Take a standard inverting amplifier: I'm unclear on what the input impedance is. At first I thought it was the equivalent resistance from the inverting input to the ground, which would be Rin || Rf, because there's a ground on the other side of Vin and Vout as well as inside the output of the opamp. However, most sources on the internet seem to claim that the input impedance is Vi/Ii, thus making it Rin. This appears to ignore the fact that the feedback wire connects to other grounds. Then there are answers like this that mention that input impedance is infinity. I'm hoping to figure out a clear definition of what the input impedance represents and a [brief] general approach to calculating it in a more complicated circuit. Thank you!
I read tons of pages like this answer and others, but at my beginner level it was hard to extract a clear answer.