Hello again guys,
Mike:
Your schematic makes this easier to talk about
The primary issue is the reference voltage in relation to the voltage drop across (your) R5, which measures the output current by providing a voltage drop.
The voltage reference provides a voltage drop relative to +Vcc (top of your V1), we'll call it Vref.
The inverting terminal is connected to the bottom of (your) R5, so it is trying to sense the voltage across R5.
When the voltage across R5 equals the value of Vref, the circuit is in equilibrium due to the fact that the voltage at the non inverting terminal and the voltage at the inverting terminal are very nearly equal, and that is the point where the op amp will stop increasing (or decreasing) voltage to the gate of the MOSFET.
Now on the other hand, if the inverting terminal went to the top of (your) R7 instead, then we would have to connect the reference voltage such that it was referenced to ground (bottom to ground, reference voltage from the top or resistor divider as needed). You'll see many circuits constructed like that, but you'll also see many circuits constructed like this one, sometimes with the reference voltage still referenced to ground in the more non critical applications (although it wont be as good if +Vcc can change much during normal operation which includes load regulation, line regulation, temperature aspects). After i thought about your replies i think this might be what seems confusing...that the voltage is a voltage no matter where it comes from. If we do reference it from ground however it is not as good as explained above (we have to consider variations in Vcc and their effect on the output current regulation).
The secondary issues are:
1. The resistances must be right in order to get the right current out or range of currents out.
2. The voltage reference must be of the right level.
3. The voltages at the inputs to the op amp must meet the common mode input range spec's.
4. The MOSFET must be of the right type (N or P channel) in order to work properly with the direction the output of the op amp takes when an error voltage is sensed. The wrong type will cause an output latch-up, and possibly other issues.
But the primary issue (voltage reference referenced to +Vcc instead of ground) is correct as is. If there are values that have to be changed to meet the practical aspects of the circuit (a few mentioned above) then they have to be corrected too in order to get it to work properly.
If you still have doubts, try simulating with 12.000v first, then with say 13.000v, and note any change in the output current with some fixed load.
Then, try referencing the voltage reference to ground by using a 10.75v reference (if that is what you measured it to be from ground up), and do the same thing: vary Vcc from 12 to 13 volts and see what effect that has on the output. I think you may be surprised
Of course i am assuming that the practical aspects of the circuit have been met already. If not, that has to be corrected before doing the two tests.