A question about a current sensing circuit
The general idea is that the opamp drives the NMOS until the voltage drop across the upper 200&Omega; resistor is the same as the voltage drop across the 0.1&Omega; sense resistor. The current required to do this (1/2000 of  the load current) also flows through the 2k&Omega; resistor, which is what gives you a ground-referenced output voltage that is 1V per load amp.

A MOSFET is preferred here because there is no DC gate current that would affect the accuracy. However, note that this requires the opamp to be able to drive the gate to a voltage equal to the highest expected output voltage PLUS the threshold Vgs of the transistor.

Since the total voltage across the two resistors is strictly proportional to the load current because of the action of the opamp, all of the remaining source voltage must be dropped across the transistor.

Note that there is essentially zero current through &mdash; and zero voltage drop across &mdash; the lower 200&Omega; resistor. It is there to cancel out any offset created by the opamp's input bias current.

It's very curious that the only resistor for which a tolerance is specified is the gate resistor. This is actually the resistor with the lowest requirement for tight tolerance &mdash; the tolerances for all of the other resistors is actually much more important!