signal generator output stage? - Page 1
Forum section: electronics / projects

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 12:34:19 am
content: I'm trying to build a simple DAC: two channels, 1 MHz sample rate, 16 bit, voltage output generation down to DC. My first try was using the circuit of theTI DAC8831/32 evaluation module, just adding an digital isolator and an isolated DC/DC converter, because the output should be isolated.This is the ciruit. Then I used the Eagle auto router, which was not a good idea,this is the layout. I used a FT2232H breakout board to send data to the DACs, test setup:The output (with averaging enabled) looks like this for a full scale sine wave:I could use a sample rate of up to about 500 kHz, with the bitbang mode and occasional dropouts because of the USB bus, which is not bad for a first test. For the final device I'll add a FPGA, which can use the 245 FIFO mode of the FT2232H for a continuous output.There were some other problems. First the DC/DC converter was awful, more than 100 mV ripple (I guess as you would expect, because it is unregulated and the layout is not perfect, datasheet says 20 mV). You could see it on the output signal, too. So I used my bench power supply to power the circuit. This was better, but still a strange 10 mV, 1 MHz signal. Turns out this was from the USB port. Depending on where I have my scope probe cables and where the USB cable is, the amplitude halfs or doubles. I hope this should be no problem anymore with a nice shielded case and higher output power.But the main problem is the output. Of course I didn't read the datasheet of the OPA735 output driver and a slew rate of 1.5V/us is too slow for me. It might not make sense, but I want a nice 1 MHz square wave, if I feed the DAC with it, so 50 ns rise time would be good (the DAC8831 has 25V/us). And then I want +/-10V output into 50 ohm. If I use 50 ohm load with the OPA735, the voltage drops to zero. How should I modify the circuit to do this? For protection I guess the easiest would be a beefy 50 ohm resistor in series. And do I need some termination resistor? Would be good to have some programmable gains as well, as known from other popular DACs, like +/-10/5/2/1/0.5/0.2/0.1V. Note: I'm an analog electronics beginner, I don't really understand how the current circuit worksAnd how can I improve the DC/DC conversion? I want a single supply 5V input. My idea is to use the same DC/DC converter type, but some higher output voltage and then a second stage with a LM317/LM337. Is this a good idea?

author: uncle_bob
author_rank: Unknown
timestamp: February 05, 2016, 01:02:15 am
content: HiOk, you want a terminated output with a source impedance of 50 ohms and you want to deliver +/- 10V into a 50 ohm load. That gives you a requirement of +/- 20V into an open circuit. It is likely you will need a >+/- 25V from the DC/DC converter to do this.You want to run square waves. They have the nice property that they have the same power as a DC voltage equal to their zero to peak. 10V into 50 ohms -> E^2/R -> 2 watts. You will put 2W into your load. If your output termination resistor on the generator will also dissipate 2 watts. If your amplifier is 50% efficient (a wild guess), it will dissipate 4 watts.So square waves are a bit interesting to reproduce. There is a bit more than just the rise time (ringing etc). You DO need to go 20V in your 50 ns. That gives you a minimum slew rate of 400V / us. You likely also need at least 20 db of gain at 10 MHz. That gives you a gain bandwidth product of 100 MHz.This is way beyond what any op amp will do. They simply will not handle the power. You can handle the voltage with op amps, but into a 100 ohm load ... not so much. You will need to dig into amps designed for this sort of thing. They are a bit unusual, so expect to pay a premium when you find them.Before I dig further -- are you sure you want to do this? Have I missed something so far?(Yes the reply that always comes up -- don't have a source resistor. That makes the amp very hard to stabilize and even more difficult to build).So ok so far?Bob

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2016, 03:02:22 am
content: FYI, DC-DC converters are always(?) rated in 20MHz bandwidth, differential only. And almost always as measured across a large ceramic capacitor (10uF or so).So it completely misses any RF crap in the 100MHz+ range, and any common mode noise (which is rather important for an isolated application!).Extra filtering, on both input and output sides, and some Y-type (across-the-isolation-barrier) caps and common mode chokes, can help with that.You should probably have a Y cap across the isolation anyway, possibly with a ferrite bead or resistor to help with damping. How much depends on the example system you test with, since it depends on factors like cable length and attached devices.Tim

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 03:31:02 am
content: Thanks Bob, I don't know what I wantI have a simple waveform generator (WG1220) which works very well for me, and if my DIY signal generator could do the same for the output stage, (only for up to 1 MHz) this would be sufficient for me. I tested it with 5 V square wave into a 50 ohm load of my scope and then the scope shows 2.5 V, so +/-5 V into 50 ohm would be ok for my generator (and then +/-10 V into on open circuit). Slew rate of the generator is 50 ns, too.BTW: strange that with my normal scope test leads it drops to 0 V when I select 50 ohm impedance on the scope (DSO-X 3012A), but when I use a coax cable it works. Does it do some detection and then simply turns off the input? I tested it again with my generator with a coax cable and then it drops to 1.7 Vpp with 50 ohm impedancem which looks right with my current 100 ohm output resistor.Another requirement I forgot to mention: I would like to have a stereo headphones output. This was the idea for the second output driver in my circuit, in parallel to the main output, because the signal should not be disturbed if there is a load or no load on the main output. Maybe there is a better audio amplifier IC for this purpose. I thought about simply using a stereo log pot for adjusting the volume, independently of the gain of the main output, which should be controllable by software.

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 03:46:24 am
content: Quote from: T3sl4co1l on February 05, 2016, 03:02:22 amOk, so the Y-type capacitor is needed for EMI/RFI suppression? I've seen this only for mains filtering so far. And the common mode choke, in series to the ouput and my circuit, would help in reducing the ripple? After this maybe another big capacitors? And the ferrite bead in series of the output to filter the high frequency stuff? I'm a bit lost, and what values should I use? The prototype of my signal generator uses less than 10 mA, so filtering should be not too difficult and maybe a small series resistor for an RC filter would be possible, too. Of course, with 10 V into 50 ohm it will go up to 200 mA. And I guess it gets a bit more complicated because of the symmetrical +/-10V supply (some volts higher, because this is the output I want). I guess I need 2 common mode chokes.

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 05:39:14 am
content: Quote from: uncle_bob on February 05, 2016, 01:02:15 amPrice doesn't matter, it is only a one-off device for me. Of course, 100 bucks for an opamp would be too expensive. But I've done a parametric search on Digikey for at least +/-10 V supply voltage and at least 200 mA output current, displayed only parts with more than 25V/us slew rate. Looks like it lists only peak current, so I sorted by current and found theLT1206. Mounted on a heatsink it should survive a continous short circuit at 10 V with the 50 ohm source termination resistor without problems. But the offset voltage doesn't look too great, so might be good to use it with gain 1 and then another pre-amplifier opamp for my programmable gain.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2016, 06:37:10 am
content: Quote from: FrankBuss on February 05, 2016, 03:31:02 amYour probe has a 9M resistor inside, so (at most frequencies) terminating it at 50 ohms gives you a 1:200,000 ratio probe.The 9M has some capacitance and other stuff across it, which makes it look more like 450 ohms at high frequencies. So you'll get a lower ratio above some kHz, down to 10:1 around 30MHz. How flat it is above there depends on the probe design.Needless to say, the probe is only valid into its specified load, and only to what bandwidth it claims.Which, speaking of: use the 10x setting by default (if it has a switch), because that has the widest bandwidth and the least loading on the circuit being probed. If you have a 1x setting, mind the bandwidth: it's typically less than 10MHz! For other probing needs (high sensitivity and bandwidth), you'll need to consider another method (direct 50 ohm 1x terminated cable; 10x 'no-probe', using 50 ohm terminated cable and a 450 ohm divider resistor; or something with an amplifier or buffer).Quote from: FrankBuss on February 05, 2016, 03:46:24 amThe CMC in line to the DC-DC converter, so, instead of tying its V+/V- pins straight to the rest of the circuit, pass them through a CMC first. (Primary or secondary side is fine. Worst case, use one each?)If the converter has dual outputs (+V, GND, -V), you'll need a pair of two line CMCs (V+ and GND, V- and GND), or a single 3+ line part.Required values range from nothing at all, to maybe a few mH. A CMC doesn't have to store energy as an inductor does, so it doesn't need to be very large; it just needs to be big enough to hold enough copper to handle the current.For an explanation: consider the equivalent circuit of the DC-DC converter, where all its input pins are shorted together, and all its outputs are shorted together. This is representative at RF, because bypass caps short those connections together with low RF impedances. Now you have one wire going in, an isolation barrier (which is not simply an open circuit, but a capacitance in series with a noise voltage source), and a wire going out. Now you can see that it's part of a series loop circuit. If there's nothing shunting across that series source, then that noise voltage will find its way out into the world, along your cables. This is an RF voltage, measured between the two circuit grounds (source ground and isolated ground). Hence, it's a common mode noise problem. Cables make fine antennas, and that noise voltage will radiate into the air, causing radio interference.Very intense noise sources (like the ~300V switching edge inside a typical mains SMPS) can be strong enough to disrupt their own operation, and of anything connected to them. (It's a fairly common sight to find phone chargers which are so noisy, they disrupt the phone's capacitive touch screen. This actually takes only a few volts -- but that's still -40dB from the mains switching edge, what some might call a good attenuation ratio. And another 60dB away from the typical FCC/CE limits on noise sources!)How much does this all matter? Well, for your own purposes, it should at least not blot out commercial television and radio, and emergency radio services. How much is required for that, who knows... For professional test equipment, most laws make an exception; but that assumes a qualified technician is operating the equipment, inside a suitably shielded laboratory. And anyway, you'll be most concerned with its own operation: a signal generator that's disgustingly noisy is worse than none at all!So a little filtering, which might amount to a Y cap, maybe a CMC or two, is probably a good idea, and fairly harmless (i.e., you're unlikely to get the values so wrong that it's actually worse).You can evaluate if additional filtering is necessary, by probing the circuit's differential and common mode voltages.Tim

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 07:39:33 am
content: Thanks for the explanation, I think I start to understand itI remember that Dave explained some of the scope probe details, makes sense that it shows 0V.Would this CMC work?http://www.digikey.de/product-detail/de/PM3700-60-RC/PM3700-60-RCCT-ND/3193337and then this ferrite bead, 100 ohm @ 100 MHz:http://www.digikey.de/product-detail/de/BKP2125HS101-T/587-1929-1-ND/1465399Circuit idea:L4 and L5 are the ferrite beads. Where do I have to place the Y-cap(s)? And if I use a metal case, to which ground should it be connected (if at all)?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2016, 10:13:09 am
content: Don't worry about L4/L5, the CMCs will have about as much leakage inductance (and it won't saturate under DC bias!).Unless it's horribly noisy, you probably won't need both of L1 and L2/L3. Might as well pick the simpler alternative (just L1). But if you want the contingency, you can lay it out as shown and jumper over the unused ones with wires.The Y cap can go in many places. Example:GND-IN to chassis/earth-VS to chassis/earth-VS to COMGND to GND-INGND to chassis/earthGrounding for the chassis depends on what's all connected to things, but hopefully it can find its way to earth at some point. What are the inputs and outputs on this thing?Tim

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 11:36:20 am
content: It will be a general purpose signal generator, for my lab. Maybe USB powered, then I can use it with my laptop, too. I just noticed that the BNC connector grounds are isolated from the chassis when I screw it in the chassis, this is nice. So maybe the input ground would be best to connect to the chassis?

author: jeroen74
author_rank: Frequent Contributor
timestamp: February 05, 2016, 11:53:07 am
content: You could check out theLT3439. It could be a nice opportunity to get into winding your own transformers as well

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 12:41:00 pm
content: Quote from: jeroen74 on February 05, 2016, 11:53:07 amNice, less than 200 uV noise and ripple looks good. There is even an application example in the datasheet for 5V to +/-12V, but not enough power. But I guess I can do lots of things wrong designing my own push-pull DC converter, using one of those bricks might be easier. But I should use a better linear regulator after the transformer, the LM317/LM337 is not very low-noise. At 0.1 V output the LSB would be 1 uV. Of course, I can't do this, less than 1 mV noise would be ok for me.

author: uncle_bob
author_rank: Unknown
timestamp: February 05, 2016, 12:48:39 pm
content: Quote from: FrankBuss on February 05, 2016, 03:31:02 amHiOk, so step back, deep breath ... what do you really want to do with this? Square waves are nice at logic levels. Sine waves can have great big amplitudes. Are multiple channels more important than a single channel? Do you have *anything* around the lab with a 50 ohm input that will not be burnt out with a > 100 mw signal?Lots to think about.Bob

author: Kleinstein
author_rank: Super Contributor
timestamp: February 05, 2016, 12:55:36 pm
content: The common mode inductors one the secondary side (L2,L3) may saturate in that circuit. You would need a common mode choke with 3 coils to make them work without saturation. In principle you should get away with having them on the primary side (L1) only. Without L2,L3 a you will need L4,L5 auf cause, even larger inductance might be better.A Y cap would be from the ground on the input side to the GND on the isolated output side.There also the option to use a royer converter for the isolation. It's not regulation the voltage but as a resonant converter it has little noise and can tolerate a transformer with less that perfect coupling.

author: dom0
author_rank: Super Contributor
timestamp: February 05, 2016, 12:59:26 pm
content: A discrete amplifier will probably be the cheapest (and most interesting way) of doing this. There are tons of service manuals of function generators available where you can take a look at the concepts if you don't know where to start.Common approaches are either- a common collector output stage with the impedance padded to ~50 ?- a common emitter (normally differential or a similar configuration) output stage with a 50 ? loadIn the ol' days these were very complex amplifiers, since the had to be fast, reasonably linear, idiot-proof and DC-accurate. Using op amps to DC stabilize a fast discrete amplifier is a feasible approach and is common since the early 80s.Other ways would be to use specialty ICs along the lines of fast current buffers, video line drivers or video current-feedback amplifiers.

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 04:46:12 pm
content: Quote from: uncle_bob on February 05, 2016, 12:48:39 pmThanks for asking, I didn't know that my Agilent scope has a protection which switches the input off, if a signal > 5 Vrms is applied, when configured with 50 ohm impedance, that's interesting. I won't test itThe maximum output of my signal generator with 50 ohm load is 5 Vpp = 2.5 Vrms (for a square wave), so it should not burn out the scope. That's the worst case, a sine wave with 5 Vpp is 1.77 Vrms.Two channels is important for me. One application is to use old analog scopes with X/Y mode to shownice graphics. I know, totally overkill for this application, but if I build one, why not using the standard which many signal generators have, 50 ohm source impedance and +/-10 V max? Most of the time I will need only logic levels, but might be handy sometimes as a component tester.

author: uncle_bob
author_rank: Unknown
timestamp: February 05, 2016, 05:57:42 pm
content: Quote from: FrankBuss on February 05, 2016, 04:46:12 pmHiWe may be talking about two different things.+/- 10V open circuit is a different thing that +/- 10V into a 50 ohm load. (like by 2:1). So far I have been interpreting your request as +/- 10V into 50 ohms.For logic levels, a function generator / signal generator is not the ideal thing to use. Logic gates are dirt cheap and they drive logic very well. No need to do fancy output stage designs. No need to worry about rise and fall times or channel to channel timing or any of the other stuff. Driving logic with a low slew signal is not a good idea. There are an ocean of MCU demo boards and FPGA demo boards at < $40 to do this with. They also have the nice property of being bi-directional. Putting a stimulus in and seeing what happens is very useful.On the overload topic - take a look at everything you have and may be getting. In general "self protect" was a rare item. There are a *lot* of devices out the that go pop when you hit them with a high level signal. Frequency counters, spectrum analyzers, signal generators, power meters are all on the list of devices that I have personally seen blown out this way.I know, more to think about and really no answers at all... Sorry !Bob

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 06:49:12 pm
content: Quote from: uncle_bob on February 05, 2016, 05:57:42 pmWould be good if my DIY function generator could do the same as my WG-1220, which is +/- 5V into 50 ohm and +/- 10V into a high impedence load (so 50 ohm source resistor).QuoteThat's right, and I need a FPGA anyway for the USB bus FIFO, so maybe a good idea to add some pure logic level outputs as well.

author: FrankBuss
author_rank: Unknown
timestamp: February 05, 2016, 06:59:58 pm
content: Quote from: Kleinstein on February 05, 2016, 12:55:36 pmThanks, so this is my new circuit idea:Parts list:C11/C12/C15/C16: 47 uF / 25 V:http://www.digikey.de/product-detail/de/C3216X5R1E476M160AC/445-8047-1-ND/2792164L2/L3:http://www.digikey.de/product-detail/de/SLF6028T-330MR69-PF/445-1993-1-ND/755400L1:http://www.digikey.de/product-detail/de/PM3700-60-RC/PM3700-60-RCCT-ND/3193337+12V regulator: LT3081, 27uVrms output voltage noise:http://www.digikey.de/product-detail/de/LT3081IFE%23PBF/LT3081IFE%23PBF-ND/4156531-12V regulator: LT3091, 18uVrms output voltage noise:http://www.digikey.de/product-detail/de/LT3091IFE%23PBF/LT3091IFE%23PBF-ND/5432251C10: 10 nF, Y rated capacitor, 300 VAC:http://www.digikey.de/product-search/de?keywords=PHE850EB5100MB04R17+5V to +/-15 V, 330 mA:http://www.digikey.de/product-detail/de/JCA1005D03/1470-1769-5-ND/4488149QuoteSounds good. The JCA1005D03 I found for the +5V to +/-15 V is a bit expensive and I don't need all the features of it like current limit, because the linear voltage regulators do this already. But then I can't do anything wrong with such a block. I would like to avoid winding my own transfomer, because then it is easier for others to build this, too, if somebody wants such a signal generator.So next would be the output stage. Is the LT1206 a good idea for it?

author: macboy
author_rank: Super Contributor
timestamp: February 05, 2016, 07:04:23 pm
content: Quote from: uncle_bob on February 05, 2016, 01:02:15 amOld LM7171 has 200 MHz GBP and 4100V/µs slew rate, and will drive 100 mA of output current. The datasheet has very extensive graphs of various performance parameters, including driving 100 ohm loads. And it's a 'easy' to use voltage feedback amp, not current feedback.

author: uncle_bob
author_rank: Unknown
timestamp: February 06, 2016, 12:41:16 am
content: Quote from: macboy on February 05, 2016, 07:04:23 pmHiThat makes more sense now that it's +/- 5 into 50 ohms rather than +/- 10 into 50 ohms.I would be a little concerned about the thermal limitations on the chip.Bob

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2016, 02:30:23 am
content: 10V into 50 ohms wouldn't even be possible on the ~2W USB input: you need external power for more.So, since this is USB, USB must be grounded, so you'll have USB ground (shield) going to enclosure ground. That's great for EMI, and it's probably grounded at the host end, so it's safe, too. And USB signals are ground (shield) referenced, so that'll also (at least) single-point the connection between circuit GND and enclosure (earth). So you'll have a nice shield there.Isolated side 'GND' floats over this, but some RF termination at least is necessary (which would look like an R+C across the barrier). Or just a capacitor, which is as shown.Since this isn't mains related (unless you'll be connecting the output to the mains..), it doesn't need to be very isolated, or use certified Y1 capacitors (though, they aren't badly priced, so it wouldn't be much of a loss). 1.5kV is a reasonable figure for how much isolation you should use. And you should use at least 1nF across the barrier.Why? Because ESD. This is mainly how Ethernet is designed. Ethernet is designed with a transformer coupled interface, with 1.5kV basic isolation. ESD applied to a cable or connector, would result in that spiking to 8kV or more, which would damage the isolation. There's a common mode filter to help with low level noise, but that hardly does anything against ESD. So, instead of trying to withstand ESD directly, it's shunted with a capacitor. ESD testing is done with a 150pF source, so 8kV on 150pF divides down to 1.5kV on 1nF, hence the isolation rating.So you'd want to do the same here. As for the exact combination of C (>= 1nF) and any ESR or ferrite beads, that will require testing to determine. Just putting one in there is a good start.Then on the BNC output side, you want that coaxial as much as possible, because any noise conducted over an unshielded length results in a voltage drop across the open section of ground lead. Which means voltage drop in the ground that's not present in the signal, thus introducing that as a noise voltage to the signal (ground loop voltage). At this point, you shouldn't be worrying much about internal noise (thanks to the CMC and filtering around the DC-DC), but it never hurts to be mindful of what might be lurking outside (susceptibility).For the same reason (ground noise), try to avoid putting the output itself through an onboard choke -- at most, use a ferrite bead around the coax cable.Tim

author: FrankBuss
author_rank: Unknown
timestamp: February 06, 2016, 02:43:29 am
content: Quote from: macboy on February 05, 2016, 07:04:23 pmNice, but I would like to have 10V with 50 ohm source impedance, and it should survive short circuits indefinitely (so 200 mA).A really nice chip is the ADA4870, 1 A output, 2500 V/us, 60 MHz bandwidth. This is overkill for my application, but one advantage would be that at 200 mA the chip would stay cool, because even without a heat sink it has a thermal resistance of 16°C/W. And if I ever want to do some high power experiments, I could bridge the 50 ohm source impedance resistorI guess harmonic distortion is better, too, when it is well below the maximum output current. The datasheet says -70 dBc for 25 ohm load and 1 MHz. For the LM7171 it is -75 dBc for 5 MHz, but with a 100 ohm load.The buf634 is also interesting, but the datasheet doesn't say anything about THD at 1 MHz. It says 0.02% @ 20 kHz, so this would be -74 dBc? For 1 kHz it is 0.015%. I guess for 1 MHz it is much worse. Looks more like an advanced audio amplifier.

author: uncle_bob
author_rank: Unknown
timestamp: February 06, 2016, 06:00:40 pm
content: Quote from: FrankBuss on February 06, 2016, 02:43:29 amHiTake a close look at the way that part is mounted in their testing. They have a multi layer board (as in 4 to 8 layers) with an ocean of via's and a 2" square ground plane on almost all of the layers. They also solder the central slug down to this giant thermal mass (not easy). Effectively what they have done is to replace a bolt on heatsink with one they have made out of very fancy (and expensive) pc board material. Because of the way the slug comes out of the bottom of the IC, you need the large number of via's (and most of the copper) even if you put a heatsink on it.Don't ask how I came to know all of this ....not a happy story.Bob

author: FrankBuss
author_rank: Unknown
timestamp: February 07, 2016, 12:43:08 pm
content: Ok, but should be still no problem. I simulated the power dissipation at the ADA4870 op amp with the 50 ohm output series resistor:So max power dissipation would be about 1.2 watt. Even if the thermal resistance is twice as bad with my board, it would only heat up to about 60°C for a short circuit.

author: uncle_bob
author_rank: Unknown
timestamp: February 07, 2016, 01:27:41 pm
content: HiHow many layers in your board and how thick is your copper?Pulling heat out of the area by the slug is highly dependent on layer count and copper thickness. You could easily have a 4:1 delta. I tend to run into this sort of thing a lot. The data sheets are often not as clear as they should be on all the details. They are targeted at a specific market that "always uses at least 8 layer boards" or some such utter nonsense. After you and the guy who did the data sheet drink a few beers, the real answer of "that's what we guessed" comes out.Bob

author: FrankBuss
author_rank: Unknown
timestamp: February 07, 2016, 02:20:12 pm
content: I planned 2 layers, 1 oz. But maybe would be better anyway to use 4 layers, for some nice big power supply planes and ground plane, and then 2 oz for even better conductivity, thermal and current. I guess this could help with noise and possible ground loop problems. And then under the op amp lots of vias and copper on all 4 layers.

author: uncle_bob
author_rank: Unknown
timestamp: February 08, 2016, 06:01:17 pm
content: HiI'm just suggesting you be cautious when looking at the AD thermal data. Their drawings could be showing a lot of layers. From real world boards, adding layers is a big help. Allowing for a heat sink is not a bad idea ...Bob

author: FrankBuss
author_rank: Unknown
timestamp: February 13, 2016, 08:10:12 am
content: Warning, feature creep startsI was thinking when I use the ADA4870, I have +/-10V @ 1 A. This would be useful for a good power supply as well, and unlike most other power supplies, with push/pull output. With an additional ADC, a simpleSMUshould be possible with not too much effort (of course, not near as precise and accurate as the commercial ones, but maybe faster).My idea is to add another 0.1 ohm series resistor to measure the current and a current limit circuit. Would this work? Is it possible to bridge the 50 ohm resistor with a switch, or is this a problem with the 1 A and high frequency (like too much noise because of the cables)? Should I use a realy?The control voltage of the current limit circuit could be fed by the signal generator. This would make it a 1 MHz programmable current source. So two modes: constant (adjustable, with a slower, simpler DAC) max. voltage, and using the signal generator for the current, and constant (adjustable) max. current, and using the signal generator for the voltage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2016, 10:14:51 am
content: FYI, using the correct ratio of voltage to current feedback (it would look something like a Howland current pump, with unbalanced resistors), you can have the amp behave as a resistor already. Downside: above the frequencies where feedback is dominant (i.e., within reach of GBW), the output impedance will go funky as it transitions to whatever the open loop output impedance is (usually an oddball, somewhat reactive value; R2R amps may be reactive and high impedance).You can switch it between whatever characteristics you like; using separate voltage dividers to set the gain and impedance of each combination, selecting between them with analog switches, is a fine way to go. All you have to mind is the switch capacitance, in the 10-100pF range, which will need some compensation (loading on the +in will reduce high frequency response and make the output capacitive; loading on the -in will add peaking, or more likely, oscillation outright).Tim

author: uncle_bob
author_rank: Unknown
timestamp: February 13, 2016, 02:54:29 pm
content: Quote from: FrankBuss on February 13, 2016, 08:10:12 amHiIf you take a look at most power supplies, they really do not do a very good job measuring current. There are a variety of reasons for this. One of the most basic is that it is harder to do than it seems. A power supply should have zero output change when you apply a load. The same thing should be true when you remove the load. No overshoot, no undershot, no odd events. The same is true when you turn it on and off. No spikes on the output.So far nothing about current ... You need to measure it without messing up any of the things above. You would like to have the current meter read zero when there is no loadHaving it read correctly when in constant current mode is also handy. Having both the voltage and current displayed at once is very nice. Just getting this much done is a struggle. Making it as accurate as a $30 DVM .... not very common at all.Now back to the topic.A power supply (or a voltage reference) with a 50 ohm output impedance is not a really handy item. Closing a servo loop around the 50 ohms is likely to give you spikes. You have a big and unpredictable capacitive load on most power supplies. Adding "stuff" in the signal path of a signal generator is not a good idea. Keep it simple is generally the best way to do it.Bob

author: FrankBuss
author_rank: Unknown
timestamp: February 14, 2016, 09:03:59 am
content: Quote from: uncle_bob on February 13, 2016, 02:54:29 pmRight, maybe better to create just a good signal generator. Things like a SMU could then be implemented as extra devices. But I think an additional high speed ADC would be nice (with an extra input). It would not cause problems for the DAC, and it can use the existing USB transfer and FPGA.

author: uncle_bob
author_rank: Unknown
timestamp: February 14, 2016, 03:16:26 pm
content: Quote from: FrankBuss on February 14, 2016, 09:03:59 amHiThere is a wonderful management theory called "cycles of learning". In an engineering sense what it means is that you get much further much faster by going through the entire process multiple times than by trying to optimize the process for a single shot solution. There is a bit more to it than that. I think it applies to a lot of home projects.I'd build one and accept that you may build another and another after that. Get the basic part working and figured out. If it works fine first time great. If it needs a tweak, redo it. When you go through the next design, add features.Bob

author: FrankBuss
author_rank: Unknown
timestamp: February 14, 2016, 06:18:42 pm
content: Yes, good idea. The basic signal generator works already, I'll do some of the try-and-error cycles on a breadboard and test ideas.