Out of PCB space, - Page 1
Forum section: electronics / projects

author: MarkF
author_rank: Super Contributor
timestamp: June 05, 2016, 07:19:30 pm
content: Does anyone see a way to reduce this circuit to 2 Op-amps instead of the 3 shown? Maybe a way to combine the summing amp (B) with the offset amp (C).Edit:Some clarification:1) The MCP4802 output is 0V-4V.2) The AD9834 output is a differential output and the op-amp (A) is +/- 2V.3) The op-amp(s) power is +/- 15V.

author: Yansi
author_rank: Super Contributor
timestamp: June 05, 2016, 07:26:28 pm
content: Sure. Try to use the opamp on the left for the job the bottom one is doing.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2016, 07:57:23 pm
content: Usually some filtering should be done before the amplification. The OPs have a hard time and can produce artifacts with all the high frequency components.I don't think it's a good idea to send the sign out directly to an output. Capacitive load, even just a cable can cause trouble.You might even get away with only one OP, if the offset does not need to be to big.

author: Yansi
author_rank: Super Contributor
timestamp: June 05, 2016, 08:04:48 pm
content: Yeah, the sign output should be better buffered. Slam together a few logic gates like 74HC04, outputs & inputs parallel and then match the output impedance to 50ohms with a series resistor (its value depends on the resulting output impedance of the gates).

author: MarkF
author_rank: Super Contributor
timestamp: June 05, 2016, 08:21:21 pm
content: Quote from: Yansi on June 05, 2016, 08:04:48 pmYou do realize the AD9834 is a DDS?The output can be either a sine or triangle waveform. All I'm trying to do is level shift the Vpp output of op-amp (A). For output (A) of 2Vpp, I would like to be able to offset it at least +/- 2V.As for the "Sign Bit Output", I'm not too concerned with. I may never use it.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 05, 2016, 08:23:29 pm
content: Why not use smaller surface mount devices (e.g., SOIC) which are not any harder to work with than through hole? Have you considered a quad version of the TL082 (TL084), its newer version (TLE2084) or the Linear "equivalent" TL1058 in a quad?John

author: Yansi
author_rank: Super Contributor
timestamp: June 05, 2016, 08:31:19 pm
content: Quote from: MarkF on June 05, 2016, 08:21:21 pmI do realize it quite well, but you don't read well.Sign output should be also buffered. Sign output is a logic output.And also don't forget about this paragraph from the datasheet:Quote

author: MarkF
author_rank: Super Contributor
timestamp: June 05, 2016, 09:01:56 pm
content: My point is that I'm NOT interested or using the Sign Output.My main interest is the differential analog output. This is a one-of after thought. I've been using this PCB for software development for another project and thought I'd add the DDS to get some limited utility out of the board.Here are a few photos. Sorry for the poor quality.

author: MarkF
author_rank: Super Contributor
timestamp: June 05, 2016, 09:16:50 pm
content: UPDATED DRAWING.Can we get back on topic now???

author: Yansi
author_rank: Super Contributor
timestamp: June 05, 2016, 09:42:52 pm
content: You can offset shift the signal right in the first opamp. Connect the vertical 2k7 resistor not to ground, but to whatever ref voltage needed. You can then sum with the DAC voltage directly, without the additional opamp to invert and offset DAC voltage.(Sidenote: It's hard to tell you what to do with the schematic, if the components are not numbered.)Also, as was already pointed out, the overall circuit topology is not the best. The lowpass filtering should be applied right after the DDS output. The filter also has to have a correct termination resistance at both ends. Otherwise the filter performance will vary with the loading of the filter output. Another reason has been already stated, the the opamps will be more satisfied working with already filtered signal :-)And BTW, the DDS is quite fast for those TL084. What maximum frequency output from the DDS are we talking about?

author: MarkF
author_rank: Super Contributor
timestamp: June 05, 2016, 09:59:38 pm
content: Quote from: Yansi on June 05, 2016, 09:42:52 pmI think I see what you mean. See drawing. The output of op-amp (C) will need to go +/- to provide both positive and negative level shifting.I'm only using a 32.768MHz clock for the DDS, so the maximum useful frequency is only going to be around 1MHz. I'm not expecting great performance. Just a signal to test with now and then.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2016, 10:21:15 pm
content: Also, the output filter is 75 ohms and 25MHz, neither of which really makes sense here (100 ohm series resistor, ?ohm output)? If you're going for an antialias filter, it should probably be placed at the DDS output pins, since the TL082 isn't going to know what to do with it anyway.Yeah, you only need one amp, and perhaps a buffer (to drive the, I would assume 50 ohm, output). There are lots of 5V RRIO amps with good performance (speed and offset) you can use, so you don't even need the +/-15V rails. Although you'll still need at least +/-5V if you want a normal bipolar output, of course...To get offset in there, just run a resistor from the '4802 to the op-amp -in. The resistor acts in parallel with the other two resistors on that node, so you should have an equal amount in parallel with the complementary (2.7k) resistor on +in.Tim

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 05, 2016, 10:22:53 pm
content: Why not go all smd? You can get tiny packages and would elimintate the need for removing an opamp

author: Yansi
author_rank: Super Contributor
timestamp: June 05, 2016, 11:02:14 pm
content: I'd suggest a few mods to the circuit, to increase the performance, significantly, for little cost extra:1) get rid of the differential output from the DDS. It can work single ended. Terminate both outputs with 200ohm resistor (as you have already) and use only the IOUT output.2) design a better lowpass filter, using for example old AADE Filter Design software tool. Very old and rusty software, but HELPFUL a LOT. Design the filter as a lowpass 200ohm at both ends, 5th or 7th order. Design the cutoff frequency as low as possible, to get the best performance. So for your maximum 1Mhz output, you can use a cutoff of about 2-3MHz (still reasonable, as your DDS clock is 32MHz, enough margin there). Place another 200ohm termination resistor after the filter (so it will be terminated 200ohm both ends).3) use some cheap and fast signal opamp to amplify a bit and offset shift the signal. Something like OPA354 or LMH6611 (with these, you can go way up with the output frequency!)4) use some suitable buffer amplifier, preferably a CFA like AD811 to power and current boost the output. Make sure you don't exceed it's power dissipation, sticking a small heatsink to it recommended anyway. Place 50ohm resistor on its output, to match the output impedance to such standard value.I know, this is not a full cookbook for making a sinewave signal gen. with a DDS, but might give you some basic ideas.

author: MarkF
author_rank: Super Contributor
timestamp: June 05, 2016, 11:47:55 pm
content: I've just thrown this together with junk parts that I have on hand. Didn't want to put much into it. I will spend some time looking at everyones suggestions. However, it all has to fit on this little prototype board. This PCB was intended for doing software development and I never intended to put the DDS on it. The output filter I just copied from somewhere and didn't even check the 3 dB freq.

author: MarkF
author_rank: Super Contributor
timestamp: June 06, 2016, 01:01:35 am
content: New output filter with 2 MHz cutoff.

author: Yansi
author_rank: Super Contributor
timestamp: June 06, 2016, 01:04:21 pm
content: Please read carefully our suggestions again. The design is still very wrong.You still have the filter in a wrong place. You still do not have well defined output impedance of the filter. It is nice to have there a 51ohm terminating resistor, but that wil work only without load on the BNC output.The filter has to be directly on the DDS output. Use only single endded output, use 200ohm filter imepdance.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 06, 2016, 03:16:39 pm
content: The outputs of the AD9834 are rather fussy due to being drains (thus high-z nodes). Throwing a lot of reactive parts for filtering right at their outputs has caused some curious behaviors as a result.OP: I haven't seen much on the "what I'm trying to achieve" front. What is the minimum frequency you're attempting to obtain? You've mentioned max of ~2MHz (given filter cut off).Could you give us a run-down of the actual specs you're after for the design? That will be the easiest to get you to your goal.

author: Yansi
author_rank: Super Contributor
timestamp: June 06, 2016, 03:49:28 pm
content: Bullshit. The outputs of the DDS are whatever but current sources and should be terminated with resistors. As it is clearly stated in the datasheet. Current source is impedance in its natural form, so is the impedance defined by the terminating resistance. So it is rally completely correct to playce the filter directly on the output! If the current source is terminated with 200ohm resistor, then the output resistance is well defined 200ohm. YOu can use that directly to feed the LC network.Quote

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 06, 2016, 04:21:16 pm
content: Quote from: Yansi on June 06, 2016, 03:49:28 pmThe terminating resistors do present a DC load of whatever the impedance is, yes. No one is disputing that. But, perhaps, in all of your masturbatory self flagellation, consider the reactive nature of that filter as it looks to the high impedance node of the drain or collector. Suppose a classic 3rd order Pi filter were used at 2MHz. The shunt Cs being 1.59nF and series L being 15.91uH. For the sake of evaluation, we'll pick 1MHz as a test case. At this frequency the caps provide -j100 ohms of reactance. The inductor gives j100. What is presented to the high impedance node? Through the transform at 1MHz, about 200 ohms.(Process:1. Admittance of output C with 50ohms: 0.02 + j0.012. Impedance of transformed shunts: 40 - j203. Add +j100 for inductor: 40 +j804. Resolve to admittance: 0.005 - j0.015. Add input capacitive admittance: 0.0056. Which is 200ohms of impedance. )Repeat this at 10kHz, 50+j0.5 ohms.It isn't uniform, and as such, quite erratic. Using a DTC or STC, or only operating over a very narrow band where the circuit is "in tune" is one thing, but as broad-band as the OP might be trying to pull off, no. If you still disagree, either show the error in the math above, or go spread your disinformation to some other forum and stop being an intellectual poison here.Supposing the 3mA swing relative to DC bias, the output node will present a 150mV amplitude signal at 10kHz, and 600mV at 1MHz. The outputs will then follow the same irregularity and non-uniformity because of the reactance that filter provides.Put into the frequency domain, and we see why:That terminating resistor is not padding the filter property due to the influence the high-impedance source has.The filtering is important for a number of reasons, but knowingwhereto filter and how to apply it correctly are also equally important. Designing a stage for the DDS' outputs to feed into in a way that provides uniform behavior is quite important. The first goal is then to translate the touchy, sensitive outputs into something the designer has control over. Thus, the questions about what he is trying to achieve.You can dismiss yourself now.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2016, 05:58:44 pm
content: Heh...You're using the wrong filter design.The CCS is an open end. You have to use a one-end-open, other-end-terminated design. There are tables for this (which I don't have handy at the moment).Or terminate it so it's a Norton equivalent source (and use a doubly terminated filter design, which is the conventional type). The downside is -6dB in voltage, but this isn't a big deal here.Tim

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 06, 2016, 06:22:46 pm
content: It was an example that illustrated the point, and yeah plenty of tables online depending on the need for Butterworth, Chebychev, etc. Their use still requires an idea of what problem is trying to be solved.And quite, terminating locally (in addition to the far-end term) to do the Norton equivalent will do it - 6dB loss be damned - but that still requires careful consideration of the high-z node. For some of my tuned amplifiers, I'll just set the bias conditions for the targeted impedance I need, use the resonant components to do whatever function I'm after, and get my output, but it still requires thinking about what I'm trying to pull.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 06, 2016, 06:49:52 pm
content: For fun, I did a quick analysis doing the doubly-terminated version, there is still a hazard of exceeding compliance range on the IOUT/IOUT# signals if the reactance isn't given careful attention still. Do a 3rd order Pi filter with Zt=400 ohms for maximum voltage, the Cs are 200pF, and the L is 63.6uH. Indeed you get a nice maximally-flat LPF at 2MHz on the output. At the input, right at around 2MHz, you get a reactance of 299-j100 ohms which generates a peak of 947mV, 347mV above compliance (the AD9834's datasheet didn't go through the rigors that some of the newer DDS parts have).

author: Yansi
author_rank: Super Contributor
timestamp: June 06, 2016, 07:51:15 pm
content: Quote from: XFDDesign on June 06, 2016, 04:21:16 pmDear XFDDesign,don't you think that your behavior is worse than a five yr old childs? What did gave you the right to call me such way, if you, yourself, can not follow simple and clear instructions for a simple filter? Where did you get that feeling, the filter should be connected one side to a high impedance current source? I've said many times, the DDS has to be terminated, 200ohms to ground with a resistor (to make a norton source equivalent). Also I have clearly stated, the resulting filter have to be terminated both ends. Which part didn't you understand?QuoteThis is a correct way to do it. Full stop.Best regards,YanEDIT: Added picture for those without basic imagination.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 06, 2016, 08:14:43 pm
content: Quote from: Yansi on June 06, 2016, 07:51:15 pmWhen your first response is "Bullshit," and then flap on in masturbatory grandeur, no. If you're going to approach as a grand wizard, and come up a fraud, you'll be treated as a fraud.QuoteQuoteI guess, from you. Both drains and collectors are high impedance nodes (in linear operation). Gluing a filter to them and then terminating it is still bolting a reactive network to high impedance nodes. Doubly-terminating still doesn't even solve all problems which I'll get to below.QuoteAs you bag on the OP for not reading, its' fascinating that you've failed to look at the schematics in my post. There you will find that, indeed, the output is terminated, be it 50, 200, or 400. In fact, it's terminated to ground. Your post mentioned nothing about norton... but given:QuoteLet us quote your post in response:QuoteThe phrase "terminated on both ends," is found precisely zero times. The DS mentions only singly terminating with 200 ohm resistors, which is due to the consideration of the high impedance nodes needing to be constrained to within compliance voltage. Thus far, you've yet to actually refute anything, justifying your claim of "bullshit."QuoteSingle-ended, OP could just ground the unused output and save the space of 1 resistor. Singly-terminated still won't solve the very same problem you've failed to rebut in the post I followed up with wherein math was used (as well as spice) to illustrate the problem with your self aggrandized claim.QuoteWhich is a statement strangely missing in the primary "Bullshit" post you made, and even then, as I pointed out just prior to your post which I'm addressing, still isn't guaranteed, andit is backed with mathQuoteNo, it's a remarkably piss poor attempt at saving face after you were just shown to be full of it. You've not actually refuted the points which called your "Bullshit" claim out as little more than a haphazard spaghetti-against-the-wall design methodology.Thecorrectway to "do it," with regards to engineering, is to show me the math, or leave. If my math is wrong, and you're right, prove it. I have applied the math on both approaches, and shown that both still have issues which result in the appearances on the drains to be sensitive. I've backed up my claims, you haven't.

author: Yansi
author_rank: Super Contributor
timestamp: June 06, 2016, 08:41:57 pm
content: I am leaving this thread, as I am not willing to compete with trolls like you. If you are so clever, why didn't you offer your masturbatory advice first?QuoteI might have missed a word there, directly on theterminatedoutput. But any person with sensible IQ should get that from the sentence before, which was repeated several times, to terminate the DDS outputs 200ohms to ground.And then, to be sure, i also stated "it will be terminated 200ohm both ends", which should give you some clear ideas.Bye,Yan

author: void_error
author_rank: Frequent Contributor
timestamp: June 06, 2016, 08:43:52 pm
content: Analog Devices hasthis nice tool.Thought it might help. Someone on the forum pointed this out to me a while ago so don't thank me.

author: void_error
author_rank: Frequent Contributor
timestamp: June 06, 2016, 08:59:44 pm
content: Foundthistoo, check page 47. Doesn't get more clear than that.

author: MarkF
author_rank: Super Contributor
timestamp: June 06, 2016, 09:08:32 pm
content: I'm just looking for a simple sine wave centered around 0 Vdc with a 2-4 Vpp maximum amplitude terminated into 50 ohm output. In addition, I would like to be able to level shift the sine wave to all positive or all negative. I believe I would need to change out the op-amp in order to drive a 50 ohm load.Since I'm using a 32.768 MHz clock for the DDS, I only expect a maximum frequency of 1 Mhz. With a low pass filter below the nyquist frequency. A cutoff frequency of 2 MHz can be the target point.I was using the differential outputs to get a +/- sine wave output without using a capacitor to block the DC component of the DDS.Also, using Analog Devices Circuit Note (CN-0304) attached as a guide for circuit layout.Bottom line: Think poor mans signal generator from spare parts.

author: void_error
author_rank: Frequent Contributor
timestamp: June 06, 2016, 09:38:41 pm
content: Finally remembered what I used to calculate resistors needed for removing DDS output offset. It's based onthis, except itdoes all the calculations for you, saving you a lot of time.I revisited my now dead*Waveform Generator Project, you may want to grab some ideas from there.Used the calculator I linked to get all the values for my level shifting op-amps.*not actually dead, part of something bigger now

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 06, 2016, 11:10:16 pm
content: Quote from: MarkF on June 06, 2016, 09:08:32 pmMy personal, general, rule with a DDS is the maximum usable frequency is 25% of your MCLK, so you can reasonably get 8MHz out. You can actually get more out, up to 16.384MHz, but you start getting a lot more spurs and other garbage. You still didn't mention what your "minimum" frequency is that you want. That is going to have ahugeimpact. There is a catch with DDS devices however, and that's the balance of your low-pass frequency corner and how much "near DC" you want. You're going to have a limitation due to your DAC, and to understand what its limitations are, you need to understand a bit about how the DDS works:Inside the DDS, there is a phase accumulator - 28 bits in the case of the 9834. This accumulator is taken, and put through a look-up table to resolve linear phase to cos/sin. The 'step' size of every tick is 2^28/(f_master/fout). Suppose you want fout=1kHz for your 32.768MHz master. This gives phase_step = 268,435,456 / 32768 or 8192. So every count of f_master, the phase accumulator adds 8192 counts to itself. Now, 2^28 = 2pi, so every step is 2pi*(8192/268435456) radians (191.7uRad) - a very small quantity. Your DAC is only 10 bits. For for all intents and purposes, just chop off the bottom 18 bits of your current accumulator word - the LUT & DACcannotresolve an output smaller than 1LSB. At 1kHz, the DAC will update at 32.768MSPS, but can only handle 1024 discrete steps. Changing the output from 0 counts to 1 count on the DAC (976.6uV rel 1V FS) wont happen until the phase accumulator has 55.953mRads of value. 0.055953/0.0001917 = 291.8 MCLK ticks - so your output will be 0 for 8.88µs. This time will diminish as you get closer to the zero-crossing value, and then expand again - you still get a stair-step output. Set your low-pass filter closer to 1kHz, and you get a nice sinewave. Set it at 8MHz, and you get a 'stairstep sine'. You can see how this gets tricky.So you need to ask yourself what you're willing to tolerate. Is a pixel-art sinewave okay? Drop in a single reactive element filter at 2MHz or whatever you wish, and call it a day. Would you like your output to be reasonably well behaved? You'll want to start looking at splitting your output path. So we still need to know what yourminimum desired output frequency is.

author: MarkF
author_rank: Super Contributor
timestamp: June 06, 2016, 11:37:59 pm
content: The minimum frequency I would like to get to is 1 Hz.The 8 MHz or even 4 MHz only gives me 4 or 8 samples. That's not much of a sine wave. That's why I figure 1 MHz, or maybe 2 MHz, would give me a pretty clean sine wave even before filtering. I'm only looking for a simple, general purpose function generator. Not looking for anything special. Just a clean output with frequency, amplitude and offset adjustments.At this point I have the pic18f2550, AD9834 and MCP4802 all working with the control adjustments. I just need to tie the analog output section together.Note. I've been doing digital signal analysis and generation for thirty years doing software development for ASW sonar systems.

author: MarkF
author_rank: Super Contributor
timestamp: June 06, 2016, 11:59:40 pm
content: That's okay.I had similar thoughts in using the differential outputs to reduce noise when the output is set to small amplitudes.As for the filter. Everyone here is saying before but Analog Devices own circuit has it after.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 12:03:03 am
content: Quote from: MarkF on June 06, 2016, 11:37:59 pmAh okay that makes it a lot easier.So you can get a "decent" sinewave off of 4 samples with a good filter - but that's the catch. Needing 1Hz makes things a bit tricky. Adding gain control complicates things a bit more too. Offset is easy.With 1Hz, fmin, capacitive coupling becomes a giant pain in the ass, but I think you mentioned you have a split supply of ±15V, so we can have a nice DC patch. I'll start doing some numbers for you. Do you need continuously adjustable gain, or can fixed-step gains work? Over what range?Quote from: MarkF on June 06, 2016, 11:59:40 pmYup. Everyone from Analog Devices is sayingafterand for a good reason.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 12:13:29 am
content: Check my last circuit. Gain control is being done by adjusting the reference voltage for the DDS with one of the MCP4802 outputs. The PIC can adjust the DDS output from 40 to 800 mVpp. I was going to amplify that to a max output voltage of 2 V or maybe 4 V. Haven't decided.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 12:17:30 am
content: Quote from: AcHmed99 on June 06, 2016, 11:46:34 pmI fight with this kind of approach a lot. The benefit for using a diff-to-single-ended amp like this (or any in the family) is simplicity. That App Note relies on a single-pole filter from those shunt RCs, which requires (to a degree) that you run the output a bit higher up in frequency to get nice shaping. The AD8130 is an older part now, and while there are newer parts like it, the issue is that you'll still get a surprising amount of garbage shooting through the part. Layout becomes very critical, as coupling into your power supply (and those 0.1uF caps can have a good amount of ESR and ESL) becomes very easy through stray capacitance. Figure even 2pF at 125MSPS (per the AD9117) may only be 636 Ohms, the PSRR of the AD8130 is only 10dB at 100MHz - you can couple in a LOT of garbage.As a result, I actually avoid using these kinds of parts unless I have to - and when I have to, I go through all kinds of fanatical gyrations to deal with these problems (and even then I do much more filtering up front).Using either two packages of single-amps, or one dual, doubles the problem. Youcando it, but you have to imagine these parts being close together means they have an easier time "singing" to one another. You also needreallygood device matching. If Device 1 has its GBW crossover at 100MHz and device 2 is at 101MHz, you're going to get phase errors which then pop up as a distortion component. So then either limit the signal that you're using for given amplifiers, or buy even faster amplifiers (which then increases the sensitivity of the parts to more high-frequency problems!)The general approach I use is _only_ my approach. It is not the universal "only way to do it." I look at the maximum frequency I need out of my source. I'll drop in a _dirt stupid_ buffer of some kind which eats all of the signal in the band of interest, and a little extra. Now that I have something I can control, I'll do something like a balanced filter to terminate all of the garbage that comes out above the signal band of interest. Once I have things filtered,thenwill I feed them into a diff-to-single-ended output amp, or even use two amps in one package (I'd never use two discrete amps). Doing this, the amplifiers are presented "well behaved" signals and it is easier to contain their behavior.Does this answer your question?

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 12:25:05 am
content: Quote from: MarkF on June 07, 2016, 12:13:29 amYou mean this one?https://www.eevblog.com/forum/projects/out-of-pcb-space/msg956095/#msg956095Sorry man, that isn't controlling your gain variably - unless you just mean a fixed gain.Just for the sake of argument, let us increase the resistors around the TL082 100x so that we can treat the output of the DDS as producing a voltage on its outputs.Let us vall the output of the MCP4802 by way of the TL082 pin 8 as "vset.We have 3 inputs, 1 output. Apply superposition.Vo = vset*(1k/3.7k)(1+2.7/1) + v_iout*(2.7/3.7)*(1+2.7/1) - v_ioutb* (2.7/1)Vo = vset _ 2.7v_iout - 2.7*v_iout_bStick it into spice and you'll see it validate this. It works as pushing some offset into your circuit, it even gains your DDS signal in a fixed way (if the '082 had enough BW, which it doesnt), but it doesn't actually gain or attenuate your sine source variably. For gain of your signal, you want to see:Vo = K*v_iout - K*v_iout_bbut you don't have this.Edit: so did you need variable gain, or was fixed okay?

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 12:46:37 am
content: Quote from: AcHmed99 on June 07, 2016, 12:32:30 amThat is what has worked for me. YMMV

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 01:22:38 am
content: Quote from: XFDDesign on June 07, 2016, 12:25:05 amYes. Output A of the MCP4802 is driving the FSADJ (pin 1) of the AD9834. Normally, FSADJ is tied to GND through a 6.8K resistor. However, driving FSADJ from 0 - 1.2 Vdc will adjust the output amplitude. See attached document.It works very well. Thank you very much.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 01:29:24 am
content: XFDDesign-Can you put forth a schematic showing how you would do the analog stage after the DDS using output B of the MCP4802 to control the offset.The output stage only needs to do amplification, offset adjustment and filtering. Amplitude adjustment is done within the DDS.I'm having a hard seeing the layout you're describing.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 02:08:50 am
content: Ohh got it, you're not doing gain adjust from the Opamps but rather though gaming the 6.8k resistor to set the current. Clever!Ok, so the most direct, and simple way I would go about it (so long as LF sine waves can be "pixel-art") is to use two NPN followers then a balanced second-order lowpass (you could do a higher order).The Ideal Source E1 would be replaced with a diff-to-single amp like the AD8130 (mentioned earlier but it has limits of ±12V supplies instead of ±15). This handles your DC offset, plus you can get DDS final-gain set as well with this one part. I'd buffer the output of the device with a THS4041 from TI - this can really dish out the current, run on ±15V supplies, and is all around a solid part. Stick a 49.9 ohm resistor on the output to your output-connector, and you're done.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2016, 02:17:53 am
content: Eh? I don't get where this thread is going...If you well and truly need a buffer, one emitter follower is enough. Or an op-amp, which can be the diff stage right there.Or you can filter it however you like. Nevermind compliance range; if that's an issue, you can simply reduce the filter impedance, or use a constant resistance filter such as this.http://jeroen.web.cern.ch/jeroen/reports/crfilter.pdfWhile the set of responses you can get from real passive filters is somewhat limited, it has by no means been exhausted from what's been discussed so far. There are equal-termination filters, unequally terminated filters, one side unterminated, constant-resistance, diplexing, and more.So the point still stands: you can put the AA filter right at the DDS output pins if it saves op-amps and PCB space.Tim

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 02:28:52 am
content: I use the darlington pair to minimize loading on the DDS output. The bias in the "output" bjt is a bit high in terms of Ie so it demands a bit much from the source driving it. I avoid opamps for reasons I stated above, until after the filtering (which is based off of my experience, YMMV). The original point I had brought up is that without having a good idea of "the problem we're trying to solve," there are all sorts of 'gotchas' that can and will come up. Yes, one can put the AA filter right on the output of the DDS - so long as they are designing for it and know the issues that can crop up (such as compliance range as just one point). It doesn't make the drain outputs any less fussy and free from careless design eh?

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 02:32:11 am
content: Quote from: T3sl4co1l on June 07, 2016, 02:17:53 amI don't either.There's no way I can add all those transistors and resistors to the current PCB. Let alone a surface mount op-amp. I only have about 1 sq inch of board space available.I only have room for the one dual op-amp in an 8 pin package. And I may just stay with the TL082. The frequency response seems to start rolling off around 500 KHz with a 2 Vpp output. The output current of the TL082 is much lower than I would have liked. For the time being, I'm okay with that.There have been a lot of different variations but nothing that really helps with a differential input plus an output offset adjustment. I guess I can just drop the differential inputs (was hoping it would reduce noise at low amplitude settings) and just do the normal summing amplifier.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 02:34:06 am
content: I was thinking the filter would be:

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 02:37:51 am
content: If your space is _that_ constrained, then yeah, skip the active stuff and take a penalty in band-flatness. Do it single-ended right on the output of the DDS as others have said (maybe go for a 150ohm normalization to keep in compliance) but at least change up to an amp with a better GBW and slew rate.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 09:16:56 am
content: This appears to be the op-amp configuration I've been looking for. I just need to work out some resistor values and do more testing.

author: Brutte
author_rank: Frequent Contributor
timestamp: June 07, 2016, 10:00:19 am
content: Quote from: MarkF on June 05, 2016, 07:19:30 pmThe op-amp arrangement takes three inputs (IoutB Iout Vout) of known and constant output impedances and has one output (100R) followed by LC. No internal states (only resistors involved), same power busses.A is differential, B is inverting, C is a buffer and D is just paperweight.For this arrangement one op-amp is more than enough, methink

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 01:28:52 pm
content: So why the hang-up onrequiringdifferential signalling, especially when it's making things more complicated?You can use a straight up diff-amp design, with a single-ended output of the DDS going into one side, and the DC offset going into the other. You could implement this in a SOT-23 op-amp so long as it was high speed. Again, this is why you really need to lay out all the requirements of the problem you're trying to solve as it keep the target in place. With the space constraints you've added in (i.e. 1"sq) I might even accept the idea of using an active filter just for size concerns.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 01:51:14 pm
content: Like I've said over and over, I have re-purposed this PCB from a small software development and test PCB to adding a DDS. There has been NO up front design or thought given to this design. See the photos of the prototype PCB I'm using. And it's also the reason SMD parts are a NO GO.I am NOT laying out a PCB to be manufactured!!!

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 01:53:54 pm
content: Quote from: MarkF on June 07, 2016, 01:51:14 pmAll the more reason to do single-ended from the DDS. You can still dead-bug SMT parts, no one said anything about turning a PCB. As to the lack of thought given on the design, it is what it is. No need to get frustrated as other issues pop up due to lack of forethought. Are you unwilling to dead-bug surface mount parts? Is the restriction 1" square, with axial resistors, radial capacitors, and DIP package amps?

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 01:57:47 pm
content: I see no reason to waste time on SMD parts if I can't get a through-hole version of them.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 02:13:49 pm
content: Quote from: MarkF on June 07, 2016, 01:57:47 pmIt lets you do more in a smaller space. But, I'll take that as affirming the constraints I listed.Ok, so then we're going to throw "good" out the window and go for workable.You can use a dual amp, something high speed enough. Filtering is done with one of the two amps. Passband won't be beautifully flat. Cutoff is 2MHz according to the calculator tool. Output is a simple diff amp. Vout=voffset_set - v_dds

author: Siwastaja
author_rank: Super Contributor
timestamp: June 07, 2016, 03:14:24 pm
content: Quote from: MarkF on June 07, 2016, 01:57:47 pmWaste time? Wut?This makes absolutely zero sense whatsoever. This choice will limit you more thananything else, and more and more so as DIP packages are getting (already are) obsolete. You pretty much can't do much anything when arbitrarily limiting yourself to through hole parts, and this has been true for about 20 years!!I remember how in the 90's people were a little bit reluctant on the News (internet forum equivalent) about going into SMD, but had no choice but to learn new things, even then there were so many SMD ICs to be missed otherwise.I'm surprised this still pops up, in freaking 2016!But it's your choice to make, however irrational.Restrtiction to DIP parts may be a fun retro game, but if you just want to get things done without wastinga lot oftime, consider upgrading to the 90's. You have so much more to choose from with modern tech.If this is about the fear of unknown, just tackle it. I promise it's going to be easy. I have seen total beginners just start soldering things together from SMD only in mere hours, because well, that's what we have today. I have taught my girlfriend with zero electronic experience to solder SMD parts in an hour. But total beginners have one distinct advantage; they are not imprisoned by irrational mental models originating from the experience of doing things in a certain way for a long time.Have fun.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 04:03:12 pm
content: Yes! A waste of time for this project.If you look closely at the two photos above, you will see that I'm not against SMD parts.The pic18F2550 and the AD9834 are both SMD parts.But, I don't have the space or any way to mount a surface mount part on this PCB.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 04:19:12 pm
content: Now, can someone recommend a higher bandwidth dual op amp that is plug replaceable with the TL082 in an 8 pin DIP package?We must work within the physical constraints of this PCB.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 07, 2016, 04:23:37 pm
content: LM6172 looks like an okay fit.

author: MT
author_rank: Super Contributor
timestamp: June 07, 2016, 07:02:56 pm
content: Quote from: MarkF on June 07, 2016, 09:16:56 amA better diff amp are the cascaded:

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 07, 2016, 07:29:29 pm
content: Quote from: MarkF on June 07, 2016, 04:03:12 pmPerhaps a picture of this "PCB" you keep referring to. Give people an idea of your problem

author: jpanhalt
author_rank: Super Contributor
timestamp: June 07, 2016, 08:14:01 pm
content: Quote from: MarkF on June 07, 2016, 04:19:12 pmAlready did, but you ignored it. The TLE2082 has unity bandwidth of 9.4 MHz compared to 3 MHz for the TL082. What bandwidth do you need? Have you tried the parametric selection tools at Digikey or Mouser?John

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 08:48:01 pm
content: Quote from: Aodhan145 on June 07, 2016, 07:29:29 pmSee reply #7 on page 1.Quote from: jpanhalt on June 07, 2016, 08:14:01 pmSorry, I guess I missed it. I thought I looked up all the parts suggested.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 07, 2016, 09:10:28 pm
content: Quote from: MarkF on June 07, 2016, 08:48:01 pmahah is that a joke. by everyone saying use SMD parts putting them on breakout boards defeats the purpose they make it larger. Make a real pcb using smd components and you will not have any problems.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 09:18:16 pm
content: Quote from: Aodhan145 on June 07, 2016, 09:10:28 pmIf I was making a PCB, we wouldn't be having this discussion.This is a one-of prototype board that has suffered serious requirements creep!

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 07, 2016, 09:52:25 pm
content: Quote from: MarkF on June 07, 2016, 09:18:16 pmLOOK AT WHAT YOU CALLED THE SUBJECT?maybe out of perf board space or out of protoboard space. I thought that was your prototype you were turning into a pcb.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2016, 09:53:21 pm
content: Most dual OPs have the same layout. Another alternative would be the NE5532 - this one also provides a little more output current. However the slew rate limits the useful amplitude at high frequencies. The choice and availability of fast OPs in a DIP case is somewhat limited. So check your source.At least some filtering should be before the amplifiers - otherwise the amplifiers get critical and need a very good layout - so need a good PCB, not prototype board. The active filtering is not a good idea at such a high frequency, as this would need high speed OPs. The filter will not stop signals higher than GBW of the OP effectively.Going differential is attractive with this type of amplitude adjustment, as the offset will not shift with amplitude.Though it needs twice the parts, am LC filter can also be used differential. One could get away with lower order (e.g. 5th or maybe oven 3 rd order). This is especially true if the cross over is chosen not so high, e.g. use a 4 MHz corner. If space is scare - SMD parts in 0805 / 0603 size parts can be used for the filter - they fit reasonably on a prototype board.

author: MarkF
author_rank: Super Contributor
timestamp: June 07, 2016, 10:01:09 pm
content: Here is my latest schematic and Reply #47 above shows the filter response with a cutoff of 2 MHz.

author: timb
author_rank: Super Contributor
timestamp: June 07, 2016, 10:48:17 pm
content: Quote from: MarkF on June 07, 2016, 04:03:12 pmJust superglue the SMD parts upside down (dead bug style) and... Problem solved?

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 07, 2016, 10:52:40 pm
content: Quote from: timb on June 07, 2016, 10:48:17 pm+1

author: timb
author_rank: Super Contributor
timestamp: June 07, 2016, 10:55:04 pm
content: I'd also like to add: The guy claims not to have space for SMD parts, but he *does* have space for DIP parts? It makes no sense...

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 08, 2016, 01:12:37 am
content: Quote from: timb on June 07, 2016, 10:55:04 pmIt's an excuse given in favor of comfort. The whole thing has a huge number of artificial constraints, and the end product isn't going to be that good. It's a work-with-what you got, and I'm waiting to see what 86uH inductor he's going to find that is small enough (and has enough Q) to not use a sallen-key active LPF...

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2016, 03:10:41 pm
content: If one wants the filter behind the amplification one should at least use a limited bandwidth for the amplifier: e.g. capacitor in parallel to the 200 Ohms and possibly also with the 2,7 K resistor(s). This at least reduces the very high frequencies before the amplifier (that might react strange to this) and support the output filter - with a poor layout the damping at high frequencies it often not as good as the circuit plan suggests.The Sllen key active filter is not a good choice - above the GBW of the OP the filter is not really effective. Quite a lot of high frequency signal just goes the way around the OP, that is supposed to do the feedback. So quite a lot of 10-200 MHz signal left. A low Q inductor is a much smaller problem. This only gives a rounded cut off, but this is not critical here as there is plenty of room between the Nyquist limit (16 MHz) and the wanted maximum frequency (e.g. 1-2 MHz).

author: XFDDesign
author_rank: Frequent Contributor
timestamp: June 08, 2016, 04:54:14 pm
content: Quote from: Kleinstein on June 08, 2016, 03:10:41 pmI never said it was, as far as quality of signal conditioning goes. The OP has placed tight constraints around physical size and only using a single dual-opamp IC. The whole thing is pear-shaped. A low Q inductor that is 86uH in value is likely to be so grossly lossy above a few kHz (maybea few 10s of kHz if he's lucky) that he may as well place a resistor there. Even a hand-wound inductor on something like a J or 77 material is going to be physically larger than the DIP 8 package. Browsing mouser, the nearest value (82uH) is 8mm diameter typical or around 1/3".