Please refer to the simplified circuit in the attached figure. The overall goal is to control the current through sense resistor R1 by adjusting a reference voltage V2, thus creating an electronic load where the power is dissipated in Q1. All load current is returned locally in the isolated loop on the right. By sensing the current differentially with e.g. AD629 and using the output as an error signal, U3 should to my mind simply drive Q1 to whatever level necessary thus closing the loop. However, in weak moments I start to think about what exactly is the relationship between the gate and source of Q1 since they are referenced to different ground domains. Is something off here?
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