Galvanic isolation means that there is no "copper" connection.... so a balanced connection is in no way equal to galvanic isolation - nor is differential.
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But perhaps a capacitor coupled differential feedback connection could do the job? Capacitor on both hot and cold - no ground - so only 2 wires between the two sections to carry the feedback signal - and no other galvanic connection.
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... the choke being the inductor in the output filter? If so it won't work as before the choke the signal varies in voltage sving which the PWM does not.
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That's a question of definition. I agree with You if galvanic isolation is defined at the interface between "boxes", otherwise a balanced or differential signal transfer can be isolated between sender and receiver. Safest is optical, but with it comes all the non-linearities of the optics, next is transformers which has their limitations(hysteresis, remanens and so on). A good solution should be based on what signal (freq, waveform, amplitude) You want to transfer and how high common-mode rejection ratio is needed. I have measured a fully functional RS-485-bus(max +/-15V) with over 100V common-mode signal.
Google says: "Galvanic isolation refers to a design technique that isolates electrical circuits, preventing direct current flow while still allowing signal and power transfer between them."
This could be on one and the same circuit board so close vicinity - there should not have to be very high common mode disturbances.
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This could be on one and the same circuit board so close vicinity - there should not have to be very high common mode disturbances.
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Before the choke, the signal is PWM and not a a filtered / reconstructed voltage as in post-filter feedback.
However, on second thoughts, I think it may not work well, as the PWM is also known to carry the inevitable voltage fluctuations from the DC bus that digital optos wouldn't pass on, affecting the PSRR of the amplifier, especially in half-bridge configurations.
It looks like TOSLINK is probably the way to go ...
You are of course correct, it's even in the picture in post #1 😎
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... and passed on to the DAC via ground... hence, "the problem" 😉
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Galvanic isolation is not an issue, but isolation without distortion is what is. A fast optocoupler with feedback has been used to isolate video signals and it might just work here, as well.
From pg 49 of:
https://user-web.icecube.wisc.edu/~kitamura/NK/Datasheets/misc/5988-4082EN designers guid.pdf
From pg 49 of:
https://user-web.icecube.wisc.edu/~kitamura/NK/Datasheets/misc/5988-4082EN designers guid.pdf