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| passing signals over galvanic isolation without an opto coupler |
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| Marco:
--- Quote from: ConKbot on October 17, 2019, 02:25:46 pm ---I like the AD appnote suggesting you interleave groundlayers so the parasitic capacitance provides a return path for the EMI. --- End quote --- It adds capacitance between the planes on the two sides, which might help a tiny bit with EMI but makes isolation worse and will defeat the purpose of the IC in some cases. Instead of less than 10 pF between either side you get more than 100 pf. Has anyone ever put an isolator on a double sided PC with simple split power planes on either side and 100 nF bypass capacitors, and actually measured the EMI with proper equipment to see if the field intensities are even relevant for regulations in the realistic worst case situation? |
| profdc9:
For low frequencies, an audio isolation transformer would probably work. It can handle frequencies above 200 Hz to 10000 Hz or more. Note if you need really high voltage isolation then you probably need a more customized solution. Some Teflon wires can have insulation strength 3 to 5 kV. For frequencies 50 kHz - 1 MHz, you can use a MnZn ferrite core with high permeability. The higher the frequency, the fewer turns you should use. For frequencies 1 MHz to 100 MHz, you can use a NiZn ferrite core, type 43 or 61. The input and output needs to be capacitively coupled, and it's advisable to detect the output using a Schmitt trigger inverter or comparator with some hysteresis. |
| T3sl4co1l:
If you pass PWM over transformer, note that you probably won't get low frequency (signal) content, and certainly won't get DC. What you need is a coupling capacitor on one side, and a DC restorer on the other. This references the signal to its peak value, rather than its mean. Follow it up with a Schmitt trigger and you have clean digital PWM back (which can be sent to a 1-bit DAC and filtered to recover the original signal). Sigma-delta is better than PWM in most cases, and reduces to PWM in the extreme cases; AFAIK there are ready-made isolators available, based on this approach. Can also do push-pull on both sides of a transformer, so that the DC voltage at the center-tap is equal -- a DC transformer. Feed a biased signal into the center tap, and there you go (or, if you're using analog switches rather than unidirectional transistors, the signal can be bipolar, no DC offset required). This has a few downsides largely due to transformer non-idealities. Tim |
| Simon:
--- Quote from: Marco on October 19, 2019, 02:48:59 pm ---Has anyone ever put an isolator on a double sided PC with simple split power planes on either side and 100 nF bypass capacitors, and actually measured the EMI with proper equipment to see if the field intensities are even relevant for regulations in the realistic worst case situation? --- End quote --- I don't know what you muean by simple split planes. I put the capacitive CAN transciever on a 4 layer PCB with 330nF each side and it passed. |
| coppercone2:
for digital isolators and EMC, look at galvanoresistive isolators, but they are more expensive.. I had it in mind to use them before I got tired of PPM designs. |
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