interest or the available facts.
    I suppose that, in the spirit that seems to pervade here, he
    should have been advised to use some op-amp having
    performance well in excess of his apparent needs. For
    example, with suitable care in its application, and a bead
    between the device and a misterminated cable, he would
    find that the AD8009 would do his job. The downside,
    less likely to be visible here, would be some extra cost,
    some extra parts, and the possibility of misbehavior at
    frequencies he may not be equipped to observe.
     
  14. Ken Smith

    Ken Smith Guest

    I believe that this is not actually correct for the op-amp you suggested.
    Based on its recovery shape, I think that it actually has a zero in its
    transfer function but for the purposes of our discussion we do not need to
    include that issue.
    Remember we are talking about an op-amp with a phase margin of only about
    45 degrees. This statement although true as far as it goes does not apply
    to the op-amp you have suggested. It does show serious peaking.
    Are you speaking here of the op-amp you suggested? If so, I'll ask you to
    state what this range is in this case.
    It does not include the unity gain buffer case under discussion here.

    BTW: there is a simple rule for the amount of peaking vs. phase margin.


    [...]
    I introduced it. It is common practice to examine what will happen to a
    signal later to see if the distortion is something worth worrying about or
    not. If the OP did have a low pass, the distortion products well above
    the signal band would matter less and the op-amp specs could be relaxed.
    As it is there is no low pass, the specs can't be relaxed.

    Now try it at 45 degrees which is what the op-amp you suggested will have.
    BTW: 60 degrees is an inportant angle.

    [...]
    If you check out the slew rate specs for op-amps you will find that in
    general they are based on the point of total limiting not the point where
    distortion passes some low value.
    My 10MHz value came from memory of reading his text.

    I would trust that about as far as I can spit it. I've been burned often
    enough to know that if the datasheet doesn't say it watch out.

    Perhaps I mis-remember his posts. I, unfortunately can't easily review
    them while I'm typing this.


    [...]
    The OP-speced 50 Ohms of impedance. The huge peaking points out that
    small variations in device parameters will change the impedance wildly.
    .... and those who have been bitten. The cut off point is way lower than
    the devices specified bandwidth etc. On the early data sheets, the
    LT1028's noise spec stopped at about 20KHz too. Guess what.
    Ok, mayby its 22KHz. Until I see it on a datasheet, I'm not going to
    trust it. Harris made some very fast op-amps, back in the stone age, that
    had two internal paths. It had a very fast high distortion,low gain path
    and a slower low distortion, high gain path. At the point where which
    path dominated changed, the distortion got bad in a hurry.

    I think the OP was hoping for a simple answer and since he didn't get one
    he bailed out on us.

    [..]
    Beads can cause measurable distortion. I'd suggest a resistor since he
    can stand a 50 Ohm output.
     
  15. Fred Bloggs

    Fred Bloggs Guest

    Oooh- wow does that *sound* technical...
    Does not apply to the unity gain non-inverting buffer- once again you
    are out to lunch, pseudo-intellectual loudmouth of subnormal intelligence.
    Backpedaling again?
    Totally unrealistic and useless assumption on G- and more of your
    pussy-algebra that you can barely handle and leads to nowhere...
    It almost always is well less than 90o....
    Face it- you are not competent to specify components, you are not
    competent to select proper circuit prototypes, and you are not competent
    at using SPICE. You are just another loudmouth pseudo-intellectual
    USENET troll and no more. The best advise to any OP is to ignore your
    bullsh_t entirely- totally erroneous gibberish and "garbage" electronics.
     
  16. My point was in response to a "not always true" statement
    coupled with an ostensibly supporting claim: "If the phase
    margin is less than 90 degrees, there is a band where the
    circuit's gain increases the amplitude of the harmonic."

    To me, it appeared that you were making a general claim,
    not one tailored to the specifics of the device I suggested.
    I don't see that in the datasheet. The open-loop gain plot
    is not credible, however, so I'm not sure what it does.
    We are in noisy agreement, again. But I notice that the
    "serious peaking" occurs with 200 pF loading. If you look
    at the "Small-Signal Overshoot vs. Load Capacitance"
    plot, you will see 5% overshoot at 0 pF, a very benign
    response from a stability perspective.
    I was speaking in general terms. My point is only that
    there is a range of phase margin between where peaking
    occurs in a mathematical sense but not to a degree that
    presents a stability issue or a real harmonic gain issue.
    I guess that depends on whether 5% overshoot can
    be said to correspond to "significant peaking", or
    whether a more difficult load than the OP mentioned
    is postulated.
    Yes. It is a rule-of-thumb since its accuracy depends
    on where the excess poles actually fall w.r.t. each other.
    Ok, I agree with that new point. Much of my own work
    on distortion performance has been concerned with odd-
    order intermodulation products which fall near the same
    frequencies that give rise to them. In those cases, an LPF
    does no good at all.

    As for "there is no low pass", it is true the OP did not
    mention one. But for his precision DAC buffer, it may
    be a good idea to have one, perhaps with 1/sinc(f)
    correction, depending on his real requirement. And for
    all we know, he already has one planned or in place.
    If '60' had been in your original general claim rather than '90',
    we might have averted this little subdiscussion.
    That has been my experience too.

    [2 vs 10 MHz, cut]
    Once upon a time, I thought all design should be done
    so as to guarantee the required performance based on
    nothing but worst-case datasheet guarantees. Since
    then, I've had to become more realistic. I do not deny
    that prudence has a place in that calculus, but there are
    too many device characteristics affecting large signal
    performance to demand they all be guaranteed within
    the datasheet specifications.

    [more 2 vs 10 MHz, cut]
    What you call "huge peaking" represents a closed-loop
    output impedance about 3 times the open-loop output
    impedance, and it occurs well outside the OP's stated
    bandwidth. The effect occurs because the feedback
    is becoming positive due to the phase margin situation.
    The stability of that effect is just as reliable as the stability
    of the unity gain connected op-amp. While I do not have
    unlimited faith in such matters, I do not believe Analog
    Devices sells many op-amps that are ready to become
    oscillators when used per their recommendations.
    I don't recall the LT1028 doing anything strange noise-wise
    at any frequency. I've used it where noise mattered a lot,
    and looked at it quite carefully, so I need not guess about it.
    We will have to disagree on the appropriate skepticism here.
    But to validate your point, I would insist on seeing some
    real parts, especially where distortion is a major concern.
    Maybe he got a simple answer, "use an op-amp", and is
    off looking into it, with better knowledge of what he needs
    than we have been privy to.
    Using only a +5V supply and delivering 2.6 Vpp to his 50
    Ohm load, I think headroom could become an issue if that
    resistor got near the load value.

    The bead I had in mind would be to isolate the reactive
    load at frequencies where it could destabilize that 1 GHz
    GBW amplifier. A bead that only cuts in a 100 MHz or
    so is not going to have much effect at 2 MHz and below.
    It need not be operated in its nonlinear region.
     
  17. Fred Bloggs

    Fred Bloggs Guest

    Larry Brasfield wrote:
    [...snip pseudo-intellectual dodging and backpedaling...]

    You picked the *wrong* part both for the bandwidth and the drive
    requirements- you are a sorry-sack-of-sh_it loser who doesn't know his
    ass from a hole in the ground...
     
  18. Ken Smith

    Ken Smith Guest

    Yes, the 90 was an error, I intended 60 as the band in which there was a
    peaking. The 90 was part of another though I removed from the post.
    It was a general claim I was making and with the correction of 60 vs 90,
    it is correct.
    Take a look at the 10th page in the upper left corner. I think it is
    obvious that the op-amp must have zeros.
    It is the increase in gain at the gain cross over frequency, increasing
    the amplitude of a selected harmonic that is at issue.
    Warning: I expect you've just opened yourself up to another FB flogging.
    1/(2Sin(X/2))


    [...]
    Your "realistic" would be called "sloppy beyond all reason" in the
    industry I work in. In the "audio" business, and a lot of other consumer
    stuff, no-one will ever know if 10% of the units don't really meet spec.

    [....]
    In that case you need to look again and much more carefully this time.
    The noise at 3KHz is just about 1nV/sqrt(Hz) at 300KHz it is about 3
    times this figure.


    [....]
    I always assume that the parts the salesperson brings as samples are about
    the best ones they make. I've been proven wrong on this twice. In one
    case the "salesmans sample" was actually defective and had obviously never
    been tested.
    Much less than 50 Ohms would be needed to improve the stability.
     
  19. Ken Smith wrote...
    It's much worse than that, according to the datasheet, rising from
    0.85nV/rt-Hz to peak at about 5.5nV at 400kHz. An interesting plot
    that raises issues of how it's measured, and what's happening.
     
  20. Jim Thompson

    Jim Thompson Guest

    I've seen that in OpAmps that were marginally compensated.

    ...Jim Thompson
     
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