Hi,Yes I DO understand HOW a summing (and differential) amp works. Do you?
What makes is SO difficult is when you try to add a bias offset to the differential amp.
The differential amp depends upon the the ratio of Rg and R2 being identical to the the Ratio of Rf and R1.
Any added resistor must not disturb this ratio.
Thus for example, in the circuit in Post #12 the equivalent parallel resistance of R3 and R4 must equal the value of RL.
The Value of R3 and R4 must also be selected to give the desired 5V offset at the output.
I don't see how to get R3 and R4 to satisfy both criteria without solving two simultaneous equations for their values.
If you can, than I would like to see how.
If you use a floating voltage reference you can do it, but the OP claims that the negative rail is available so we can actually use that to bias the (other) input. It doesnt work as a 'summer' because we arent just summing as you know, but on the other input we can bias the 'ground' and that is not a summer. That's one way to do it, but not sure if you want to explore more ways too which would be interesting.