I suspects the issue might be semantics. IMHO the output signal reference is what I would call ground. In this case the output reference (aka common) is: U2 pin -Vs and U1 anode.
I suspects the issue might be semantics. IMHO the output signal reference is what I would call ground. In this case the output reference (aka common) is: U2 pin -Vs and U1 anode.