Why not use digital isolators in hobby microcontroller and USB designs? - Page 1
Forum section: electronics / general

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 30, 2024, 01:47:17 pm
content: I like to use digital isolators in my own hobby designs related to microcontrollers and USB. Sometimes the isolation itself is the purpose, sometimes it is only a tool to achieve what I want – for example, power domain separation, or safety against experimental circuit failure. One particularly good not-for-isolation-per-se example is I²C voltage level translation, with their supply requirements and low level threshold differences: I prefer to just grab a TIISO1640to avoid all those issues, even if I end up tying/shorting the two grounds together.Why u no use digital isolators?I mean, I see people complain about digital supply noise and ground loop issues all the time, but it is extremely rare to see e.g. the I²C DAC and amplifier circuits to be digitally isolated, even though it'd completely avoid the problems ahead of time. For quick one-time tools, isolating a SPI ADC frontend lets you reference it to the target 0V rail even when running, without having to rely on AC coupling via a capacitor or some such. To me, the digital isolator chips seem like a vastly underused tool that would solve a lot of the daily problems many are encountering,if used in the design in the first place.Just like trying to add security on top of an existing system, isolation tends to be expensive and ineffective if added on top afterwards. So, why not design it in, from the get go?Edited to add: Let's exclude stuff like RF and high-frequency analog stuff that requires linear supplies anyway. I'm talking about simple hobby stuff like your own USB audio projects, interfacing your computer to your own USB or microcontroller projects, and similar that will anyway be using switchmode supplies and the noisy USB +5V.

author: coppice
author_rank: Super Contributor
timestamp: June 30, 2024, 01:57:49 pm
content: If someone could get the cost really low the volumes would soar. If someone could get the cost low for devices with integrated isolated power the volumes would skyrocket.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 30, 2024, 03:02:28 pm
content: Is it really the cost?(A honest question: no doubts or subtext intended.)Mouser sells ISO774x four-channel isolators (capable of voltage translation and very high data rates) for about 2€ apiece in singles. ISO1640BDR is 3€ in singles.Isolated DC-DC 5V to 5V unregulated 2-watt converter modules cost between 4€ and 8€ in singles, these being sufficient after filtering with a low-drop regulator like MaxLinear SPX3819M5-L-3-3 (0.53€ in singles) to get a low noise 3.3V rail.For bipolar ±3.3V to ±4V rails (max. 200mA each), one can use something like Traco TBA 2-0521 (4.40€ in singles at Mouser) to get unregulated ±5V rails.For a low noise ±9V or ±10V bipolar rails (80mA max), one can use something like Traco TBA 2-0522 (4.40€ in singles at Mouser) to get unregulated ±12V rails, and filter and regulate down. Or TBS 2-0523 (±15V 65mA max).My preferred design is such that the microcontroller is always powered from the host USB 5V, but the isolated side is powered using an isolated DC-DC converter, preferably hardware-switchable (DPDT) between host 5V and a separate power input. Notably, when using a separate power input, it should not be tied to the USB ground, and should be completely separate from the USB host.Getting a low-noise isolated 3.3V rail is easy; +5V rail is much harder. It is easier to use an external AC-DC supply, really. However, Mouser now carries Murata MGN1S0508MC-R7 for 3.62€ in singles, which can provide 125mA at 8V from 5V 310mA, which one can easily filter and linearly regulate down to a low noise +5V rail. I haven't used this one myself yet, but it and the MGN1D050603MC-R7 for 3.90€, +6V and -3V at 111mA each definitely look like a good match for my needs. (+6V filtered and regulated to +5V using SPX3819M5-L-5-0, to -2V or -1.8V using TPS72301 for example.)I understand that the cost is the driving factor behind commercial designs, but for hobby designs, I'd expect to see isolation used much more, considering how many problems one can exclude or mitigate with them, for relatively little increased cost.

author: coppice
author_rank: Super Contributor
timestamp: June 30, 2024, 03:10:55 pm
content: Quote from: Nominal Animal on June 30, 2024, 03:02:28 pmYes, its the cost. i haven't seen real prices for these for several years, but I doubt they have changed that much. If Mouser sells you one IC for 2€ you might expect to pay a few cents when you buy a decent volume. That is not the case with isolators. They still cost tens of cents, which is a big hit on a high volume product BOM.

author: nctnico
author_rank: Super Contributor
timestamp: June 30, 2024, 03:27:02 pm
content: Quote from: Nominal Animal on June 30, 2024, 01:47:17 pmBecause they are pushing RF currents around and can cause other problems. The isolator chips need really carefull layout to create enough capacitive coupling between the side so the RF current isn't going to seek other paths.Same for DC-DC converter modules; these have significant RF currents between the primary/secundary side which needs to be diverted (=shorted between in & output) and filtered. All in all galvanic isolation causes more problems than it solves. Especially if you tie the grounds together. In that case it makes no sense to isolate.The worst example I've seen is a open-collector comparator followed by an opto-coupler which also creates an open collector output. Everything tied to the same groundFor I2C level translation, TI has a whole bunch of bi-direction I2C level translators which work really well.For power supply separation, you can use filtering between supply sections. In RF equipment where filtering is critical you often see an inductor or PI network followed by a local, low noise regulator. A simple RC filter before a regulator can also help but needs line regulation analysis.To avoid digital noise from entering a DAC or ADC, you can filter the digital signals to & from in order weaken the edges (=reduce high frequency components).Oh, and do not split grounds!

author: coppice
author_rank: Super Contributor
timestamp: June 30, 2024, 03:31:46 pm
content: Quote from: nctnico on June 30, 2024, 03:27:02 pmIf you are doing something like digitising an analogue signal specifically to be able to put it through an isolator into a different DC domain what you say is true. If you have high speed digital data you already have those noise issues. If you put an isolator in the path, to create two separate DC domains, you really haven't made things any worse.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 30, 2024, 04:50:28 pm
content: Quote from: nctnico on June 30, 2024, 03:27:02 pmYou haven't used them in a situation where the other side logic level varies from lower than to higher than the microcontroller side? Nor been bitten by the asymmetric low-level thresholds and limited pull-down abilities of them?The most common one isPCA9306, I believe. VREF2 > VREF1 is required, and bias current from VREF2 to VREF1 can play havoc with VCC1 side regulators. So, to use it right, you need to use a VREF1 pulldown and a current-limiting resistor on VREF2. If you don't know beforehand what logic level voltage the device uses –– for example, you want to use it for a couple of different modules you can switch between during experimentations –– good luck getting that right.Quote from: nctnico on June 30, 2024, 03:27:02 pmMost of hobby projects where digital isolation would helpalreadyalready use switchmode supplies. USB in particular is quite noisy in that sense, both from computers and from wall warts.Let me amend the initial post to reflect what I actually meant: hobby projects using USB, not RF stuff that require linear supplies.On typical machines, when you have your microcontroller and a mouse connected to the same hub, you can usually tell from the supply noise when the mouse moves (look at glitches occurring at very steady 1/2/4/8/16 ms intervals). On many USB sound cards, this translates to audible noise.Another case where I'd prefer to see more digital isolation is in stepper controllers. There is no need for the very electrically noisy motor supply and the digital control circuitry to share the ground. If you isolate the step-and-direction signals, you can use beefier AC-DC supplies for the motors, and a separate AC-DC low-voltage supply for the logic, or even allow safely connecting a computer via USB to the logic.

author: coppice
author_rank: Super Contributor
timestamp: June 30, 2024, 04:53:22 pm
content: Quote from: Nominal Animal on June 30, 2024, 04:50:28 pmFrom what I've seen motor control is a big part of the isolator market. That's why you see a lot of isolated sigma-delta modulators now.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 30, 2024, 05:06:41 pm
content: Quote from: coppice on June 30, 2024, 04:53:22 pmHigh-end, yes. Hobby projects, absolutely not. This is exactly what I am wondering about. You don't see isolation used in OSHWy projects likeSmoothieboardor RAMPS. For four drivers (enable, step, dir) you only need two TI ISO7760's, which Mouser sells for under 2.50€ in singles, 1.80€ in lots of one hundred.Most of the higher-current drivermoduleson heatsinks have optoisolated inputs, but for hobby projects –– including small servo robotty things –– digital isolators just are not used. Is it the same kind of "they bring more problems than they solve" assumption/attitude that we can see in nctnico's response above? For me, digital isolators are very useful components, and I honestly haven't been bitten by any issues yet.

author: ajb
author_rank: Super Contributor
timestamp: July 01, 2024, 03:03:46 am
content: Quote from: Nominal Animal on June 30, 2024, 05:06:41 pmA high power motor driver module is exactly the kind of hobbyist/dev-board sort of thing that SHOULD have digital isolation at the IO port, but more for controlling motor return current paths than for noise. Many an arduino (et al) have met their end when a ground connection to a motor driver came loose and motor return currents found their way back to the battery via the signal wiring harness of some hobbyist/prototype project. So isolating digital signals where they cross from a 'safe' domain into a high power/high risk of dramatic failure domain makes a lot of sense, since it eliminates a path for escalating failure.If you're putting everything on one PCB, though, like on the smoothieboard, you don't have to worry about that fault mode. The best thing to do in terms of noise is to put everything on one solid ground plane across the whole PCB, so there's minimal impedance between everything, and carefully place everything so that large return currents and noisy signals are kept well clear of anything sensitive. You do not split the ground plane, to be clear, but you do split the layout between digital/analog and/or high/low power so that noise and return currents stay in their respective zones. In some rare cases you might slot the ground plane to help control DC return currents, but you *never* route tracks across that slot. You can of course add filtering between the power domains, or on signals that cross from one domain to the other wherever needed. You may need to protect signal lines from backfeeding into logic supplies in a partial-power state (like if the motor controller logic supply goes out with the motor supply fuse but the MCU keeps going), but that can be done with level translators or just a transistor + resistor -- you only need to block the signal connection, there's no need to break its return connection.Even if you want to use separate supplies for the motors and the logic, you can still do that perfectly well by bonding the two supply grounds at the PCB in most cases. If you have a larger, possibly distributed system of many PCBs and a big harness between all of those things, then you're back to the first scenario where maybe isolation does make sense.As far as optocouplers instead of digital isolators, an optocoupler input has the advantage of flexibility -- you can make it work with a sourcing or sinking output, and with active high or active low logic quite easily. (Of course, it's not so flexible if the cathodes are just connected to the local ground, which I've also seen on those cheap relay boards!)As far as why you don't see digital isolators in hobby projects specifically, even presupposing that they are the right solution in a given scenario....well there are a lot of hobby projects that get away with far worse!

author: exe
author_rank: Unknown
timestamp: July 01, 2024, 07:38:30 am
content: I'd say cost and board space are significant for isolation.If we add a digital isolator, DC-DC converter, output LDO to filter the noise, it's already more complex than most of my projects) and take quite some real estate. Also, the isolators mentioned in this thread are just simple digital isolators, while I'd prefer to use a usb isolator. Those, unfortunately, even more expensive (or please point me out to a suitable device).Though I do like the idea of isolation from PC. I just wish there was a simple and cheap USB module to achieve just that.Just the other day I connected my oscilloscope to a circuit powered by my laptop. I had horrible waveforms, and my hdmi monitor went black. I spent quite some time troubleshooting. The moment I disconnected scope from the charger (it's battery powered) all problems were gone. I'm glad I didn't get any damage, especially to the laptop and monitor.

author: tszaboo
author_rank: Super Contributor
timestamp: July 01, 2024, 07:43:24 am
content: Quote from: Nominal Animal on June 30, 2024, 03:02:28 pmWhat you are describing competes with the total BOM of many designs.I understand why you would think that this cost is insignificant compared to your hourly wage, hunting down issues.But most hobbyist will buy this from their already taxed salary, and they might work in different jobs as you.Plus, you can buy these as separate blocks, and place it at the end of any USB cable.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 01, 2024, 10:04:21 am
content: Quote from: Nominal Animal on June 30, 2024, 01:47:17 pmBecause it violates rule #1 of..., well, everything. KISS.

author: EPAIII
author_rank: Super Contributor
timestamp: July 01, 2024, 10:26:45 am
content: Not the price? I recently purchased a combo package of two power strips. They had three AC outlets, three USB outlets, surge protection, and even a power indication light. The TWO together PLUS FREE SHIPPING cost $10 US. That's $5 each. And they were made in China so they had a lovely ocean voyage to get here.When I opened one of them up I found three PC boards in addition to all the power sockets. Three separate PC boards! I can't imagine what each part on those boards must have cost. FIVE DOLLARS each! That is the world we live in.Now tell me that a ten cent part is a negligible addition to a product. You and I can add $5, $10, even $50 to the price of a personal project and think nothing of it. But anyone designing a product today will be pricing each part down to the 1/1000 of a cent, if not better.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 01, 2024, 11:45:10 am
content: Quote from: tszaboo on July 01, 2024, 07:43:24 amI am poor, and have less than 50€ each month for my projects. As I've described in other threads, I suffer from repeated burnout and depression, and have been unable to work for years now. I am only a hobbyist on the electronics side too, with some formal education on it, but not that much practical experience.For me, it is not so much a cost calculation, but a simple observation on what I have had to do to get my projects working.One previous similar step for me a few years back was level translators. Other hobbyists struggle with all kinds of transistor circuits. I checked the datasheets, and realized I could use 74LVC1T45 –– my Odroid HC1 had a 1.8V UART –– and avoid all that: in SOT23-6, it's even dead-buggable directly to wires. Later on, I discovered the TI TXU0n0m, and shifted to those for my UART and SPI shifting needs.Why not bidirectional ones? They typically have a low-level "glitch" when "erroneously" shifting directions. If the MCU or device has a sufficiently high logic low threshold, that glitch does no harm, and everything works fine. But, when you do have low logic low thresholds, or you really don't want the voltage level translator to try and drive your MCU output pins, using unidirectional level shifters like the ones I mentioned aboveavoids all the problems beforehand.Thus, you could describe my own design and implementation strategy as the opposite of throwing components on a breadboard and seeing what seems to work: I want to avoid the problems I can foresee. It is why I like to use NX138AKR MOSFETs in my12V PWM fan controller–– they're like BSS138, but slightly faster ––, why my small display module LED backlight driving scheme needs the possibility of individually adjusting the LED intensitiesin casethey're not well matched, and so on. Because I can easily and cheaply avoid the problems I can foresee at design time, I like to take the time and do so, instead of having to later redesign (or worse, bodge a workaround) because those problems become a showstopper.Quote from: tszaboo on July 01, 2024, 07:43:24 amThe cheap ADuM3160/ADuM4160-based full-speed (12 Mbit/s) isolators suffer from poor DC-DC converter choices. They are typicallythis, and their output voltage without external load can shoot well over 6V and break things. Wagiminator hasa much better design, with a 5.1V Zener limiting the output voltage to about 5.3V even when not loaded. I do find its 1W (5V 200mA) DC-DC converter choice limiting, though. A good 2W output one (5V 400mA) at 80% efficiency should only draw 500mA from the host.ADuM3165/3166/4165/4166 and TI ISOUSB211 -based high-speed (480 Mbit/s) isolators are much more expensive, with the cheap ones off eBay suffering from the exact same DC-DC converter choices. What we really need, is a design where the DC-DC section can be omitted completely for self-powered designs, and the DC-DC input can be isolated – say with a pair of jumpers – from the USB host.I have thus far not seen a single one with a boosting DC-DC converter to allow for filtering the switchmode noise and linear regulation to a fixed voltage.Thus, while I personally do have some cheap ADuM3160-based isolators and use them quite often, they're not exactly a panacea; and the high-speed isolators are either quite expensive or suffer unnecessarily from similar limitations. DIY solutions arestillbetter, exactly due to commercial solutions having to be the cheapest possible, and the niches having users who don't know exactly their own requirements. (This is best exemplified by the Audio segment ones, which don't even describe the technical properties of their USB isolators, only their effects to the audio...)

author: tszaboo
author_rank: Super Contributor
timestamp: July 01, 2024, 12:02:22 pm
content: Quote from: Nominal Animal on July 01, 2024, 11:45:10 amSo take a soldering iron, and replace those converters. Or add a through hole Zener to the USB connector.It's not like you would need more than 2-3 of these isolators unless you work with everything connected all at the same time.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 01, 2024, 01:21:25 pm
content: Quote from: tszaboo on July 01, 2024, 12:02:22 pmIt is still cheaper to DIY one. For example, JLCPCB sellsADUM3160BRWZfor assembly for USD $4.45 apiece right now.Quote from: tszaboo on July 01, 2024, 12:02:22 pmAt most, two or three full-speed ones and two or three high-speed ones.Probably the most common scenario for me is to connect my computer to the console (UART) of one of my SBCs or routers. Their logic level varies; I have seen 1.8V, 2.5V, 3.3V, and 5V ones. If I connect it to my laptop, I can just run it on battery power, and tie the laptop and SBC/router grounds together. Or, I can use a full-speed (12 Mbit/s) USB isolator. Or, I can use a TI ISO6721/ISO7721 for RX+TX or TI ISO6742/7742 for RX+TX+RTS+CTS, for both isolation and level translation. Mouser sells ISO6721 for 1.39€ apiece in singles, 11.80€ for a set of ten.Considering comparable voltage translators (TI TXU0202 is 6.93€ for a set of ten), why would one stick a voltage level translator at the end of their USB-UART cable, and not a TI ISO6721/7721/6742/7742 isolator? I only need at most two of these, though. As I run Linux, instead of one of the dedicated USB-serial converter chips, I can just use a cheap Cortex-M0 or RISC-V MCU with a native USB interface, so that if I need some kind of translation or software handshaking, I can implement it on the MCU instead of on the host. Back in the days when ATmega32u4 "pro micro" clones were cheap, I used those. Later, I used Teensy LCs.I'm thinking of making a new slim-line one with WCH CH552G, as JLCPCB has both TI ISO6721 and CH552G in stock for assembly ($1.19 and $0.58, respectively; with AP2112K-3.3 for the 3.3V regulator at $0.08, plus some supply bypass capacitors, maybe a button and a resistor for boot mode/flashing). I'll solder a cable directly to through holes, scavenging a cheap but known-okay USB A-B cable I no longer need. It will be dirt cheap, and be much better than any ready-made USB-to-UART cable you can buy.

author: shabaz
author_rank: Frequent Contributor
timestamp: July 01, 2024, 02:00:57 pm
content: Just to add an (unrelated) idea for your USB-UART board, perhaps it could be something of interest: break detection. I don't know how useful it will be on a USB-UART board, but it could occasionally be a good problem-solver.The attached circuit (untested) shows what I was thinking about; the break detection output is made available on the output connector, along with 3.3V and 5V break-interruptable outputs, so that optionally circuits-under-test can be easily reset from the serial console connection. As mentioned, it is probably not an extremely useful feature, but it is possibly worthwhile on a DIY USB-UART for development purposes.I've been using a DIY USB-UART board for ages too. Mine happens to be based on CY7C65213 (no real reason, just that it supported reasonable speeds, and had battery-charge current negotiation capability which I wanted to experiment with but never got around to). I'm hoping to add that break detect circuit next time I modify it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2024, 02:16:58 pm
content: This is actually a perfect application for this meme...Those who don't know about them, might only know where they've seen them (not often) and assume they aren't very useful generally, or only used in special places (when ya gotta, ya gotta).Those who know everything about them, know they're useful, but also know the hazards that can be created by them (ground slots, EMI), and are also aware of mitigation to avoid them (dealing with ground loop voltage/current, differential design, etc.), even if it costs extra time and effort to plan around (hey, we're into that kind of stuff~).Those who know a bit, can use them, at added cost of course (and really, it's not much, not for few-offs anyways), and maybe run afoul of EMI issues, but often EMI isn't a functional requirement for little projects, and yeah, that can ease a lot of potential annoyances with hooking things up.Tim

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 01, 2024, 03:03:07 pm
content: Quote from: T3sl4co1l on July 01, 2024, 02:16:58 pmYou meanie!I'm definitely in the "isolators should be used everywhere it makes things easier" camp, but even I don't use themeverywhere; only where useful. Tool for the job, really. It's just that I don't see them used in hobby designsat all. For example, consider the isolated USB-to-UART example above, and try and find a similar hobby project.Furthermore, I donotagree that digital isolation is closely related to EMI issues; only isolated DC-DC converters are (and only to the same degree as any other switchmode DC-DC converter). There just isn't enough energy involved/lost in digital signal isolators to generate/cause much EMI.

author: coppice
author_rank: Super Contributor
timestamp: July 01, 2024, 03:17:49 pm
content: Quote from: Nominal Animal on July 01, 2024, 03:03:07 pmSome isolators inductively couple. Some capacitively couple. I seem to remember people having issues with the inductive types, but its a few years ago.

author: Karel
author_rank: Super Contributor
timestamp: July 01, 2024, 03:32:48 pm
content: One reason: EMI.I designed a medical device (ECG, cardiac floating) that uses a multi-channel digital isolator andbecause of the patient leakage current constraints you can't use capacitors between the differentsides to suppress EMI. It was very difficult to get the device within the limits.Using a variant that provides also power to the secundary side was a nogo.It was impossible to get it within the limits without using capacitors or overlapping groundplanesthat caused the leakage current going through the roof.So, for power I used a HF transformer.Ofcourse, for non medical devices that are connected to a USB port of pc, there's no requirementfor isolation so you can apply capacitors between both sides to suppres EMI but then, how usefulwill be that isolation. I guess it helps against ground loops and short circuits but any HF will leakthrough.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2024, 03:38:47 pm
content: Quote from: Nominal Animal on July 01, 2024, 03:03:07 pmYeah, to be clear, the meme is written with some hyperbole.I am, after all, in the right-hand camp writing about the middle and left. I am, right?...Usually EMI is low, but some can misbehave. Keep in mind for example the ADI family are monolithic transformers, pulsing in the 100s of MHz; the isolators I think are generally well-behaved while the DC-DC ones can potentially be problematic, though I haven't used and tested any of the latter (and mostly used the former in industrial applications where I wouldn't notice EMI anyway). The TI ones are monolithic capacitors, differential, and pulsed or otherwise coded at similar frequencies (I haven't actually seen what frequency and coding they use, would like to know), which should generally do well but in the event it ends up off balance, that would be a cause.The thing is more that, isolating a cable or set of cables, makes a high impedance gap where voltage can build up, and where CM currents can't flow. Except when the barrier capacitance is dominant, which -- depends. In short, it's more reactive components in play, more considerations, and unusual conditions. Maybe one is accustomed to dealing with common-ground EMI situations well enough, but doesn't know as well how to deal with this, and maybe it turns out alright, maybe it mucks up. (Most people aren't familiar with either, anyway; EMC is such an obtuse topic. They might well be helped by using isolation more often? dunno.)For example, I'm fond of putting a R+C across most isolation barriers, just to add some loss to the isolation capacitance. Maybe this is justified (de-Qs cable resonances, makes ferrite beads more effective), maybe it's worse (exacerbates CM-DM mode conversion, increases CM current in high-CMRR or dV/dt applications), but in any case it depends on application.And yeah, the nearly-inevitable DC-DCs are a big concern, but I mean independent of them, which is why I didn't mention it.(I actually built one of those applications the other day... no DC-DC, that is. Just a little test equipment project with a logic level trigger input, figure why not make it a 5-30V universal input ala SSR, simple enough to add an opto with current limiter. If something can provide power to the opto LED, that's all that matters. And as a bonus, I provide local 12V on nearby pins so I can trivially plug in a pushbutton for manual activation when I don't need it sequenced electronically.)Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: July 01, 2024, 04:52:36 pm
content: Quote from: Nominal Animal on June 30, 2024, 01:47:17 pm

author: coppice
author_rank: Super Contributor
timestamp: July 01, 2024, 04:56:32 pm
content: Clearly the ethernet approach is the right choice for this discussion. No tough decisions. Its just isolated however you use it.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 01, 2024, 04:57:23 pm
content: Quote from: Karel on July 01, 2024, 03:32:48 pmThis is very interesting to me; I'd like to learn more. Could you roughly compare the EMI involved there to say that of a typical Cortex-M microcontroller?If I understand correctly, the EMI issues are focused on the power rails (as opposed to signal rails), with the isolator current draw being very spiky, containing high-frequency switching noise for capacitive isolators like TI ISO67xx/77xx series. I was under the impression (from the datasheets) that 100nF C0G/X7R bypass capacitors placed very close to the isolator IC suffices to bring that down to typical microcontroller levels, and a pi filter using ferrite beads on both supply and ground as inductors would suffice for e.g 16-bit ADC use (say, isolating I²S or SPI data lines), given a separately regulated reference voltage and analog supply for the ADC. (I do not know enough to say if such a pi filter can turn each side of the isolator into a radiating antenna.)Did you end up having to use overlapping but non-connected shield cans (each connected to one ground)? Did you find a sufficient filtering scheme? Or did you switch to less noisy optoisolation? (I've experimented a bit on using plastic optical TOSLINK transceivers and a pair of cables for signal-inverted UART a few years ago. It worked surprisingly well for occasional short control type messaging at high baud rates.)

author: Karel
author_rank: Super Contributor
timestamp: July 01, 2024, 05:15:16 pm
content: Quote from: Nominal Animal on July 01, 2024, 04:57:23 pmI cannot go into the specifics because it's a commercial and proprietary design.What I can say is that I experienced all the problems described in these documents:https://www.analog.com/media/en/technical-documentation/app-notes/an-0971.pdfhttps://www.analog.com/media/en/technical-documentation/application-notes/AN-1109.pdf

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 01, 2024, 05:44:44 pm
content: Quote from: Karel on July 01, 2024, 05:15:16 pmCould you at least say how much EMI you initially saw from the digital signal isolator alone, roughly compared to a Cortex-M microcontroller?I'm only looking to understand the magnitude (power of ten!), nothing specific.Quote from: Karel on July 01, 2024, 05:15:16 pmThanks!ADuM3160/4160/3165/3166/4165/4166 USB isolators use the same iCoupler technology with embedded transformers (inductive coupling) and those documents will help me (and anyone else interested!) understand the EMI issues with them much better.However, TI 67xx/77xx signal isolators and the ISOUSB211 use capacitive coupling, which I understand generates significantly less EMI.It would beso useful😍 for us hobbyists to have someone describe the rough magnitudes involved, so we'd have an intuitive understanding of when we should go ask an experienced EE for help, instead of just implementing the datasheet design.Quote from: joeqsmith on July 01, 2024, 04:52:36 pmTo be clear, you don't need to use USB isolators for digital isolation in USB projects, so that picture is very much a low blow; deliberately misrepresenting my question and points. It might be funny to you, but you're doing a disservice to all hobbyists with that: making them avoid digital isolators even more in their own designs, because they don't want to be made fun of by the likes of you.I already mentioned using a cheap TI ISO6721/7721 for an USB serial interface, to act both as the isolator and level shifter (from 1.8V or 2.25V–5V logic levels). For audio projects, I would recommend using something like TI ISO6740/7740 to isolate the I²S bus (between MCU and DAC+amplifier), not just whack an USB isolator in there. With audio projects, getting low-noise analog rails for an USBy project is already pretty difficult, most failing and getting USB supply noise in their output. My own approach would be to use an isolated boosting DC-DC converter to filter and linearly regulate down to get low-noise rails. It isn'tthatdifferent to using a non-isolated boost DC-DC converter for doing the same. Proper EMI suppression with isolated DC-DC converters and their input and output filtering is a separate question, though; all I know on that front is what I can read from the DC-DC module datasheets.While Ethernet is extremely nice hardware-wise, it is an utter bitch to implement the software side both securely and efficiently. Wiznet W5500, for example, has a hardwired TCP/IP stack and is thus quite robust, but it tops out at somewhere around 20-30 Mbits/second. Compare that to my Teensy 4.0, which can easily exceed 200 Mbits/second over USB 2.0 (high-speed) using plain USB Serial and a simple Arduino sketch. My Teensy 4.1 does have a 10/100 Ethernet port (DP83825 PHY and a MagJack), but the software side is not nearly as robust as the hardwired Wiznet stuff is, neither the lwip nor the qnet implementations. I can easily make it technically work for my own projects, but I can just as easily write a simple program on my Linux machine that will utterly jam up that stack so it won't be able to process most of the legitimate packages at all (i.e., DOS it).There Is No Such Thing As Free Lunch. Every solution, including using an isolator, has its costs. I am not wondering why isolators aren't used everywhere, I am wondering why I'm basically never seeing them in hobby projects, even though they have helped me solve some of the issues I see other hobbyists getting frustrated with all the time.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 01, 2024, 06:14:54 pm
content: Quote from: Nominal Animal on July 01, 2024, 05:44:44 pmIf I am working in an environment where I have to resort to isolating USB to get it to survive, I am going to choose a more robust solution. Not try and bandage the problem. You may see it as a low blow and have your feelings hurt but having been down that path a few times, I take it serious. I would never design in USB for any device that I would plan to use in a hostile environment.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2024, 06:58:57 pm
content: Quote from: Nominal Animal on July 01, 2024, 05:44:44 pmIt perhaps is a sign, that you could improve your electronics design techniques, a little bit. By sorting out any issues, that not using an isolator, seems to be causing your circuits. By tweaking the design, as necessary.Electronics can be a bit cruel at times. Not only do better, low cost changes, that fix every day technical challenges work.But also, changes, which add significant cost and possible problems, in other areas, might also appear to solve an electronic circuits, design issue. Even if that solution, is not the best of ideas.

author: coppice
author_rank: Super Contributor
timestamp: July 01, 2024, 06:59:54 pm
content: Quote from: joeqsmith on July 01, 2024, 06:14:54 pmI would avoid isolated USB in a volume production environment. However, most development tools are now USB based, and a lot of things we develop are pretty hostile. So, we have little choice but to use isolated USB to develop them. I think that may be the main market for the various USB isolator devices on the market today.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 01, 2024, 07:43:51 pm
content: Quote from: joeqsmith on July 01, 2024, 06:14:54 pmTake the aforementioned USB-to-UART connection between a host computer and an appliance, say a TV box, with the appliance sharing ground with your TV/display, with the UART pins using 2.5V logic levels and directly connected to the SoC.How is using a TI ISO6721/7721 or Si8621/8622 capacitive signal isolator on the RX and TX lines for both ground separation and logic level translation "bandaging the problem"?Bandaging the problem is to tell the developer to use a laptop without connecting it to mains, to avoid ground loops. Here, we do not have polarized mains connectors. Typical electronics devices use CEE 7/16 "Europlug": two unpolarized pins with 240 VAC, no ground connection. Because of EMI suppression Y capacitors in the supplies, the ground is close to the negative AC peak voltage. If the two devices are in different circuits, they may have a potential difference (when mains derived from three-phase power, common outside cities), causing significant (several mA) ground current loops.The same happens if I want to connect a desktop computer to my stereos using normal unbalanced audio cables, and the computer and stereos are powered from different mains circuits – typical if they're in different rooms. A ground lift passive DI box is audiophoolery and a band-aid, and approximately halves the output power (volume). Isolating the line out circuitry is a robust option, as long as you do the required DC-DC power supply correctly.What is exactly the "non-band-aid solution" here you so strongly advocate for?Quote from: MK14 on July 01, 2024, 06:58:57 pmWhat you are saying is that any circuit using an isolator to solve a problem is the wrong solution, because you have never used a digital isolator to solve a problem you haven't been able to avoid otherwise. Let me guess: you have never used a digital isolator at all, outside an occasional optocoupler, right?If I am correct, then your retort is just saying that "if you don't avoid them like I do, your skills are inferior", which has negative informational value in my opinion. I am not interested in using digital isolators unless they are the best tool for the job; and I've described above at least two problems where I use them for. What exactly is your "design tweak" there that makes the isolators unnecessary?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 01, 2024, 07:46:14 pm
content: Quote from: Nominal Animal on July 01, 2024, 05:44:44 pmYeah, not a good application of the meme...It's not an easy meme to use, it's surprisingly subtle. I only *think* I've used it correctly, but maybe not quite still.QuoteTo be clear -- for my part anyway, I've implied that hobbyists are frequently in the "inexperienced" camp, and that they might benefit some if they knew. That basically sums it up as an answer.Which part of the curve you consider yourself on, is another matter (I would assume closer to middle). Give or take what the actual "hobbyist" distribution is, it might still be a misused meme, say if the inexperienced category is the actual population peak (which seems likely). But that's hopefully taking a meme a little too seriously, and an excusable error.There is also the converse case: using isolators when no isolation is actually even used. You see optos in novice designs from time to time (I say that, but I've seen them in industrial designs too..) where it's not even doing a logic inversion or level shift, just 5V to 5V noninverting or whatever, but sometimes at least invert or level shift too, but something that fairly trivially could be made with a single BJT instead since it's still common ground. These are likely mostly copy-paste designs, i.e. a result of imitation without understanding. How this reflects amateur/hobbyist knowledge of isolation, I mean, it's not encouraging, but it's at best only suggestive overall.Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: July 01, 2024, 08:08:16 pm
content: Quote from: Nominal Animal on July 01, 2024, 07:43:51 pmI don't consider my home TV a hostile environment.QuoteEthernet, CAN.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2024, 08:14:08 pm
content: Quote from: Nominal Animal on July 01, 2024, 07:43:51 pmIt's tricky to comment, about your circuit(s)/schematic(s), which desperately need digital isolators, without seeing the schematics.In terms of a minimum, circuit configuration. What would a hobbyist MCU circuit, need to do, as a minimum. Which suddenly, makes digital isolators, a useful circuit solution, to your issue(s)?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 01, 2024, 08:29:17 pm
content: Quote from: coppice on July 01, 2024, 06:59:54 pmUsing USB based development tools to work on something designed for a hostile environment is fine. It's only applications where I would expose USB to high common modes where I would steer away from USB.At home, I have not ran into a case where I was for example, attempting to design a circuit that rode on top of the mains where I need some type of debug port to communicate with a second device that was grounded to the same mains. Some sort of AC motor control for example with a USB/JTAG interface attached to my PC. I have had to do this in the dark ages with 422 using an isolator but that's been many years ago. Still that's different from designing in the isolator.With Ethernet, we have the advantage of transformer coupling and fiber if needed.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 01, 2024, 10:56:40 pm
content: Quote from: T3sl4co1l on July 01, 2024, 07:46:14 pmThat is what I thought, too, but seeing the many answers that imply that any use of digital isolators is a sign of a bad design has made me question myself.Quote from: T3sl4co1l on July 01, 2024, 07:46:14 pmYeah; Iamguilty of that with I²C level shifting, but only because the I²C isolators likeISO1640allow me to put my 3.3V logic level MCU on side 1, and interface to any logic levels between 2.25V and 5.5V on the other side. Compare to the standard PCA9306: with MCU at 3.3V, you can use PCA9306 for 1.2V/1.8V/2.5V, OR for 5V, but not both. And if you have close to 3.3V on both sides, you need to remove the PCA9306 from the picture altogether. With PCA9306, I'd need to make it a module I can insert and remove whenever needed; with ISO1640 I can spend about 2.5€ more and make it monolithic.Mouser sells cheapest PCA9306 variants for about 0.57€ in singles, whereas ISO1640 is 3€. If I know the logic level beforehand, or can use I²C devices that work with 3.3V logic levels, then of course I will do that. But, when say testing various I²C sensors or sensor modules, that is not always possible; but I can still spend a couple of euros more to simplify my designs and misuse the ISO1640 isolator as a "more robust I²C level translator".(I have looked, but have not found a comparable I²C level translator, one without requiring one supply to be lower than the other, or having very asymmetric logic level thresholds and outputs –– in particular, one side output low being higher than the low threshold on the other side.)For the UART cable, I like TI ISO6721/7721 being able to handle both level translation (from 1.8V/2.25V-5.5V)andisolation whenever I happen to need one or both. This way, I only need one cable for all my USB-to-UART needs.Quote from: MK14 on July 01, 2024, 06:58:57 pmIs it? You already confidently implied my use of digital isolators is an indication of my lack of design skill, so how come it is now suddenly tricky to back up your claim?Here. This is the core part of my USB-to-UART cable.On the right side, you have either a cheap microcontroller with a native USB interface like CH554G or a cheap Cortex-M0, or a dedicated USB-serial chip like FT230X, CH340, etc. In my case, I've used various microcontrollers at 3.3V and 5V logic levels, but nowadays the right side VCC is always 3.3V.On the left side, you have the serial console UART pins on various single-board computers, routers and switches, and even 3D printer mainboards. The logic levels vary, from 1.8V (many Linux SoCs) to 3.3V (common) to 5V (older things using 5V logic). The idea of this cable is to be able to safely power both my host computer and the target SBC/appliance, while accessing the Linux terminal console on the target. Note that such pins are often just through holes on the target board, only sometimes labeled in the silkscreen.With this cable, all I need to worry about is the baud rate, bits, and stop bits. Even if the device uses a cheap wall wart that floats the negative side to peak negative mains voltage (being double-isolated with a class Y capacitor between the isolated side and the mains side), with a couple of hundred volts between its ground and the ground on my properly true-earth grounded PC, this cable is safe to use. I don't have to first use a multimeter to check if there is a potential difference, and check if I bridge their two grounds I see a ground current I need to worry about.The digital isolator costs $1.21 at JCLPCB assembly, 1.46€ in singles at Mouser, and the two 0.1µF capacitors are bog-standard X7R 16V or higher ceramic caps in 0603. You can save something like $1.5 or 1.6€ if you bodge the level translation using transistors or whatever components you have at hand,like most do when they need level translation with their USB-UART cable, but you likely won't get very high baud rates with such, and if the target has a particularly low (< 0.3V) logic low threshold voltage, bidirectional translators low-level glitch when they temporarily confuse the actual direction of the signaling can ruin your transmitted signal (TX to the SBC/SoC).My suggestion is to avoid both voltage translation and ground loop/ground current issues by spending an additional $1.30/1.60€ here. My question is about why one can find dozens of hobbyist DIY USB-UART projects, but none using such a digital isolator. Is it because you and joeqsmith are right, and using a digital isolator is only papering over a bad design? Or for some other reason?I am fully aware that price is a key factor in commercial designs, well exemplified in the Audio USB Isolator niche, where they don't even bother to describe the isolator characteristics, only the effects they claim audiofiles will benefit from. The lack of digital isolators in hobbyist and custom-purpose projects is surprising to me – as well as suspicious, because I am fully aware of the risk of falling into the Dunning-Kruger trap. As I often repeat, on the electronics side, I still am only a hobbyist.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2024, 11:00:18 pm
content: Quote from: Nominal Animal on July 01, 2024, 07:43:51 pmActually, I'm disappointed with my posts, in this thread. I failed to spend enough time, reading it through and digesting the information properly. Sorry.I skim-read bits of it, far too quickly, and then jumped in and made some posts.As long as the grounds are NOT shared, and significant interfaces are involved, such as USB. Then using isolators, makes perfect sense, and seems very reasonable.If the grounds are shared, but the interface voltages are different, or even if you want to well protect your circuitry. It still makes sense to use isolators, unless you are confident and/or there are easy ways of converting or coping with the differing interface voltages.The 'Tweaking' I was referring to. Was techniques, such as for allowing MCU inputs to safely handle much higher input voltages, in a well protected manner, or their outputs to do the same. Such as by using transistors and op-amps, as necessary.You may well know all that, already.Ideally, I'd try and avoid such problems, in the first place. By using modules, which run from the same supply voltage, and are basically compatible with the circuit. E.g. all 3.3V.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2024, 11:28:09 pm
content: Quote from: Nominal Animal on July 01, 2024, 10:56:40 pmAs I've mentioned in my previous post.I'd NOT read this thread properly, and just jumped in.I thought, you were talking about straight forward, hobbyist projects. Which only had to deal with supply voltage differences, between your MCU circuit and an external module, or similar situation.But going back on topic.At a very quick glance at the circuit you kindly have drawn, and before spending time, to check my thoughts.I would have thought line senders and receivers, would make more sense here.I.e. These:https://www.digikey.co.uk/en/products/filter/interface/drivers-receivers-transceivers/710With appropriate input/output voltages, and suitable baud rates.But don't necessarily assume, I'm knocking your solution(s), as they could still be better.EDIT: I'm assuming the grounds are either common between the two circuits, or can be. If not, then some form of isolator, may well be a good solution.EDIT2: ok, I stand corrected, after rereading, and better understanding your post, I quoted here.Yes, if they have different power supplies and you don't want to risk joining them (grounds), which if they are floating, probably is not a good idea, anyway.The isolator, makes a lot of sense.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 01, 2024, 11:33:04 pm
content: Quote from: Nominal Animal on July 01, 2024, 10:56:40 pmRight? All designs are a compromise. I typically start with the requirements, which include common mode. Again, I am only concerned when using USB in cases with high common mode. USB is well suited for single ended devices, like mice for example. Why? Because you avoid any common mode, in most applications. So if I am designing for cases where I know the common mode may be a problem, I am not going to choose USB. In the majority of cases where this is not a concern, USB is fine.So, why the big deal about common mode? Can't we just ground the two devices to keep the signals within the range? In my experience, that has proved to be far more difficult in practice. Some attempts to make USB work under such conditions were out right comical. In every case, these have failed which included attempting to isolate the two devices.For your TV/audio applications, will it matter? I doubt it. For the environment I work in, I cringe at the thought of USB. For my home projects, I just use Ethernet. Don't like my responses, don't ask in a public forum. Problem solved.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 01, 2024, 11:48:51 pm
content: Quote from: Nominal Animal on July 01, 2024, 10:56:40 pmThat claim is mind-blowing.

author: MK14
author_rank: Super Contributor
timestamp: July 02, 2024, 12:03:59 am
content: Quote from: Nominal Animal on July 01, 2024, 10:56:40 pmGood question!I suspect, it is poor/marginal designs, on their part. Hobbyist designs, can often be of substandard quality.But also, it is because (I suspect, educated guess, rather than fact based), much of the time, such a solution would work, just fine.It is just poor practice (opinions may vary), to connect, two relatively unknown, floating (i.e. unearthed power bricks) devices, together.So, you may have a better design.But also, those digital isolators, are a bit pricey, and possibly harder to obtain, compared to many other electronic parts.

author: BrianHG
author_rank: Super Contributor
timestamp: July 02, 2024, 12:34:25 am
content: Quote from: Nominal Animal on July 01, 2024, 10:56:40 pmWarning about these type of capacitor based isolators, even though they say they can withstand 3kv isolation, their operating conditions are way down below 450vac.If all you require is <1mb, but you want the robust isolation operating at >3kv, you will need to use 2 optocouplers, 1 for each direction.I have used the 2 optocoupler for my isolated RS232 link in one of my test exercise bike designs where my brushed DC motor's commutator noise with the added human rider acting as a random static ground erased all data glitch errors. Though, the modern ISO6721 should be fine if you don't expect regular 400v spikes between the grounds of both sides in your design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2024, 03:42:39 am
content: Quote from: BrianHG on July 02, 2024, 12:34:25 amBut not 6N136.Takinghttps://www.ti.com/lit/ds/symlink/iso6721.pdffor instance:We have CMTI min 100kV/us at 1200V step (p.12); that's more than good enough for me, that's like offline GaN converter gate drive territory. And that's a characteristic, it's measured. (Optos are not tested equally in these parameters; beware!)The insulation specs (p.8 ) are type ratings under various agencies: 450Vrms might be a nominal maximumplus transientsof some category implied. It may be a safe long-term (sufficiently long, or ~forever, depending on statistics of the barrier) rating. You'll have to check the standard to see. (Fig. 9-8 suggests the latter. And that it's perhaps the plastic package barrier limiting lifetime, not the SiO2 barrier.) 450Vrms isn't enough for 480V industrial US, but US isn't subject to DIN and the UL rating likely suffices for that application, perhaps even for 600V, or give or take what lifetime you [the manufacturer] expect from it.As I recall, the ADI parts use a polyimide barrier, which does degrade over time, at most any voltage (so, following a similar life curve), but that they are suitable for comparable impulse and long-term voltages.Incidentally, notice they're only type-rated for basic insulation (relevant for both CE and US markets, e.g. under coordinated standards like 60950-1; and perhaps UL 1577 or others use such a type system as well, I don't know). That means, for example, earthed logic on one side, mains on the other.Hm, I don't recall optos usually having such a lifetime calculation or tradeoff reported. I wonder if that's because they're so old, they've stuck with old standards (that just dictated some impulse or short-duration test for some other given nominal type-rated voltage), or does anyone rate them the same way (under the same standards and clauses)? I admit I don't read safety tables in enough detail to recall, I'll have to take notice next time I'm looking at one.Tim

author: Berni
author_rank: Super Contributor
timestamp: July 02, 2024, 05:24:25 am
content: Yep usually the bigger problem is power.Isolated DC/DC modules are large, expensive, noisy, come in not so convenient packages... etc, building one yourself takes a fair bit of parts too and can cause a lot of noise problems if you don't know what you are doing.Isolation can also cause other issues where things float up to some high voltage and then you get an arc over in some spot that kills things.So for short distances isolation usually does not make sense. You can still blow up isolated interfaces if you wire them up wrong enough, while protocols meant to go over a reasonable length of cable are designed with some sort of immunity to common mode noise (be it large signal levels or diff pairs). If you have ground loop issues corrupting data on external digital interfaces then you are doing something wrong, disconnect your stick welders output from that ground. Where ground loops are a serious problem is analog things like audio (where microvolts can cause interference). If you are going large distances, just go ethernet (Can still stick non TCP/IP data trough it).Isolation can start to make sense in more cases if you lower the barrier to entry some. Like for example if you only need to pass a binary on/off signal to something, you can receive it with a optocoupler , no isolated power needed as the signal is the power for the isolated side LED. Similar for using an optocoupler as a open collector output, no isolated power needed. So you see this done a lot in PLCs, heck even MIDI does it.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 02, 2024, 11:43:01 am
content: Quote from: Berni on July 02, 2024, 05:24:25 amI don't think you can fix a USB common mode problem with a protocol. Or, maybe let me ask. What do you think happens when you exceed the common mode voltage when using USB? Do you think it just causes some errors in the data stream?

author: Karel
author_rank: Super Contributor
timestamp: July 02, 2024, 11:47:33 am
content: Quote from: Nominal Animal on July 01, 2024, 05:44:44 pmAll measurements we did in test labs where performed during acquisition of signals.(EN 55011 group 1 class B)Here's a pic of a pre-compliance measurement:And this one is the official one used for certification:They were done in different test labs. First one was done in peak mode. Second one was done in quasi-peak mode.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 02, 2024, 12:32:01 pm
content: Quote from: Karel on July 02, 2024, 11:47:33 amMicro with 50MHz clock? If so, you couldn't improve that area? Just curious.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2024, 01:20:42 pm
content: Quote from: joeqsmith on July 02, 2024, 11:43:01 amThe hidden (and real / more important) question being, is the above question even meaningful.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: July 02, 2024, 01:38:05 pm
content: Quote from: Nominal Animal on July 01, 2024, 01:21:25 pmThen do it.Though the only time I need an isolator is when I work with project with power+data 2 wire protocols.Like 4-20mA, DCC, Wiegand interface. None of these have shared ground, so that's the reason to use it . It's very rare to actually need an isolator. Aren't you a bit paranoid? What's your actual use case?

author: Karel
author_rank: Super Contributor
timestamp: July 02, 2024, 01:50:58 pm
content: Quote from: joeqsmith on July 02, 2024, 12:32:01 pmPIC32MX@96MHz, clock coming from a 66.6MHz oscillator block with spreadspectrum, SPI bus @12 Mbit/sec. through the digital isolators.No shielding, plastic enclosure, no metalization.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 02, 2024, 02:16:31 pm
content: Quote from: Karel on July 02, 2024, 01:50:58 pmNice. Are you required to meet some sort of radiated susceptibility requirements as well? If so, I'm curious about the field strength and frequency range required? Then, criteria?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 02, 2024, 02:41:34 pm
content: Quote from: T3sl4co1l on July 02, 2024, 01:20:42 pmIf the USB chipset locks, it's possible to add support to the protocol / hardware that allows hard power cycling the device to clear it. Before you laugh, I've not only seen that proposed .......Your question on if my question was meaningful, only in that it was an attempt to determine if Berni had ran into similar problems and how they tried to mitigate them. Again, working with your TV and audio system, who cares. Most likely things will be fine.

author: Njk
author_rank: Frequent Contributor
timestamp: July 02, 2024, 02:46:43 pm
content: The isolator is not a decoration feature. It's not wise to use it when it's not needed. But I'm going to build a battery eliminator device for Hantek CC-65 current clamp (which is extensively discussed here). So it can be powered from USB socket on the scope. Chinese provided me with a nice isolation DC-DC converter module in the branded box, all for about $1, with shipping. That's fantastic. I'm hoping all the components, including the filter inductors, will fit the battery housing, greatly simplifying the connection.

author: djacobow
author_rank: Super Contributor
timestamp: July 02, 2024, 03:47:59 pm
content: I'm not sure what the debate in this thread is about. The digital isolator parts are nice when they are appropriate.I used an ISOW7841 in an AC power meter project, and it worked like a charm. It provided an isolated power supply so that I could power the ADC chip I was using (ATM90E26) directly from it. It was a nice a really nice one-chip solution for isolated SPI slave with a power supply.Part was pricey, but was a good choice for a low-volume hobby project.

author: coppice
author_rank: Super Contributor
timestamp: July 02, 2024, 03:58:39 pm
content: Quote from: djacobow on July 02, 2024, 03:47:59 pmNow more isolator parts are becoming available with built in DC isolation these things are becoming really flexible for small scale and space limited systems with multiple DC domains. Most isolated requirements only need a small amount of power in the remote domain, to power some kind of sensor, so things work out really well. if they can just get the cost down....... which seems unlikely. The parts aren't that simple when you are looking for sufficient isolation for an AC mains system, with standard surge tolerances and so forth.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2024, 04:03:05 pm
content: Quote from: joeqsmith on July 02, 2024, 02:41:34 pmHeh...Ah, I had read it as a leading / didactic question. But perhaps the "real answer" has more general appeal.The angle I was hinting towards is:USB is differential during a data packetonly; it's not clear to me whether interfaces implement it as actually differential in practice, or just logically merging two normal-mode inputs together as the functional equivalent, but in any casepackets are delimited bySE0symbols, which is a both-lines-low bus state. Thus,the common mode is an intentional signal component of the USB interface.Receivers have 0...VCC input range, whether CM or NM, so the noise margin is small (a volt or two). A shielded cable is therefore obligatory in a commercial (3V immunity) environment.(Immunity is a bit different in High Speed mode, as both ends are terminated to GND with internal switched termination resistors, and a relatively small signal current is driven into the bus at either end. Sort of an upside-down ECL or CML connection. The low voltage means an even poorer noise margin in terms of raw voltage (~200mV), but the low impedance changes the gain from an EMI/RFI source. Low and Full Speed modes are just your basic LVCMOS source-terminated connection, with the receiver behaving open-circuit, hence being likely more sensitive to E-field induction for example.)One can still be clever about CM filtering: note that, since the voltage of each individual line is important, and it's not a strict differential pair, what is important is the voltage of each line with respect to common. At minimum, a three-line choke is therefore required, preferably four so that VBUS and GND can be current-compensated. These (of data line type) are rare or custom, so the next best option is to use a bunch of two-line chokes, with one winding wired in parallel (say to GND) and the other windings for data. But data type CMCs saturate with little current (upper 10s to lower 100s mA) so it's still more complicated than this: you need to choke the power lines separately, then AC-couple the EMI difference into the data chokes to avoid saturating them.But it's 1000% easier to just hard-ground (to PCB plane and/or enclosure) the USB shell and call it done, and perhaps use a cable with ferrite bead(s) if needed. And if you needanymore immunity, or isolation (including in the RF sense, as afforded by a CMC), might as well just use the digital isolator chip(s) and be done with it.Tim

author: Berni
author_rank: Super Contributor
timestamp: July 02, 2024, 04:23:10 pm
content: Quote from: joeqsmith on July 02, 2024, 11:43:01 amUSB uses diff pairs to combat common mode noise problems. This already makes it more robust than just running a UART or SPI bus over a cable. Indeed every diff pair receiver has limits to the common mode voltage it can work with, but you can help it out with a common mode choke to filter out a lot of the high frequency noise that tends to be picked up by cables.(Low frequency tends to point towards too thin wires or someone forcefully feeding extra current into your grounds)Is USB the most robust protocol? Not really, but it is designed for consumer electronics and for that it is more than good enough. It can handle the common ground loops just fine, things like 2 mains powered devices that are plugged into the same power strip.There are cases where isolated USB is a good idea, especially around devices that handle high powers and voltages, like say a VFD drive. But in most cases it doesn't really improve the safety or reliability very much, so it is just extra cost and complexity, adding just another thing to potentially go wrong. I seen some cheap isolated DC/DC modules be so common mode noisy that they could jam out a strong FM radio station at close range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2024, 04:29:07 pm
content: Quote from: Berni on July 02, 2024, 04:23:10 pmIt does not(Possibly you were writing that while the above post came in, and didn't see it, it happens)

author: joeqsmith
author_rank: Super Contributor
timestamp: July 02, 2024, 05:13:22 pm
content: Again, don't laugh but I have seen some crazy custom made USB cables in an attempt to work around the shortcomings.Quote from: Berni on July 02, 2024, 04:23:10 pmWhile I agree for 99% consumer electronics, its good enough. Your next statement is where I disagree. Not that it wouldn't handle this basic condition in pretty much all home-use-cases, but you need to consider that there are special cases where that statement falls apart.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 02, 2024, 07:04:46 pm
content: Quote from: SiliconWizard on July 01, 2024, 11:48:51 pmIn what sense?#4:Because they are pushing RF currents around and can cause other problems. [...] All in all galvanic isolation causes more problems than it solves.#12:Because [using digital isolators breaks] rule #1 of..., well, everything. KISS.#16:No need and don't design by sprinkling in parts.#24:[implies that only idiots use USB isolation using a "funny" meme]#29:[USB isolation is not a solution, only a bandaid]#30:It perhaps is a sign, that you could improve your electronics design techniques, a little bit. By sorting out any issues, that not using an isolator, seems to be causing your circuits. By tweaking the design, as necessary.Have I misunderstood the abovementioned posts, making my understanding of them (or claim what they imply) so ridiculously wrong it is mind-blowing?Quote from: BrianHG on July 02, 2024, 12:34:25 amFor me, 340 V (240×√2) is more than enough. It is the difference in ground potentials (0V reference) in many Linux-based appliances' power supplies the serial RX/TX isolation helps with. Plus it also acts as a voltage level translator at the same time, all of my SBCs having a suitable logic-level voltage pin and a ground pin next to RX and TX pins. It is very easy to essentially brick your SBC/router/appliance by damaging either pin due to e.g. overvoltage, as they are typically directly connected to the low-voltage domain of the SoC, with zero protection.While the default/initial baud rates used in terminal consoles vary, many can do up to 2Mbaud. USB 1.1 Full Speed microcontrollers can do about 1 Mbyte/s payload over USB Serial, and many of my own SBCs can do 2 MBaud very reliably. Teensy 4.0 can sustain 200+ Mbit/s payload data (25+ Mbytes/sec) in one direction indefinitely, over high-speed USB 2.0. Using a TI ISOUSB211 (or ADuM3165/66/4165/66) isolator has zero effect on that.Optoisolation is related, but a bit different use cases in my opinion. I do like Toslink (digital data, typically S/PDIF audio, over plastic optical cables of up to 10m length), and it is my preferred audio transfer method. (Good luck trying to sync ethernet-based audio to your host computer. S/PDIF is synchronous, with the master DAC clock being a multiple (64×, some multiple of 64 of the S/PDIF bit clock.). I've even experimented with using level-inverted high-baudrate UART over a pair of Toslink cables, about a decade ago or so. My favourite microcontroller, Teensy 4.x, has both I²S and S/PDIF support, too.Quote from: tszaboo on July 02, 2024, 01:38:05 pmI doshenaniganswith Linux-based appliances, from TV boxes to routers to my own single-board computers. In that domain, I'm only limited by my imagination and time; I'mnota hobbyist when it comes to Linux integration and software.Routers' and TV boxes supplies are class I isolated wall warts, and because of the class Y capacitor between the output 0V and the mains, and the mains not being polarized here, the output 0V can differ between devices, especially if powered from different mains circuits. Here in230V 50Hz C, Fland, we do not have "live" and "neutral"; we only have two AC lines, and optionally a protective ground. Most appliances being double-insulated class I only use the two AC live pins (usingCEE 7/16 Europlugs), and are not connected to the protective ground at all; they are referenced to some potential derived from the AC.If I connect the ground of my computer to the ground of one of those appliances, I often get ground currents. It is my understanding that this is the leakage current through the class Y capacitor, occurring because the outputs' 0V or ground references are not exactly at the same potential. Do correct me if I'm wrong here. While the potential difference could initially be a couple of hundred volts, the leakage current will rise until it balances out the potential difference, so it all depends on the power supplies used.I can work around that either by using a laptop not connected to mains at all, in which case the entire system will have its 0V or ground reference derived from the appliances' power supply; or I can use an isolation barrier to leave those ground potentials be whatever they normally are, and only pass the information across. Most common form of that information is bog-standard UART, although sometimes I do sample interesting digital data streams, just to figure out how the appliance does what it does, or what a specific chip on it might be all about.In many cases these pins, including UART TX and RX pins, are directly connected to the SoC on the appliance with zero protection, and may have very low current capability, too. For example, my Odroid HC-1 has 1.8V UART pins directly connected to the Samsung Exynos5422 SoC, which I believe can only source a maximum of 1.5mA on those pins.I do not want to simply short the two grounds together, because of the ground current. It is enough to upset things, and could break things; it has for others. Me, I've avoided breaking things, by being an idiot and using an isolator here.(The Odroid HC-1 is a bit special in that I can use my own supply (it wants regulated 5V at up to 6A if I use a spinny-rust SATA 3.5" drive with it), and I can choose to use e.g. Mean Well GSM60E05-P1 class-II low-leakage supply, the 0V output of which I know I can safely short to the ground of my host computer. I do not yet have the skills with switchmode supplies and AC-DC supplies to tell if the same is true when I see the circuits. In particular, I know they're not overprovisioned, so I worry that the leakage current will thermally or electrically stress the supply circuitry over its intended operational range.)I do believe my own isolation needs boil down to two basic issues. The first is working with devices that have their own AC/DC supplies I know very little about, and wanting to interface them to my own microcontroller projects, which in turn are connected via USB to my host computer. While I do use a variety of microcontrollers, most of my ad hoc tools are based on Teensy 4, which as I've already mentioned can sustain over 200 Mbit/s to my host computer over USB, using plain USB Serial, and no extra code in the Arduino sketch. (I do need to ensure I write in chunks of a few bytes, as writing individual bytes to the USB buffer limits the bandwidth to under 40 Mbit/s.)The second is working with devices using steppers or many servos. Their current draw is horribly noisy, and filtering it out from the logic supply and ground is not easy. Just plunking down a few millifarads of bulk capacitance means either the logic supply rail will fall very slowly, or it will have a shunt resistor wasting power. I see isolating the control signals –– enable, step, direction for steppers, PWM or I²C bus with a PWM controller for servos –– a simple solution to protecting the logic from the motor noise and related issues. It is also the only way (aside from an USB isolator, doing the same isolation at a different point) I know that allows one to connect the logic side to a host computer via say USB, while the motors are powered by a separate class I AC-DC supply, without risking leakage current between the host computer and the device due to disagreements on what 0V potential really is. For the logic side, one can use a class II 5V supply, say a Mean Well IRM-05-5, with a pair of Schottky diodes (the other from USB 5V) to some filters and a low-noise 3.3V linear regulator, which not only simplifies things, but also allows the logic side could be powered either from the host USB (say, for programming), or from its own AC supply, regardless of whether the motor supply is enabled or not.For me, this scenario opens up very interesting possibilities, for example speeding up prototyping by keeping most of the control logic on the host computer, and have the device microcontroller only handle timing sensitive stuff. Exactly the kind of stuff an interested hobbyist might be doing.I am not an expert on any kind of isolators, and I know it. I also know that I've successfully used digital isolators according to the datasheets to successfully avoid the problems others complain about. That is the reason why I wonder why other hobbyists are not using them to solve the same problems.Even here at EEVblog, ground loops –– or, to be more precise, leakage currents through the class Y EMI suppression capacitors in class I AC-DC power supplies when devices don't precisely agree to what the 0V "ground" potential is –– have bitten people,HobGoblyn for example. If their USB sound card had Toslink output, and their Eris speakers a Toslink input, the problem would not exist, because the plastic optical S/PDIF cable acts as an isolator. Granted, their speakers seem to have an actual fault causing leakage current via the audio inputs, but still: these problems are not as rare for us hobbyists as you professionals believe.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 02, 2024, 08:34:06 pm
content: Quote from: Nominal Animal on July 02, 2024, 07:04:46 pmBased on the horizontal axis and Goddard's definition, I believe the proper term is moron, not idiot. That said, yes I believe you are only reading into it what you want. Again my goal wasn't to damage your sensitive ego. You asked for feedback and I provided it. I have stated in various ways that design choices are not cut and dry. Parts exist for a reason. If they were never designed into products, they would not survive. Do I have a use for isolation with USB at home, no. It's not what I consider a hostile environment. For work under these conditions, based on my previous experiences with USB, I would choose a different bus all together from the start. Because my choices don't line up with yours, so what? Do you really need me to validate your choices?As I stated, I typically start with the requirements and don't just randomly start pulling parts and sprinkling them into my designs. I'm not sure why that upsets you. Would it make you feel better if I told you that for home projects, I typically know what I want and just toss something together based on the parts I have on hand? Of course at home, I am not held to the same standards.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 02, 2024, 08:54:47 pm
content: Quote from: joeqsmith on July 02, 2024, 08:34:06 pmUseful feedback is not general statements and memes. What you posted was pure garbage, what with your "hostile environment" nonsense. I do understand now that that is the best you can do, and are unable to go into any technical details, because you're all opinion and no practical use. Truthiness at its most obvious form, really.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 02, 2024, 09:00:23 pm
content: Quote from: Nominal Animal on July 02, 2024, 07:04:46 pmNo, that wasn't your misunderstanding that was mind-blowing, but many of the answers you mention. I think you got them right - apart (maybe) from just the "meme", that was (IMO) to make fun of people having closed opinions on either side, while hinting that reasonable people are those who'll use a given solution if they just fit the requirements. But the link with IQ (if we refer to the same meme) was pretty unfortunate, IMO.Many of these statements that you listed and that were made in the thread don't make any sense, at least without context: and that's the whole problem, a very common one in online discussions. Many will just come up with their own experience (which per se is what you were after here), but each of these experiences is in a given context. Making them an absolute is cargo cult. They even miss, for the most part, the point of your topic, that was for *hobby* use. Even if any of the above was true in all contexts (which it's not), we're talking about hobby designs here (if I got it right), and quite frankly, for hobby work, as long as something works, fits requirements and is not dangerous, I don't even see how that could ever be a problem.And it's a sign of "bad design" only if it actually is. Stating that in general doesn't make sense, it's again cargo cult engineering. (Which it would be as well if one were throwing isolators everywhere all the time, just in case.)Regarding EMI (which in most likelihood, you don't usually care about for hobby stuff, unless it was completely horrific), it's probably way, way out of context. The only kind of digital isolators that I've seen EMI issues with are the ones who embed a small isolated power supply to power the "isolated" side. And even so, with some care you can deal with it. But that's only one specific type of isolators, and I don't know for sure, but don't think you even talked about these. Simple digital isolators, that just isolate digital signals, require a power supply on each side (without embedding any), basically cause absolutely zero EMI issue. I'd be curious to see a design where they did, I have never run into this problem.Isolating from USB is done to isolate from the host your device is connected to. One common use case is when you don't want your device to share a common ground with the host. In that case, you don't have a choice. Note that when isolating in such contexts, you need to always keep in mind the whole setup: if you use PCs and lab equipment, that are usually all connected to Earth, they'll all share a common ground, whether you like it or not. So, if you isolate some device from USB, but connect an oscilloscope to your device, then you break isolation (unless said oscilloscope is itself isolated). You get the idea. So, that takes some care.You mentioned USB-UART adapters, I frequently use isolated ones, which give the benefit of not only isolating (if needed), but also level shifting at the same time. Handy and relatively cheap. Note that these require a power supply from the isolated side (which is the whole point anyway for level shifting), so you have to connect a Vtarget pin to your digital supply.Finally, I knew your thread could turn problematic with your mention of "hobby". "Hobby electronics" covers a very wide range, from total newbies that barely know how to connect wires together and just follow some videos on YT, to very experienced people that just design stuff outside of a professional setting, which qualifies as "hobby". So, the concept is too wide not to trigger a wide range of odd opinions, and also too wide to be able to make a clear opinion of what a "hobby design" is. IMHO.As to the question of what's available on the market for hobby use, it's also a question that's hard to answer. Depends on where you look entirely. Depending on the perspective, with this approach, one may be tempted to think that all there is for hobby use in terms of MCU dev boards are Arduino stuff with an Atmega, 8-bit stuff with just a few KB of RAM. That's obviously not the case at all. Isolated USB-UART adapters are relatively easy to find, and priced adequately for hobby use, so you tell me if this doesn't qualify as hobby stuff. You have a 4-port version here:https://www.tindie.com/products/saimon/4-ports-isolated-usb-uart-converter-with-usb-c/, but they do sell variants with just one port. Handy and cheap.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 02, 2024, 09:11:37 pm
content: Quote from: Nominal Animal on July 02, 2024, 08:54:47 pmROFLAMAO!! I always try to give my best.

author: nctnico
author_rank: Super Contributor
timestamp: July 03, 2024, 08:34:00 am
content: Quote from: Nominal Animal on June 30, 2024, 04:50:28 pmI don't know the pca9306. Never seen it, never heard of it. I'm always using the TI TSX0102 (and related, >2 channel versions) which just works from higher to lower and from lower to higher levels. I don't rely in the internal pull-ups though for I2C. There are probably other chips which work equally well but I never had problems with TSX0102 (including in high volume products) for low-speed level conversion so never needed to look elsewhere.

author: tszaboo
author_rank: Super Contributor
timestamp: July 03, 2024, 01:10:52 pm
content: Quote from: Nominal Animal on July 02, 2024, 07:04:46 pmSo why would people place these isolators in their designs? You are not going to place an Arduino or whatever inside the television. Nor do an average hobbyist do anything near the 230V, yet alone understanding X and Y capacitors. What you are searching for are USB isolators, and USB serial converters with different voltage levels.So the bigger question is: Why don't you run your media box DUT from a lab power supply?Why don't you just buy a bunch of USB isolators and different voltage USB UART converters to connect to your DUT?All the EMC and other discussions are beyond the scope of this. DUTs not supposed to work properly and pass EMC tests while gutted and connected inside.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 20, 2024, 05:58:47 pm
content: Apologies for the delay, I was basically offline for a couple of weeks.Quote from: tszaboo on July 03, 2024, 01:10:52 pmNo, but I do interface my Arduino/Teensyduino/microcontroller projects to routers, single-board computers, and other appliances. (I don't have a TV, actually; I have no use for one right now.) Many of them do export a Linux/Android serial debug console (on unpopulated pins, i.e. pads or through holes only). For example, one might add a microcontroller to control large-format 12V PWM fans for quiet operation on a modified telly, and for programming and monitoring purposes wish to connect it to a PC to verify everything is okay (say, a few well-placed digital temperature sensors) for the first couple of weeks before removing the PC connection.Quote from: tszaboo on July 03, 2024, 01:10:52 pmI do not own one I'd trust to behave correctly; the only supply I have is a Gophert NPS-1601, which I mostly use to power my temporary experiments on breadboards and such (as it does not have the kind of voltage overshoot on power-on as many other supplies do have, that can b0rk low-voltage ICs).(I don't even have a proper oscilloscope; Analog Discovery 2 is what I have.)More importantly, I want to limit the number of changes to the DUT to the minimum, when investigating possible issues. If it is an interaction with high current draw due to software configuration and a borderline supply (say, high noise at high load), I'd like to see that behaviour even when instrumented. Simply put, the UART isolation is the simplest and least invasive/minimally differing configuration.Quote from: tszaboo on July 03, 2024, 01:10:52 pmBecause isolation at the proper boundary is cheaper. Also, because of my background in physics and security-sensitive software development, putting interface boundaries "where they belong" (i.e., maximize the benefits while minimizing the downsides) pleases me.So yes, you are completely right in that my behaviour is nonstandard and odd, with most others doing similar stuff using bog-standard off the shelf solutions. It was my intention to find out why and how the current typical solutions came to be; to discover and find out, not convince anyone else to do what/how I do stuff.I do intend to build my own LS/FS/HS USB isolator based on Adum3166/4166 or ISOUSB211, for non-bus-powered devices, though. I have minimal funds to spend on my electronics hobby, and tend to spend it on microcontrollers and SBCs instead of test equipment, because I want to get most fun per buck spent.

author: nctnico
author_rank: Super Contributor
timestamp: July 20, 2024, 08:51:47 pm
content: Still, it would make sense to replace the Gophert switching PSU with a linear one from Korad. PSUs from Korad (like the KD3005D) don't break the bank but a linear PSU has a lot less output noise issues so you'll be able to make better measurements. I do like the Gophert PSUs, but only as a universal wall-wart to power non-critical stuff.PS: Not owning a TV is nothing to be ashamed about. I have been fine without one for 15+ years and likely won't buy one again ever.

author: John B
author_rank: Frequent Contributor
timestamp: July 21, 2024, 04:53:31 am
content: Lmao this thread feels like a personal attack.I've made pretty good progress on my home automation system, where I opto isolated all the major sections. The RPi is isolated, the microcontroller slave modules are isolated, all connected through an opto isolated RS485 network. Each micro also connects to a 36V lighting PSU, isolated naturally.I had a bad experience with a previous lighting project where 60V got backfed through my project......

author: nctnico
author_rank: Super Contributor
timestamp: July 21, 2024, 08:50:29 am
content: Quote from: John B on July 21, 2024, 04:53:31 amFun story: Back in the home computer days someone demonstrated a Commodore 64 controller lamps at a computer fair. He had made some controller boards with triacs and stuff but didn't have any isolation or casings. At some point the boards shifted on the table and mains got into the Commodore 64 and fried the thing instantly. So he had to go shop around for a new Commodore 64 which he managed to find and continued his demonstration

author: amwales
author_rank: Regular Contributor
timestamp: July 21, 2024, 10:30:09 am
content: I got some of these the other dayhttps://www.lcsc.com/product-detail/Digital-Isolators_2Pai-Semi-122U30_C5187288.htmlfor 25centsI plan on using them along with a ch340 to separate the usb/uart side of a board, just adding it to the TX/RX lines. I tested it on a breadboard interfacing two separate usb/uarts on their own power rails, not shared ground. They seemed to work well.I plan on isolating a 12v side of a circuit from the USB in the event that someone plugs the 12v in the wrong way around. I already have polarity protection in the form of a p-channel mosfet but that doesn't stop someone driving 12v to the ground of the usb side.I just a hobbyist and a relative newbie.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 21, 2024, 06:02:23 pm
content: Quote from: John B on July 21, 2024, 04:53:31 amA big part of that is how I stated the question. My intention was to learnwhyisolators are avoided in hobby designs, and to express how I've used them as a cheap solution to a few problems affecting many hobbyists (but perhaps only a tiny fraction of all hobbyists, those doing specifically similar things that I myself do).A small part is that many members here do either metrology or RF (radio frequency) stuff, where switchmode noise (including capacitive and inductive isolators) and electromagnetic noise and interference (here, especially from inductive isolators) would be a show-stopper; and naturally reason from that point of view.A tiny part is experience/knowledge of mains transformer use, and cargo-cult-like extension of those risks and dangers to digital isolation.I personally don't have "proper" test equipment: only a cheap multimeter, an Analog Discovery 2 (which is limited to ±25 V!), and a cheap switch-mode supply (Gophert NPS-1601, which does not overshoot the voltage when output is enabled, and thus is quite suitable for powering experimental setups and breadboard circuits without risking blowing up ICs which do not like overvoltages). And I have very limited budget, too: for the price of a "cheap" Korad KD-3005D supply, I could get an Odroid M1 or a couple of Radxa Rock Pi E single-board computers to play with. I have aneedto optimize the "bang for buck" I get; to be very practical while still having fun – and that includes efficient use of isolators, even if I have to do the boards myself (at say JLCPCB prototyping and assembly services).As to "misusing" isolators, for example theTI ISO1640 I²C isolatorto allow any voltage between 3V and 5.5V on one side, and any voltage between 2.25V and 5.5V on the other side, the idea is that when prototyping designs, such an interface board (for bread-board use) gives youextra optionswith much less cognitive load –– what with the input thresholds and stated output low voltage levels when using bidirectional/I²C/SMBus voltage level translators –– means I can use the same module for all different cases, even when tying the grounds together and thus having no isolation at all. Similarly, usingTI ISO6721-Q1orISO6742-Q1for UART isolation means I only need one USB-UART cable (or, rather, don't need to worry about the logic level voltages as long as the device provides a suitable logic level voltage), rather than a set where each is dedicated to a specific voltage. Of course, I could useTI TXU0202orTXU0204voltage translators instead, but the price difference in singles at Mouser is about 1-2€ apiece, so it makes financial sense (with less cognitive load, in considering which cables would besafeto use in which scenario) for me to have a couple of USB-UART cables with isolators, as they can do the job even when I short the grounds together. (The exception is 1.2V - 1.8V and 1.9V - 2.1V logic levels, which the isolators cannot do, but the voltage translators can.)In other words, the things I'm discussing have proven extremely useful for me in practice, although I am well aware of the downsides. (Some of the downsides mentioned here, like the EMI issues wrt. ADuM inductive isolators, were new to me and very useful to know.) In particular, the digital isolation schemes I've described let me do more with much less, both in terms of equipment and money spent, leaving my meager budget for stuff I really like to play with: SBCs, Linux-based appliances from TV boxes to routers and everything in between, microcontrollers with native USB interfaces, BuyDisplay/EastRising display modules, and Human Interface Device experiments (arcade joysticks, keyboards, novel input devices like 3D trackballs and e-ink reprogrammable macro keypads with capacitive touch, and so on).It can be frustrating when threads go this way –– and I did get a couple of comments via PM's because members didn't want to put it in public and risk being criticized in the same vein –– but the key is to remember thatIt is all about communication, and nothing personal.Writing style and even grammar matters a lot, because the text is the only thing we use for interaction here.Opinions do not matter, and can be safely ignored. What matters, is the reasons behind those opinions; the risks one wants to avoid, and how; ways to mitigate the downsides; and ways to pick a good solution for a particular problem. To get there, one needs to poke and prod a bit, which can annoy and anger some, but which will more often than not yield more useful and applicable information. Thus, it is worth the effort of getting over the tone of the discussion, and to get into the true meat of the things hidden beneath.Of course, this is just my opinion and experience and current belief, and I could very well be wrong, too. But I do not think so, based on the practical results I have gotten with both the circuits and the discussions and what I've learned on these forums in general.