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| passing signals over galvanic isolation without an opto coupler |
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| ConKbot:
--- Quote from: floobydust on October 17, 2019, 12:38:45 pm ---Optocouplers aren't good with age or temperature, almost none of them are AEQ. IC's using digital isolation with tiny air-core transformers are sexy but a challenge for EMC. Their high carrier frequency is not mitigated in any app notes or datasheets, as far as AD iCouplers. It's a great place to get bit in EMC testing with them. For a single analog, using a transformer and PWM works for lowest cost. It depends on your isolation voltage, regulatory requirements and accuracy. --- End quote --- I like the AD appnote suggesting you interleave groundlayers so the parasitic capacitance provides a return path for the EMI. I'm not familiar with regulations and industry practices, but interleaved ground planes if you need significant isolation or a proper reinforced barrier seems a tad suspect. |
| tggzzz:
--- Quote from: Simon on October 17, 2019, 11:44:33 am ---I am not comfortable with opto's after a few failures and looking at opto couplers as a whole (I went through a few Datasheets) I would prefer not to use them. The leakage over 80C changes from 2nA to 200nA. They are ideal to cheaply solve some problems but I do not feel comfortable with them in the application I am involved with and with every other part rated to 125C the opto's are now the weakest link. --- End quote --- Optos have a long-term degredation in their current transfer ratio. I would have thought leakage was only relevant in a DC coupled circuit. You could probably get around both issues by modulating the DC signal onto an AC carrier (at say 1MHz), and having an AGC in the receiver. Without knowing the application, threats, and constraints, it is difficult to offer a solid recommendation. |
| coppice:
Some digital isolators use inductive coupling. Some use capacitive coupling. The ones using capacitive coupling seem to cause a lot less problems during EMC testing. While most opto-couplers have issues with time and temperature, some are designed for demanding medical isolation requirements. The fading output of the LED over time means their analogue performance will change with time, but if you have a digital application that won't matter much. You need to look around if you have demanding requirements. |
| SiliconWizard:
Never had any problem with the AD digital isolators. The ones giving me occasional issues with EMI were the ones embedding a small isolated DC/DC converter, which are pretty noisy. Other than these ones, I found those isolators pretty robust and trouble-free. AFAIK, if you want to go for capacitive-coupled ones, go for the TI ones. SiLabs also makes some that are quite good (Si8xxxx). They are based on yet another approach. No microtransformers, no capacitive coupling, but some kind of internal RF carrier. |
| Simon:
I used a TI isolated CAN transciever that past militarry spec EMC and that uses capacitive coupling. |
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