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.
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:
formatting link
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.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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.
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.
compensation.
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.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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.
have enough slew rate to avoid large distortion.
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.
compensation.
that damned cable looks like its characteristic
[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]
transistor?
Asked and answered in the appropriate thread.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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...
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.
compensation.
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?
"Ken Smith" wrote in message news:d2jo57$376$ snipped-for-privacy@blue.rahul.net...
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.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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...
enough slew rate to avoid large distortion.
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.
compensation.
that damned cable looks like its characteristic
Nah- you got caught on that one, pseudo-intellectual, and it is obvious you still don't get it....
transistor?
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?
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.
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.
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.
"Ken Smith" wrote in message news:d2l3t5$30b$ snipped-for-privacy@blue.rahul.net...
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.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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.
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.
"Ken Smith" wrote in message news:d2nbo8$th6$ snipped-for-privacy@blue.rahul.net...
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.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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.
I've seen that in OpAmps that were marginally compensated.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
[>> Based on its recovery shape, I think that it actually has a zero in its
I hesitate to draw such conclusions from the large signal step response, especially considering that the circuit used to get the response plot is not shown. The "Open-Loop Gain and Phase vs. Frequency" plot, if interpreted as "40 dB nominal Closed Loop Gain and Phase vs. Frequency", (which reveals the open-loop response where it gets interesting), shows no such zero.
Yes. If you say the few tenths of a dB implied by that performance is "significant", I'll not argue the point. I would argue that said peaking occurs a high harmonic of the OP's highest stated frequency of concern.
Thanks, I guess. I cannot imagine running my life so as to please that guy. I'd have to be crazy to try.
That is silly. I am stating nothing more than the commonly understood fact that some parameters upon which a design relies are not found in the min or max columns of a datasheet. If you believe that has never happened in your industry, or only happens when "sloppy" engineering is done, then you have led a life sheltered from reality.
That frequency is a ways beyond where I was looking. (The application was delay control in an analog phased array beamformer, where an LPF was necessary to keep noise at such frequencies out of the RF signal.) I suppose that bump could be called strange, although if you look at the bias current cancellation scheme in that part, it is not too surprising that it would happen.
....
The issue then would be what it did to the frequency response. Again, we are left to speculate what will actually solve the OP's real problem.
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
"Multiple zeros" tends to fill my bill of "marginally compensated" ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
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