Floating probe! For $2.50 - Page 1
Forum section: electronics / beginners

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 08:09:21 am
content: Hey, I'm trying to build a floating probe, this design is based on the tek A6902B, for the optocoupler LF insulator. Tweaking the design a bit I was able to get 230kHz BW from it using a TL082, a NE5532 and two 4N35. The cost I mention is for those four ICs, of course, the complete build cost will be higher but still quite cheap compared to anything on the market. 200kHz BW is nothing to write home about but quite useful for any audio applications among others.I leave you a schematic and the rising edges with two different compensations, for 100kHz and 230kHz -3dB bandwidth. As you can see for the 230kHz BW there's a bit of a jiggle so I tweaked the filter to get the other compensation. Also, for the 230kHz the delay in the signal makes way over 90º phase shift at -3dB point. I didn't tried to trim for gain or offset, that shouldn't be too hard to make and having a spare amplifier in each side leaves the option for tweaking. I didn't tested linearity either, but it doesn't look too bad from the waveforms I've observed.There are faster linear optocouplers, like the 6N136 I'd like to test but I don't have the sim model and can't source the part locally. I'd like to run a test with it!I think I could find this quite useful, as for higher frequencies using a transformer is not too hard to get around. As you could observe in the 6902 schematics, TEK is using a transformer to get from the optocoupler (which goes up to a few kHz) to 20 MHz, so something similar could be added, but I can't source HF opamps locally either so I gave up about that for now. It doesn't seem to have trouble running from 6V or even lower (other than the clipping level) so it could very well run from two 9V batteries to makes things easier, or build a few channels with a 9V battery for each input and a dedicated PSU for the output. The battery on the floating side should last about 100 hours in this condition, I guess I could try to get that longer using lower DC currents for the optocouplers or use two 18650 for the input and a dedicated supply for the output, which could also work without much hassle.JS

author: mikerj
author_rank: Super Contributor
timestamp: September 10, 2018, 10:40:40 am
content: It's not an isolated design if the input and output op-amps are powered from the same voltage rails.

author: StillTrying
author_rank: Super Contributor
timestamp: September 10, 2018, 02:00:43 pm
content: Interesting, reminds me of my flashing LEDs at a photo diode experiments!Why is it inverting, intuitionsays non-inverting would have better stability, a bit better bandwidth, and as high an input impedance as you want.

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 02:30:39 pm
content: Quote from: mikerj on September 10, 2018, 10:40:40 amI did mentioned the battery for the input side at least, not much of a different running the sim to check how it went but essential for the actual device. That's the next step and time to check CMRR.Quote from: StillTrying on September 10, 2018, 02:00:43 pmI have an extra amplifier to buffer the signal at the input, in the prototype I used a tl082, but I wanted the virtual earth circuit to keep the output of the optocoupler to that node, making equivalent circuits in both sides to get the best tracking between them and get the best transfer response as the output of the optocouplerd only have signal current but not signal voltage.JS

author: edavid
author_rank: Super Contributor
timestamp: September 10, 2018, 02:42:32 pm
content: I think you would need to select matching optocouplers. They have a very wide spread of CTR.

author: StillTrying
author_rank: Super Contributor
timestamp: September 10, 2018, 03:29:25 pm
content: Quote from: JS on September 10, 2018, 02:30:39 pmI see, makes a bit of sense.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 10, 2018, 03:54:52 pm
content: If I remember properly, Avago, or whatever is nowadays called, had a dual matched optocoupler for precisely this type of circuits.As other posters have mentioned the CTR tracking of independent optocouplers will show significant differences depending on their bias point.

author: Gyro
author_rank: Super Contributor
timestamp: September 10, 2018, 04:10:07 pm
content: You did pretty well for 4N35s.One of those linear optocouplers with the built-in feedback phototransistor is the TIL300. It's quite an old part now but on paper, it's slightly faster than the 4N35, and with none of the CTR variation issues of course. I'd be surprised if there isn't a faster equivalent available these days.

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 07:35:13 pm
content: Quote from: Gyro on September 10, 2018, 04:10:07 pmThanks! I was doubious about this design but I had the things to set a breadboard and test, it did surprised me. I wish I could source new and good parts but here is pretty painful to do. Once in a while I make an order overseas for specialized parts but for now I have to do whith what I can find aroubd here.Quote from: schmitt trigger on September 10, 2018, 03:54:52 pmI couldn't find any locally, I should see what parts I can get and test again. The concept working with this is a nice start, and even to an useful degree I think, so I might build one or two and upgrade when a better solution appears. I should check for stability but trimming the offset in each use only takes a few seconds and I expect gain to be even more stable.Also, one think I like to take in my projects are easy to get parts, if I can source them locally most likely anyone will so they can get a good use of my design. TL082 and 4N35 are jelly beans that can be found anywhere... If something useful comes from them seems great for me, this could help a lot of beginners not blowing their scope and getting much more comfortable with them.Then I of course would like a better behaved version, with faster response and better stability.The limiting factor for the speed seems to be the input side compensation, I should run some more tests to see how far I can improve this, the virtual ground makes a much better reaponse than just the ootocoupler, as there's no signal voltage at the outout device and that takes the BW from about 40kHz up to over 200kHz (simulated and real world confirmation) but at that frequency the input side compensation starts to be a problem, having a peak which gets compensated by the LPF of the output side to get a flatish response up to 230kHz. At 100kHz the output LPF dominates, and the response is slower but much smoother.Having the optocoupler inside the feedback loop makes it tricky, as it's expected, let's see how far I can take it...JS

author: Yansi
author_rank: Super Contributor
timestamp: September 10, 2018, 07:38:55 pm
content: I don't see too many uses (if any) for this thing.At least switch the coupler to something appropriate, such as IL300.Or just buy an analog isolation amplifier! For example Si8920 costs under $4 and has pretty darn good linearity, is even differential and the bandwidth is at least as good as your circuit. (probably better).//EDIT: BW is a bit under 1MHz. Also the CMTI of 75kV/us is far better than what you would ever get from a nonshielded (even shielded) optocoupler.

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 07:45:45 pm
content: Other use could be injection amplifier, for the LF wiuld be better than any transformer coming down to DC.Quote from: Yansi on September 10, 2018, 07:38:55 pmYes, better and specialized parts makes for better circuit and easier to design, no doubts! If I could get some I will.JS

author: Gyro
author_rank: Super Contributor
timestamp: September 10, 2018, 07:49:07 pm
content: If you want to continue with the opto approach, the Broadcom HNCR200 linear opto is good for about 1.5MHz.

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 08:13:34 pm
content: Quote from: Gyro on September 10, 2018, 07:49:07 pmI couldn't find any reference to the propagation delay in the data-sheet, that will limit the application of this circuit inside the feedback loop. Taking it out of the feedback loop will make tracking and offset compensation a real problem, there are some designs with that approach and might work reasonable fine for some applications, but drift will be bad. In any case it looks like I'm close to the limit of the 4N35. 6N136 is much faster too and I think I can get them, but not locally either. I've seen a project here with one of those, I should check how well it did, the switching takes 0.3µs typ and 1µs max, but this is not switching so specs means very little. All specs are with output voltage with a finite resistnace, not current in a virtual earth config as I'm using them, so I expect to get a bit over of those specs.JS

author: Gyro
author_rank: Super Contributor
timestamp: September 10, 2018, 08:35:59 pm
content: No, you're right the datasheet is light on information. The 1.5MHz bandwidth is quoted for the Fig 16 application circuit (the transistor one). I'm not sure how linear that one would be.It sounds as if sourcing the part isn't going to be an option for you at the moment anyway.

author: JS
author_rank: Frequent Contributor
timestamp: September 10, 2018, 11:38:01 pm
content: Quote from: Gyro on September 10, 2018, 08:35:59 pmFig 12 makes a more similar thing to what I've made, using opamps would make for a more libear design I guess. Not too crazy about linearity for an 8 bit converter anyway... If they are using it like this, the propagation delay must be really low, otherwhise it will become unstable, note the lack of compensation other than the optocoupler itself, I think that's pushing a bit too much.I could get a HCNR200 for the small amount of $20 plus shipping... The 4N35 is $0.50 here for reference. I will anotate that part for my next order overseas, but it's likely to be for the next year or close to that...JS

author: Yansi
author_rank: Super Contributor
timestamp: September 11, 2018, 10:11:00 am
content: HCNR200 is well under $4 as far as I can tell. (Mouser, TME)If you go for speed, then don't 4N35. It is a "standard slow" coupler if I remember correctly. Any photo-transistor coupler will be likely very very slow with low collector currents.//What kind of distributors are there in Argentina?

author: JS
author_rank: Frequent Contributor
timestamp: September 11, 2018, 01:35:59 pm
content: Quote from: Yansi on September 11, 2018, 10:11:00 am4N36 was what I had in the bin. The kind of distributors we have here are mainly the old man who used to repair TVs so they know eventeverithing about vertical deflection but very little about any modern-ish components. Then you have the bigger stores which probably have the same old man in the desk behind the scenes and a bunch of kids dealing with the public, then you have some individual selling dedicated components for 20x what it should cost and some major importer that will get you anything you want for 10x what it costs but within the month (or two). In my city only the first two options, and 300km from here, in Buenos Aires, all of the mentioned flavors.When I was building guitar pedals and tube amps everithing was great, find an old man in a slow day or a kid like me interested I what I was doing and they would allow me to go into the pile of junk searching for all the good NOS they had... JRC4558DD, TA7136P, NE3101, silver mia caps, a wide variety of germanium devices, etc.With all that, any serious development here gets really frustrating pretty fast and working on my own becomes a thing about what can I do with the components I can find knowing it will be pretty easy with modern components. Building small gadgets to sell as a kickstarter seems like a very distant future seen from here, so to calm my thist of development I run into this projects useful for the beginers to help them in tooling their lab. For a day job I do industrial automation where all this is not usually a problem and I can work more freely.JS

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2018, 02:27:26 pm
content: Using 2 optos in push-pull instead of any feedback, and using just the photo diode part, I can get the BW well over 1MHz.At least in the simulation it's linear with IN between +/- 2V @ +/- 10mA, OUT is unfortunately +/- 18uA, +/- 80mV.

author: JS
author_rank: Frequent Contributor
timestamp: September 11, 2018, 03:33:22 pm
content: Thanks for that, I'll try it in the breadboard!Did you observed the propagation dekay?JS

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2018, 05:02:29 pm
content: "Did you observed the propagation delay?"I think it depends on how you define it. If I put a 1us wide 1.5V pulse through the sim it comes out looking like that, the half signal height delay is 120ns.There some real 1us pulses through red, green and white 5mm LEDs viewed by a photo diode here.- the last image.https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg1751210/#msg1751210I still don't know why the light shape(and current) through a red LED is so curved for the first ~0.5us, I suspect IR LEDs might be similar, but not tried one yet."I'll try it in the breadboard!"In the sim the current through the LEDs side is about 6mA, +/-5mA on the signal peaks, and on the photodiodes side about 7uA +/- 6uA, good luck!

author: JS
author_rank: Frequent Contributor
timestamp: September 11, 2018, 07:00:19 pm
content: I'll run a sim now to define a full circuit to take to the breadboard. I guess some discrete transistor should be used to take those 6µA up.The fast response might make it suitable to work inside a feedback loop as well, but the differential approach also makes sense to retrieve higher signal at the output, many things to check now, thanks for the idea!JS

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2018, 09:30:59 pm
content: "I guess some discrete transistor should be used to take those 6µA up."I'd just view the +/- 80mV across a 5k1 or 5k6 resistor for now myself. You're going to need at least some resistance from the photo diodes mid point to 0V because a 0.5uA difference in the PD quiescent currents would make that point go to near one of the rails.It's a good place for a CFA amp, but I've not had a massive amount of luck on a breadboard where every pF at the summing point counts."the differential approach also makes sense to retrieve higher signal at the output"I used 2 optos working in anti-phase to hopefully cancel their non linearity between 2mA-12mA, of course I've no idea how well it might work in practice.If I swap the sim to use the photo transistors rather than the base-collector photo diodes the BW is only 12kHz.

author: JS
author_rank: Frequent Contributor
timestamp: September 12, 2018, 02:27:36 am
content: Quote from: StillTrying on September 11, 2018, 09:30:59 pmWell, I made it, I came up with this circuit which simulates nicely, ~600kHz BW, good linearity, decent step response and all that. I leave the schematic here.I started with another approach which didn't polarized the output of the optocouplers properly, on the sim seemed all great but it worked only up to 40kHz or so in real life.Once on the breadboard I started messing around and came up with the circuit in the schematic, which simulates up to 600kHz but I can only observe a bit over 200kHz in the breadboard. The good thing is that the step response is much better than the one in the previous 200kHz circuit, no ringing at all. Changing R6 and R12 from 10k to 1k takes the BW up to 250kHz in real life, to 2MHz in the simulation. Rise time still at about 1.2µs. I'm observing this response at the output of the optocouplers, so the bandwidth limit is there, didn't improved much using it as diodes, next test could be a dual virtual earth on the output, try with diodes or transistor output connection. I don't know if changing Q4 and Q5 for jfets will make any difference. I also leave a capture of the step response for your joy!Thanks for all the help and I listen to any ideas of how to take the BW even further, I'm liking the open loop approach, much cleaner step response and seems it can be pushed a bit more. I'm happy with it as it is so this will end in a PCB inside a box, but the more BW the better!JS

author: Gyro
author_rank: Super Contributor
timestamp: September 12, 2018, 07:00:01 pm
content: Nice iteration.If the bandwidth that you are seeing is still at the outputs of the optocouplers, then you could go ahead and box it with them socketed and then replace them with faster ones when you can obtain them as they are pretty much all pin compatible.There is a circuit floating around somewhere on the web that splits the LF and HF frequency components, the DC/LF being handled by a linear opto (an IL300 I assume). The HF portion is coupled by a very low value, high isolation voltage capacitor coupled circuit. Both are then re-combined to produce a high fidelity waveform. I though I had seen the circuit on either the EDN or Electronic Design publications' websites but haven't been able to locate it again yet.Of course this combined approach would reduce the CMRR that your purely opto approach gives you.

author: JS
author_rank: Frequent Contributor
timestamp: September 12, 2018, 07:11:18 pm
content: Quote from: Gyro on September 12, 2018, 07:00:01 pmThe tek uses a transformer for the HF, to get from a few kHz upto 20MHz, I haven't seen a cap approach.CMRR is a major factor for this amplifiers, the problem for HF is the capacitance between all the input amp and the rest of the world, hence the optic fiber coupling seen in Dave's vid lately. You don't need that separation for 1kV insulation but the higher the capacitance the lower the CMRR and the higher the load for fast common mode signals as they were measuring at the gate driver.JS

author: StillTrying
author_rank: Super Contributor
timestamp: September 12, 2018, 08:22:45 pm
content: Well I'm glad the anti-phase optos seem to work in keeping the linearity without any feedback, because AFAIK that was untested in practice.But, (there's always a but) have you seen my photo diode amp step response at the same 500ns/Div speed. LEDphotoTRopamp.gif"the more BW the better!"Yep, it's quite possible to get it much higher than that, I'm setting the target at 1.5M+.I simulated the 5 transistor long tailed pair arrangement in the end. The problem is the high 2k5 impedance at the bases, it's much too high, together with the PD capacitance it limits the BW at that point to ~600kHz.Everybody will know more about that 5 TR arrangement than me, so is it possible to get the base impedance down to a few 100 R. After about an hour of trying I gave up and drew this 1 transistor instead.I've got the impedance at the base ~160R, and the BWdown to3MHz, because I think that might be about the best the opto can do.The input current variation is 7mA, -3.5mA to +3.5mA, and the collector OUTput 138mV pk-pk, I've doubled the frequency of my test waves to 200kHz, and they still look good!A problem with that simple 1 transistor is that the output ~2V DC level is not very well defined and varies a bit with temp.I'll add the LT.asc and hopefully someone will suggest simple improvements.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2018, 02:11:30 am
content: The 4N35 and similar transistor output optocouplers which provide access to the base connection are a lot faster if thebase connection is used.I suspect for a linear application it might be best to treat the output transistor as a photodiode driving a transimpedance amplifier.

author: BrianHG
author_rank: Super Contributor
timestamp: September 13, 2018, 02:14:52 am
content: Quote from: StillTrying on September 12, 2018, 08:22:45 pmPlace compensation caps across R1 and R2, maybe around 100pf. You will improve bandwidth further.Change that BC546 to a MPSH10 or MMBTH10LT1.Also, Toshiba's optocoupler TLP2719 is around 10x faster than the 4N25.

author: JS
author_rank: Frequent Contributor
timestamp: September 13, 2018, 03:16:23 am
content: Well, I got -3dB at 550kHz but with some ringing.. +0.8dB at 440kHz. I can observe the AC component on the ringing in the virtual earth node, so I think here is the NE5532 not being able to keep up, I could try an NE5534 which is a bit faster but I'm not sure if it's going to be stable in this config without external compensation.Compensating with 200pF still doesn't solve the ringing, 270pF completly eliminates it but with a BW limit of 280kHz. Using a 220pF seems to be the sweet spot with very little ringing and the -3dB point is at 350kHz. Here is the response with the 220pF cap with some series resistance, -3dB point a bit over 350kHz.JS

author: BrianHG
author_rank: Super Contributor
timestamp: September 13, 2018, 04:58:11 am
content: 2x J113, or J112 jfet in a push pull source follower/constant current configuration replacing the TLO81 opamp and a bias adjustment on R1 and R2's values could improve bandwidth and get rid of some ring.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2018, 05:03:56 am
content: The transistors acting as photodiodes add shunt capacitance at the inverting input of the NE5532 operational amplifier destabilizing it. A capacitor across the feedback resistor cancels the shunt capacitance but also lowers bandwidth.There are application notes about how to improve performance of that circuit configuration. Search for photodiode transimpedance amplifiers.

author: StillTrying
author_rank: Super Contributor
timestamp: September 18, 2018, 11:53:32 pm
content: Quote from: JS on September 13, 2018, 03:16:23 amWith NE5532s in simulation that's the about the best I could get, with NE5532 and opto I think that might be the limit.I've speed tested a cny1711 opto, just it's base-collector junction. It's over 10 times slower than a separate LED and photo diode, even a white LED + PD is 10 times faster than the opto.

author: JS
author_rank: Frequent Contributor
timestamp: September 19, 2018, 03:59:38 am
content: Thanks for the ideas, I'll try them out when I find some time on my hands...Quote from: StillTrying on September 18, 2018, 11:53:32 pmAre you suggesting discrete LED and photodiode are faster than the collector base junction as photodiode?JS

author: BrianHG
author_rank: Super Contributor
timestamp: September 19, 2018, 05:38:15 am
content: Quote from: JS on September 19, 2018, 03:59:38 amYes, take a look at the spec on this photodiode:https://www.digikey.ca/product-detail/en/finisar-corporation/P850-2124-001/P850-2124-001-ND/4416177It has a rise and fall time of 30 PICOSECONDS, however, it is pricey at 23$. This makes an optocoupler look like watching paint dry by comparison.Ok, for a reasonable priced photodiode:https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/SFH-2701/475-2967-1-ND/2794398At 87cents, and a 2ns rise and fall time, it makes the optocoupler look like a snail speed device. And there are 50k units in stock.I'm sorry to say, but, 2 blue leds, or 2 high speed 820nm IR leds, with 2 of these 87cent photodiodes in a push pull drive can reach beyond 200mhz bandwidth with a proper discrete pre-amp on the IR led, and similar for the photodiode.However, you will need to mount them in a small black straw/heat shrink tube. Though, with a little space, a narrow beam blue LED, you can now have 10s of KV isolation.You might make it to 500mhz bandwidth with a laserdiode...

author: StillTrying
author_rank: Super Contributor
timestamp: September 19, 2018, 04:10:04 pm
content: Quote from: JS on September 19, 2018, 03:59:38 amWell that's what I've found in one opto test in practice. In simulation with the 4N25 I can get passed the 1 - 2us speed limit by using just the base-collector photo diode part and get the analogue BW up to 10M. But in practice with a cny1711 the 1-2us speed limit seems to be on the b-c photo diode itself, even 8V reverse bias made no noticeable difference to the rise time.I had to make the opto's pulses 5us and 10us wide to match the shape of the 0.5us and 1us pulses from a 5mm super bright white LED and SFH213 PD - so opto's PD was ~10X slower!It mostly matches what you've found, a simulated BW of 2M becomes closer to 200K in practice, although I still think the NE5532 is a limit.A super bright white ~1MHz looks the easiest ATM, it might have 250ns rise and fall times but it reaches the flat top of the 1us pulse before the red, and is 10-100X brighter. A super bright green, red or cool white might be faster, but I don't have any loose ones to test - yet.I'll probably test the linearity of the super bright white around 3mA - 30mA soon. I'mexpectingthe open loop linearity to be very good, and the BW ~1MHz.Theblue traceis the 1us through a super bright white.

author: BrianHG
author_rank: Super Contributor
timestamp: September 21, 2018, 02:18:58 am
content: Quote from: StillTrying on September 19, 2018, 04:10:04 pmSuper bright white is a blue led with yellow phosphor to make it look white. The super bright blue should be the fastest as it has no phosphor and it's a fundamental color generated by the die directly. A true super bright 405nm violet led or UV ones should be fastest.

author: StillTrying
author_rank: Super Contributor
timestamp: September 25, 2018, 06:39:49 pm
content: "I'll probably test the linearity of the super bright white around 3mA - 30mA soon. I'm expecting the open loop linearity to be very good, and the BW ~1MHz"At 8.5mA +/-5mA the super bright white LEDs light output seems very linear.Max. input(and output) is 200mVpp, with the ca3140 10%-90% rise/fall times ~250ns, -1dB at 280kHz so -3dB is going to be <1MHz.Green traceis the light output.

author: BrianHG
author_rank: Super Contributor
timestamp: September 27, 2018, 04:30:11 am
content: Quote from: StillTrying on September 25, 2018, 06:39:49 pmWhat does the throughput of a triangle wave look like?

author: StillTrying
author_rank: Super Contributor
timestamp: September 27, 2018, 07:41:45 pm
content: "What does the throughput of a triangle wave look like?"I don't know, my simple AF SG doesn't do triangular, but as there's already a CMOS 555 on the other end of the breadboard, I might produce and capture some rough triangular before swapping the SB white. I'd expect the linearity to be OK, but the different rise and fall times of the SB white to become visible around 10kHz.The first start of clipping on the bottom of sines was the 3140 struggling to pull down against the 1k2 pull-up (which was there help it!), rather than the LED current going <3mA. By simply removing the 1k2 and letting the 3140 supply all the LED current the dynamic range before clipping increased ~240mV to ~350mVpp, and the speed, with a slight increase of the 8.5mA the SB white BW must now be close to 1MHz, probably about the best a white LED is going to get at these low LED currents.With a -ve supply availiable, an NE5532's higher output current and BW might be quite good there, the voltage gain needed is very low because the voltage across the LED only varies ~100mV + the 20R.SuperBrightGreen,OrangeandBlue'shave arrived, it might be a day or 2 before I test them.

author: BrianHG
author_rank: Super Contributor
timestamp: September 28, 2018, 01:12:08 am
content: According to these guys, bandwidth tests done, blue really the best at 800mb, then green, then red:http://www.spie.org/newsroom/3941-optical-wireless-network-built-on-white-light-leds-reaches-800mb/sThe problem with red is that there are so many different formulations, from 620nm through 670nm, some slow, some fast. You can at least count on super bright blue always being the same formulation.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 28, 2018, 03:13:41 am
content: Quote from: BrianHG on September 28, 2018, 01:12:08 amHmm, I'm missing something in that article. They say "The maximum data rates at which the BER did not exceed a 2·10−3 forward error correction threshold were ~ 294 (red), ~ 223 (green), and ~ 286 (blue) Mb/s", meaning that red was fastest and green was slowest. Granted these were three dies on an RGB LED, and they were using a modulation scheme to push the bit rate past the raw LED bandwidth, but presuming the same tests were run on all three dies it appears red was the fastest. I honestly *expected* blue to be the fastest... what am I missing?

author: BrianHG
author_rank: Super Contributor
timestamp: September 29, 2018, 03:30:50 am
content: Quote from: IDEngineer on September 28, 2018, 03:13:41 amThat's because they are trying to communicate with overall white light so your office LED lighting is the network and people wont be working in an all blue lit environment. Look at the first table with the axis " Demonstrated rate  ". You see on that table, the red is the slowest while the blue is 800Mbps all on it's own. Get rid of all other light colors in you office and illuminate it pure blue, and you will get 800Mbps all in one led, one wavelength of light.If you want your employees the luxury of white lighting, mixing 3 colors to always achieve a single brightness and color of white, WHILE TRANSMITTING DATA, then the figures they quote for their trick to keep one brightness of white with one color temperature are the maximum speeds they can achieve.Pure white LEDs are slower due to them being blue LEDs with a yellow phosphor which has an inherent afterglow to it slowing down the maximum Mbps.

author: StillTrying
author_rank: Super Contributor
timestamp: September 29, 2018, 12:22:44 pm
content: I joined a string of LEDs and with 5mA through the string, measured the uAs produced by a SFH213 in contact with the LED, they're the highest blue and green uAs I've seen.uA235 SB Blue225 SB Green140 Warm White080 Medium White070 SB Red050 SB Orange026 Std Red025 old IRWith the SB blue or SB green the BW now seems to limited by what the 3140's output/top of LED can do, if I scope the 3140's output, the voltage shape is a close match to the received light shape.

author: BrianHG
author_rank: Super Contributor
timestamp: September 29, 2018, 12:34:46 pm
content: Quote from: StillTrying on September 29, 2018, 12:22:44 pmWhat doe this mean?Are you driving all these LEDs in series? Parallel?Different LEDs have different capacitance and current rise and fall times based on it's internal switching time. Wouldn't testing 1 led at a time yield better results.

author: StillTrying
author_rank: Super Contributor
timestamp: September 29, 2018, 01:35:15 pm
content: "What does this mean?Are you driving all these LEDs in series? Parallel?"5mADCfrom a PSU through the series string, and a bare SFH213 connected directly to DMM to measure the max. head on uA photo current from each 5mm water clear LED, 3 times around the string and averaged, so not exact uA, (depends on the exact head on alignment), the 5mA might have changed a bit, butthe order of sensitivitywas the same for each of the 3 times the string was measured.I'm surprised no one asks what I'm using to receive the light, most of the time I'm using the PD +1 transistor from the light bulb thread, I'd add the imperfect 1 TR receive on to the schem in the previous post #42 when I find it.

author: StillTrying
author_rank: Super Contributor
timestamp: October 15, 2018, 12:14:47 am
content: I tried a 5mm UV LED, it's light output had horrible curves lasting microseconds.https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/?action=dlattach;attach=547853;image