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[ATTACH=full]198090[/ATTACH]

All the currents that flow in individual inputs flow, in their sum, thru the fdbk circuit.

Keep in mind, because of virtual ground , each input thinks its current is going into ground.

And writing KCL Ir1 = Va/R1, Ir2=Vb/R2.......

Is the OpAmp summing currents ? I like to think its steering them, and isolating them from each

other at the input side.

Not sure why you have R15. Because its got virtual ground across it no current flows thru it, hence

its irrelevant. However there is a small current thru it due to OpAmp input offset V, and G effects,

eg. Aol (open loop gain) of OpAmp finite so virtual ground is not exactly = 0.

Regards, Dana.