Zero-crossing distortion for high frequency sinusoidal signals on final OPA2830 - Page 1
Forum section: electronics / projects

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 13, 2018, 04:18:53 pm
content: Hi,I've built a function generator that has the output stage as in the picture attached.The OPA2830 works without a problem when amplifying sinusoidal signals up to 2MHz. Above that, if the output is terminated with 50 ohms, it starts to show zero-crossing distortion. This distortion worsens with frequency. However, it can be alleviated, and eventually completely eliminated if the signal amplitude is decreased. This shows me that the distortion seems to be slew rate dependent. What confuses me, is that the OPA2830is being used within the specification.This issue is only caused by the output stage. If the output is left unterminated, or unloaded, it can go well above 4MHz without showing any kind of distortion. What is happening?Kind regards, Samuel Lourenço

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 13, 2018, 06:22:30 pm
content: I see it specified down to 150 ohms, not 100. How does it do at 150 total (you have to include both the output resistor and the feedback network)?

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 13, 2018, 07:07:49 pm
content: I suggest taking a look at the datasheet, p. 19 and 20.As Conrad suggested, the behavior you're seeing seems to be related to the output load. With such low loads, at high frequencies and high output voltages relative to power supply, you'll get relatively "high" distortion. This can be seen in the datasheet.It may even get worse in "real life", since you may have some input capacitance in parallel with your 50 ohm output load, that will further decrease the total output load as frequency increases.If you really want a 50 ohm output impedance, there is nothing much you can do apart from choosing another opamp with higher output current capability, such as the LT1206.Another option that would work nicely as well would be to buffer the output.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 13, 2018, 08:09:58 pm
content: Hi,The opamp is able to deliver up to 60mA without an issue. If it was over-current, I would see some clipping at the top and bottom. But that doesn't seem to be the case. Besides, at the maximum peak voltage of 4V, I would expect it to deliver 40mA.Nevertheless, I've tested the output stage with a 100resistor as termination (that gives a total of 150load resistance) and I see the same thing again (a tad better, but not significantly better, actually). The amplitude was set to 7Vpp without load, which gives 4.6Vpp with the 100resistor. Please, see the picture attached.I don't see any issues if the frequency is set to below 2MHz, but above that, the distortion worsens the higher the frequency. A workaround is to lower the amplitude. I've inquired Texas Instruments, and they find this strange.kind regards, Samuel Lourenço

author: danadak
author_rank: Super Contributor
timestamp: February 13, 2018, 10:00:33 pm
content: Am I correct that you are running part at +/- 4.2V ? Datasheet specs seem tobe focused at +/- 5, even though min is 1.4. There are also specs at 3Vsingle supply, you can see Iout is quite a lot less. Makes one wonder if partis running out of gas/output stage bias much below +/-5V. Might just try it at+/-5 and see what results you get.Also input CM is kind of limited, any violations of this ?Regards, Dana.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 13, 2018, 10:10:11 pm
content: Hi Dana,Unfortunately I can't do tests at other voltages. I'll have to find a way to test the opamp on its own board. Surely, it is just the last opamp, so I think I can replicate the issue and test it out. I could also test the LMH6612 to see if it fares better.As for the input common mode, there are no violations. The input voltages are well within spec (between 0.015V and 0.3V). During zero crossing, the inputs should cross 0.15V. I should refer that the inputs are complementary signals with an offset of 0.15V.Kind regards, Samuel Lourenço

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 14, 2018, 05:33:08 am
content: 1) which supplier you get your opamp from?2) what is the source feeding the ana4_n and ana4_p?

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 14, 2018, 12:11:09 pm
content: Hi,I've got the OPA2830 from Mouser. I suspect that these may be knokoffs, although the laser markings seem to be legit. They are way out spec, as they should be capable of outputting a signal without distortion under those conditions. I've inquired TI, and they find it strange.ANA4_N and ANA4_P is being fed by the signal generated by the AD9834, after passing through the reconstruction filter and after being amplified with a gain of two. The signal is clean as a whistle until passing through the last OPA2830 (1/2).I've uploaded the schematic.Kind regards, Samuel Lourenço

author: danadak
author_rank: Super Contributor
timestamp: February 14, 2018, 01:00:05 pm
content: If I look at the test conditions for slew rate its stipulated with 150 ohm load, +/- 5V supplies.8 Mhz, 3.4V, requires 171 Mhz SR, @ 150 ohms. Your effective load stage 2 is ~ 47 - 48 ohms,and of course the lower supply than spec calls out.Also notice harmonic distortion looks like specs don't fit your conditions, but imply maybe itsdegenerated under your operating conditions significantly ? Maybe high 30 db, low 40 db range.....Maybe OpAmp just a tad marginal at 8 Mhz, 3.4V ?All that being said still odd that distortion looks like crossover. I am stumped.Whats your Cload look like, and what do you think your parasitic C looks like atinverting input. Try tacking in a few pf on either inverting or output to see ifdistortion is aggravated.Note the OpAmp is comped at unity G stable. That tells me significant comp capinvolved, and LV operation maybe bias I sources are getting marginal as well.Just thinking out loud......rambling.........Regards, Dana.

author: Kalvin
author_rank: Super Contributor
timestamp: February 14, 2018, 03:18:59 pm
content: What kind of probes you have? Check that you are using 10x attenuation for the oscilloscope probes and the probes are calibrated. Even at 10x attenuation the probe will exhibit some capacitive load the output at higher frequencies ie. at high slew rates.What happens if you remove the TVZ-diode IC17 from the output? Or if you remove R24 and solder it directly from op amp output to the ground and then probe from the op amp output? The idea is to isolate the op amp output from the TVZ-diode and the output connector and see whether they will affect the measurement somehow.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 14, 2018, 04:18:36 pm
content: Quote from: danadak on February 14, 2018, 01:00:05 pmHi Dana. How did you get the 48figure?Quote from: Kalvin on February 14, 2018, 03:18:59 pmI'm not using probes of any kind, but coaxial cables. However, what I could do is to couple the terminator directly and use the probe to measure that. At least this would eliminate a few pF from the cable plus the scope.I don't think it is the diode's fault, as its capacitance is very tiny, and the opamp output is isolated from it (and the cable) by R24. Plus, without the 50load I can go to 20MHz and still no distortion (just attenuation above 8MHz due to the DAC's sampling envelope). Note that the OPA830/2830 family handles relatively high capacitances very well (up to 1000pF), without an isolating resistor between the output and the load.Nevertheless, I've built a board to test the OPA2830 output capability under different conditions (supply included). The main purpose of the test board was to compare this opamp with the LMH6612, and see if I have a similar problem too. I'm hoping that this issue is particular of the OPA2830, but the LMH6612 might not fare well since it doesn't handle capacitive loads that well (if it is the case).It will take a while to get the boards done, but I'll surely post the results.Kind regards, Samuel Lourenço

author: David Hess
author_rank: Super Contributor
timestamp: February 14, 2018, 04:38:54 pm
content: The low load impedance seems marginal for the operational amplifier. Unless I could use a different circuit or part, I would use two in parallel with 100 ohm loads each, or 3 with 150 ohm loads since they are dual parts, to make a 50 ohm input impedance and lower the power dissipation of the amplifiers.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 14, 2018, 09:02:14 pm
content: Quote from: David Hess on February 14, 2018, 04:38:54 pmWell, on second analysis, so it seems, but only for such high frequencies (> 2MHz). I've been testing the function generator again, and I noticed that the voltage before R24 is a tad smaller when the output is loaded. I've also been testing it with a 100pF capacitor at the output, in parallel with the load, and actually it seems to improve a bit the distortion. Having a single 100pF capacitor and no termination, I don't see any distortion. That rules out the TVS diode, as its capacitance is much smaller.See the images attached for more details.Kind regards, Samuel Lourenço

author: Kalvin
author_rank: Super Contributor
timestamp: February 14, 2018, 09:19:38 pm
content: Good analysis!Have you probed the op amp power supply and checked that it is nice and clean ie. the bypass is sufficient and behaves well enough.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 14, 2018, 10:11:18 pm
content: Quote from: Kalvin on February 14, 2018, 09:19:38 pmIndeed I did. Although the LDOs have low headroom, the supplies show noise in the order of a few mV. No major ripple there. The current spikes caused by the charge pump are well mitigated in this design, tooThe next step is to test this opamp alone, and the LMH6612 in the same circumstances. Lets see if it fares better.Kind regards, Samuel Lourenço

author: danadak
author_rank: Super Contributor
timestamp: February 14, 2018, 11:32:52 pm
content: 48 ohms comes from parallel combo of R24 in parallel with seriesof R22 and R23.I assume you placed 100 pF at output side of R24....not OpAmp.Regards, Dana.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 15, 2018, 08:38:49 am
content: Quote from: danadak on February 14, 2018, 11:32:52 pmR22 + R23 = 1.2K ohm. he intentionally put the R24 as source impedance and he's using 50 ohm termination (load) in the last setup making R24 + Rload = 100 ohm. if combined all, still nearly 100 ohm (150 ohm in earlier setup).Quote from: danadak on February 14, 2018, 11:32:52 pmi assume that too since he has R24 there. from the way he posted his result, he seems to know what he's doing.btw, adding capacitance load to improve distortion maybe is another hack for the day, but i dont think its wise, it could worsen at another frequency. from page 10 in datasheet, 3rd harmonics distortion is -53dB (-58dB 2nd harmonics distortion) at 100 ohm load (that is 50 ohm source impedance and 50 ohm termination), from the OP result, i guesstimate the (crossover) distortion is -20dB or worst. something is wrong as admitted by the TI member. i suggest trying another opamp and see if distortion still exist. fwiw.

author: danadak
author_rank: Super Contributor
timestamp: February 15, 2018, 11:25:44 am
content: I see your interpretation of load, mine was that R24 was terminated to groundfor testing purposes.Regards, Dana.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 15, 2018, 01:42:21 pm
content: Hi,Try a resistor direct from the opamp output to the negativ power rail, chose 3 a 4 mA as current, and then check the distortinon again.Kind regard,Blackdog

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 15, 2018, 02:46:30 pm
content: Quote from: danadak on February 14, 2018, 11:32:52 pmHi Dana,The 100pF was placed at the connector side, and not directly at the ampop output. Also, mind that the load resistance is in series with R24. Of course, in practice, the effective loading resistance will be a tad less than 100, especially for that frequency.Quote from: blackdog on February 15, 2018, 01:42:21 pmHi Blackdog,I quess I could try that. But what is the reasoning?Kind regards, Samuel Lourenço

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 15, 2018, 03:00:29 pm
content: Quote from: Mechatrommer on February 15, 2018, 08:38:49 amHi Mechatrommer,I guess that trying another opamp will be the fix, if proven. I'm yet to try the LMH6612. It is a direct fit, and the stats look promising. The output capability is definitely better (120mA instead of 60-80mA), and has a bit more bandwidth, although less slew rate. Only the capacitive load drive capability is a minus, in comparison with the OPA830.However, I'll not hack it in just yet. I want to test this first on its own board.As for the TI support on this case, no definitive answer yet. The post is here:http://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10/t/663718Kind regards, Samuel Lourenço

author: blackdog
author_rank: Frequent Contributor
timestamp: February 15, 2018, 03:16:15 pm
content: Hi bloguetronica,This resistance puts the output stage of the opamp more in class A...It is an old trick to reduce cross over distortion in opamps.You see the distortion more and more at higher frequency because less and less loopgain is available.I Like this fast opamp THS3061, also available as dual opamp.I used this THS3061 in the next project, see the link below, use google translate if you cant read the Dutch languagehttps://www.circuitsonline.net/forum/view/134726#highlight=tiKind regards,Blackdog

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 15, 2018, 03:39:25 pm
content: Hi blackbog,The THS3061 is quite a beast, but perhaps a bit overkill in terms of $$$. The slew rate is really impressive.I'll try what you suggest. A 1K resistor should do, although the typical crossover distortion is flat through zero. This seems to be more of a phase issue. This distortion is not noticeable on the FFT, by the way.Kind regards, Samuel Lourenço

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 15, 2018, 04:00:28 pm
content: Quote from: bloguetronica on February 13, 2018, 08:09:58 pmIt will do so but with some distortion. The distortion figures, which are fully part of the specs in the datasheet, are actually not that great in some cases including low output load and high frequency. The datasheet doesn't mention crossover distortion specifically, but crossover distortion gives harmonic distortion (as well as non-harmonic, which unfortunately, the datasheet doesn't seem to care about. It only gives HD, whereas THD+N may have been more interesting.)What you see here is actually called crossover distortion and is more common than you may think when using opamps. It comes from the fact that there is a dead band due to the use of push-pull configurations for output stages, the biasing current of which isn't high enough in some use cases for them to remain linear around the cross-over point.As to the output load, you have to take the feedback resistors into account, since they are actually loading the output as well. And in your case, the feedback resistors have kind of a low value. If an high bandwidth is not what you are after, you could try increasing the feedback resistors and see what you get. For just a few MHz of bandwith you can probably increase them significantly.Quote from: bloguetronica on February 13, 2018, 08:09:58 pmIt's not over-current in itself, and it's not just about clipping. You're just more likely to have higher bias currents in opamps having higher output current capability, thus less crossover distortion with low loads.And again, the max output current given in the datasheet doesn't imply you can get that without distortion. You have to look at the distortion graphs to see that it worsens with increased frequency and lower output loads.The trick consisting in forcing the biasing of one of the output transistors with a pull-up or pull-down resistor from the opamp output to one of the supplies may work, but it's not without its potential issues. It may make your opamp overheat and may also degrade overall distortion (except crossover) in some cases.So what I would try first if you're not aiming for very high bandwith is to increase the feedback resistors (like doubling their value).You can also try adding a pull-down resistor from the opamp output to your negative supply, as someone suggested, but then carefully check your output signal and the opamp temperature.You may also try increasing the power supplies, but as I gathered, you might not be able to in this design.As I said before, if adding a part is a possibility, adding an output buffer such as the OPA693 is an option.If you are free to select another opamp and can't add any component, it's probably your best bet.

author: mikerj
author_rank: Super Contributor
timestamp: February 15, 2018, 04:03:17 pm
content: Quote from: bloguetronica on February 15, 2018, 03:39:25 pmI'm not too surprised about that. This isn't classic crossover distortion since the output slew rate isincreasingat the crossing point, rather than reducing, so tricks with pull down resistors etc. may not help.It's probably within spec for the amp if you look at the distortion vs output load charts in the datasheet.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 15, 2018, 04:45:03 pm
content: Quote from: bloguetronica on February 15, 2018, 03:39:25 pmI agree this doesn't exactly look like your typical crossover distortion. But looking closer at your screenshots, it looks like the positive part of the signal has reduced slew rate when approaching zero. The negative part, then, seems to catch up a bit later and thus we can see a steeper slew rate until the opamp gets linear again in the negative region. So this may just be that the dead band is not symmetrical. Zooming on this part of the signal would help seeing this.The fact that it's not noticeable on the FFT (which I assume is your scope's FFT) is not surprising either. From your captures, the distortion is obvious. But your scope resolution is probably not good enough to be able to show that. Remember that most scopes have 8-bit ADCs. If you can't see any peak in the FFT apart from the fundamental freq, that would suggest that distortion is probably under -50 dB (or even a bit higher) or so... which seems congruent with the graphs in the datasheet.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 15, 2018, 04:57:31 pm
content: Quote from: bloguetronica on February 15, 2018, 03:39:25 pmif the eye can see it and the FFT doesnt, then something wrong with the FFT, is it a 100MHz DSO's FFT?. the correct FFT can easily sees -20dB and below, which are mostly invisible to the eye in time domain, ymmv...Quote from: mikerj on February 15, 2018, 04:03:17 pmimho, it is related to crossover distortion, except with feedback system, something is turned off and then something is trying to catch up.Quote from: bloguetronica on February 15, 2018, 02:46:30 pmbecause your problem is related to crossover. loading the opamp to negative rail will make upper half of the push pull bjt to always on. even if you load the opamp so badly to negative rail (without any load to ground), you should not be seeing this crossover type distortion, if you do, then something else, not crossover, with badly loaded opamp, you'll start seeing decreased magnitude on top and bottom sine, that too will equally increase THD. loading the opamp to both rails may reduce the THD but to what extend is for you to test. any modern opamp should not be demonstrating this type of classic problem, thats why my suspect is knockoff or out of spec (bad or damaged) opamp, ymmv.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 15, 2018, 05:01:29 pm
content: Quote from: SiliconWizard on February 15, 2018, 04:00:28 pmHi SiliconWizard,I overlooked those figures, so I have to see the graphics. Adding an extra part is not an option in this case, because the board doesn't have the necessary space. I'll have to consider replacing the part instead. Every opamp is an option, given it has the sufficient specs.Increasing the power supplies would be ideal, but in this case not doable. The board is fed via USB. I could add a second charge pump and used them chained. The first would double the 5 volts, and the second would invert the 10V generated by the first, to give the minus 10V. The problem is, again, lack of space.Increasing the feedback resistors would worsen the bandwidth. The parasitic capacitances at the opamp inputs would have a greater effect. I'm aiming to get 8MHz, and without attenuation (0.1dB is acceptable, though). Probably, considering another opamp is the best fix.Quote from: Mechatrommer on February 15, 2018, 04:57:31 pmHi Mechatrommer,My DSO is a 100MHz, 1MSa/s scope. It is a Owon SDS7102V, a crap according to Dave Jones. It has been serving me well, but the FFT is not to be trusted.By the way, I've tested the circuit using a 1K resistor from the OPA2830 output to -4.2V. Seen no improvements, but it does affect the distortion. You can see the images attached.Kind regards, Samuel Lourenço

author: David Hess
author_rank: Super Contributor
timestamp: February 15, 2018, 05:32:54 pm
content: Quote from: SiliconWizard on February 15, 2018, 04:00:28 pmIt is more complicated than just a dead zone. In the area near crossover, which is actually crossover of current instead of voltage, both output transistors are conducting at close to the same current and their transconductances and output resistances combine creating a dip or hump in the transconductance and output resistance which alters the open loop gain and output resistance. If the idle current is high, than it is a hump and if it is low then it is a dip.I do not know exactly what is going on in this case but it is the sort of thing I would expect with excessive output load. Maybe the part is running hot which is to be expected causing the thermal protection to cut off idle current to the output stage. Using a more powerful part or circuit or paralleling several of the same amplifier to distribute the current and power dissipation should solve it.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 15, 2018, 05:50:09 pm
content: Quote from: bloguetronica on February 15, 2018, 05:01:29 pmno entry level DSO can be trusted on this. the noise floor (or distortion) due to the 8bit limitation alone is already -50dB, that if given a perfect sine wave to it. and the cpu is not have enough power to do long memory FFT (greater than 1Mpts) to have good enough resolution to catch all harmonics. retry with proper spectrum analyzer to get a trusted figure.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 15, 2018, 06:05:04 pm
content: Quote from: David Hess on February 15, 2018, 05:32:54 pmHi David,The part is not running hot, and perhaps is barely warm to the touch. Notice that this distortion worsens with frequency and amplitude, above 2MHz. If it was a case of overloaded output, I should see it below 2MHz, right? I've used a T to put two terminators in parallel. At 2MHz, still there is no distortion. At 8MHz, it does get worse than before.However, I did some quick calculations after viewing the harmonic distortion graphs on the datasheet. For 5MHz, and 7Vpp output, I should have -45dBc for the 2nd harmonic and -50dBc for the third harmonic. This might explain what I'm seeing.Kind regards, Samuel Lourenço

author: duak
author_rank: Super Contributor
timestamp: February 15, 2018, 06:09:21 pm
content: I tried to find an internal schematic of the OPA2830 but no luck. To get the rail-to-rail output voltage swing it uses a common emitter output stage rather than the classical op amp emitter follower design. I would bet the output stage is a current mirror or some some variation and it can't supply any more current due to drive current limitations and/or hfe fall off in the output transistors. I would try loading the output more directly, say another 100R, to see what happens. The overall amplitude should decrease and the crossover glitch should increase in area.I ran across another distortion effect in rail-to-rail op amps - input crossover. If memory serves, R-R op amps are really two complementary single amplifiers in parallel. Under some conditions the transition from one to the other is not so smooth. Here's a vague write up:www.ti.com/lit/an/sboa181a/sboa181a.pdfCheers,

author: blackdog
author_rank: Frequent Contributor
timestamp: February 15, 2018, 06:11:43 pm
content: Hi,Can you show us some good quality pictures of the test setup you are using?Your scoop is not the probleem, one of my 4 scoop's is theSDS7102VIt is fine for analog design and mine is about 160Mhz bandwith, and i usen it on a regular basis on its battery so is have no commonmode problems.Kind regardsBlackdog

author: Damianos
author_rank: Frequent Contributor
timestamp: February 15, 2018, 09:49:31 pm
content: I try to follow this discussion but I'm confused!Some measurements seem peculiar: unloaded and loaded output has the same level instead of half? What is the measurement point?Somewhere it is stated that instead of a probe is used a coaxial cable (a transmission line). Is it terminated? A different length will give different results.At the right side of the product page there is the "WEBENCH® Designer OPA2830". If the requirements are entered there, what it suggests for the components values?

author: David Hess
author_rank: Super Contributor
timestamp: February 15, 2018, 10:31:13 pm
content: Quote from: bloguetronica on February 15, 2018, 06:05:04 pmThat is why I said maybe. I do not have a good hypothesis describing what is going on as I have never seen this exact problem except when the idle current is low. One of the first audio power amplifiers I designed did something similar above 1MHz but in a much more extreme way which was probably due to storage time in the slow output transistors.In this case, it looks like what I might expect do to overload into a capacitive load but a 50 ohms source termination into a parallel terminated transmission line is not capacitive. Into a resistive load, overload shows up near the peaks instead of at the crossover.QuoteThe sudden discontinuity just does not look right to me.Even a cheap DSO FFT should match what the datasheet says unless it is broken.Quote from: duak on February 15, 2018, 06:09:21 pmTexas Instruments is not forthcoming with data needed for engineering like schematics and this is one reason I try to avoid their products. It ends up being a waste of time trying to get their undocumented stuff to work.QuoteWithout the schematic it is difficult to guess what is going on but even common emitter output stages can suffer from odd effects if the idle current is low. The audio amplifier I mentioned above used a common emitter output stage driven by current mirrors so it had a lot in common with integrated rail-to-rail output amplifiers built on complementary processes.QuoteIt could be but the crossover point is usually near one of the supply voltages. I try to avoid complementary input rail-to-rail input stages because of this problem.Another possibility is *input stage* slew rate limiting which is unusual but possible. I have only run across it in comparators.

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 15, 2018, 11:05:55 pm
content: Hi,Well, I think only by experimenting another opamp I can draw some conclusions. I've ordered the board attached to test these opamps as buffers. I've done some pretesting and concluded that the gain is not a factor. In these boards the opamps are being used as voltage followers.So, lets wait and see. It can be that TI screwed up somehow (or I got screwed up by fakes).Kind regards, Samuel Lourenço

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 16, 2018, 12:59:38 pm
content: Hi,TI confirmed me that the part looks legit. Now it is a matter of waiting for the boards to arrive and solder on.Kind regards, Samuel Lourenço

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 27, 2018, 06:26:04 pm
content: Hi,I've decided to replace the problematic OPA2830 (IC14) by the LMH6612. It basically solved the distortion problem with a 50load. The function generator can output 5MHz, 8MHz, and even 10MHz with this new IC soldered on, and loaded, without distortion. There is a bit of attenuation at the higher frequencies, but that is due to the DAC envelope.However, the LMH6612 requires a higher supply voltage to prevent clipping (the output voltage is too close to the supply rails). It clips very hard, as you can see in the last image (output frequency set to 10KHz). Nevertheless, this can be solved.I still have to make those three breakout boards, two with OPA2830 ICs from different batches, and a third one with a LMH6612. I'll post the results here too.Kind regards, Samuel Lourenço

author: blackdog
author_rank: Frequent Contributor
timestamp: February 27, 2018, 06:41:44 pm
content: Hi bloguetronica,Nothing wrong with the clipping, looks good :-)No abberations visible!To get a better picture for showing on the web, use the averaging function of your OWON scope.This wil get rid of the noise on the signal and you wil have aa better view on the distortion if it's there, not masked by the noise.Kind regards,Blackdog

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 27, 2018, 08:35:19 pm
content: Thanks blackdog,I'm not a fan of the averaging, but I've tried and I really don't see any distortion, even past 12MHz. Attached is one taken with averaging, at 15MHz. The LMH6612 is really good and really sharp, by the way. I can get a nice triangular wave at 3MHz. This shows that the function gen will work as intended, as soon as I fix the supply rails.About the clipping, it is an easy fix. All I have to do is to increase the supplies to +/-4.5V. I've temporarily increased the negative supply rail to -4.58V by replacing one resistor with a close enough value (I was aiming to get -4.5V). You can see the results.Kind regards, Samuel Lourenço

author: blackdog
author_rank: Frequent Contributor
timestamp: February 27, 2018, 09:05:16 pm
content: Hi bloguetronica,Use averaging when it is sensible, so e.g. for documentation.When you are developing, you set the scope so that you have the best image for your application at that moment.Do not use a power supply voltage that is just above wat its necessary, take somthing extra.Kind regards,Bram

author: bloguetronica
author_rank: Frequent Contributor
timestamp: February 28, 2018, 12:14:18 am
content: Hi Bram,Unfortunately I don't have more room to increase the supplies. My negative rail is not very stable, since the LDO no longer has sufficient voltage drop to work with. Thus, I need to decrease it to -4.5V instead of the 4.58V that I've got after the fix with the resistor values I have. This means a proper feedback resistor to allow for the required minimum dropout of 500mV.The voltage supplying the LDOs is +/-5V. I'm planning to have +/-4.5V at the LDO's outputs. My headroom is none. But I'm still studying the possible values.Kind regards, Samuel Lourenço

author: blackdog
author_rank: Frequent Contributor
timestamp: February 28, 2018, 11:48:26 am
content: Hi SamuelMaybe put the power supply for the output opamp directly on the buffer capacitor.Or first a small resistance and then an extra buffer capacitor of say 4700uF.Just 2 cents..Regards,Bram

author: The Chump
author_rank: Contributor
timestamp: February 28, 2018, 07:18:23 pm
content: The distortion you are seeing with the OPA2803 is crossover distortion, but in this case it is being caused by the speed limits of the output transistors of the IC.As frequency goes up, more and more current is required to switch the transistors on and off. They also take longer to turn off than to turn on.And as the current increases the turn-off time increases. Add to this the feedback atttempting to correct the error and you get the waveforms you see.Unfortunately, as you have found, the only way to fix it is to figure out where this has been hidden in the Datasheet so that you can make a better choice.CheersAlex

author: bloguetronica
author_rank: Frequent Contributor
timestamp: March 17, 2018, 01:03:18 am
content: Quote from: blackdog on February 28, 2018, 11:48:26 amHi Bram,Unfortunately this is a USB powered function generator I've been working on. Using a that beefy capacitor would pose an issue and, probably, would violate the USB spec.Quote from: The Chump on February 28, 2018, 07:18:23 pmHi alex,That seems to make sense, but mind that only happens when the opamp is loaded with a total of 100. I wonder how they claim the slewrate and the bandwidth they claim.The LMH6612 fares better, although the specs are comparable to the OPA2830. I guess that the LMH6612 beefier output capability wins here. Never expected the former Burr-Brown part to perform poorly in this situation.Kind regards, Samuel Lourenço

author: bloguetronica
author_rank: Frequent Contributor
timestamp: April 06, 2018, 05:25:59 pm
content: Hi,The boards I've mentioned earlier arrived yesterday, and thus, I was eager to assemble them. So eager that I've soldered some of the BNC connectors in the wrong orientation. However they worked well for the purpose of testing the OPA2830 versus the LMH6612. I've assembled two of them with OPA2830s (chips from different batches) and one with a LMH6612.As expected, I confirmed that the OPA2830 distorted when loaded. Seen the same distortion on both of these boards. Conversely, the LMH6612 presented no distortion at all! All boards were tested using a supply voltage of +/-5.5V . A 50termination was used (this corresponds to a load resistance of roughtly 100). Notice that both OPA2830 and LMH6612 are dual opamps. The two opamps inside each chip were used as voltage followers.You can see the photos attached. The tests were done using 8MHz and 16MHz sinusoidal input signals. Note that, in the case of the OPA2830, the distortion worsens with frequency. The attenuation you see at 16MHz, on both cases, is not caused by the boards, but is a "characteristic" of the function generator, that is not able to generate frequencies higher than 8MHz without attenuation. After all, it is a 80MSa/s device.Kind regards, Samuel Lourenço