HCPL3700M optocoupler: why is pulse generator connected to both AC and DC pins?
This is just a pulse speed test circuit to minimize measurement errors with a common ground and simple DC positive pulse input.

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Although intended for line frequency it's max. toggle rate of 4kHz is not speedy by design in order optimize transistor parameters for high current gain yet low bandwidth. Yet with all the optical coupling loss factors the hFE or CTR is still &lt; 1.