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.
What 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.
I 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 TXU0
n0
m, 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 above
avoids 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 my
12V 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 intensities
in case they'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.
Plus, you can buy these as separate blocks, and place it at the end of any USB cable.
The cheap ADuM3160/ADuM4160-based full-speed (12 Mbit/s) isolators suffer from poor DC-DC converter choices. They are typically
this, and their output voltage without external load can shoot well over 6V and break things. Wagiminator has
a 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 are
still better, 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...)