Self Powered Current Sensing Circuit

This is a design circuit of current sensing circuit sense the current at the high side (between positive supply and load). This circuit is take the benefit of the microampere supply current and Rail-to-Rail input of the LT1494 op-amp. Here’s the figure of the circuit;

Look at the op-amp, the inverting input is connected directly to the output, and this negative feedback will keep the inverting input to be in the same level with the non-inverting input. This means that the voltage across RA will be kept equal to the voltage across R sense.  This voltage make the current flows from the power source to the output pin of the op-amp through RA, and finally return to ground via RB. Almost all of the current at RB comes from RA since the internal circuit draws only very small amount of current through Vcc pin, so the output at RB is valid. [Circuit diagram source: Linear Technology Application Notes]