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High Side Sensing - The output voltage at the drain of the MOSFET has a 1:1 ratio to the current (1V/A), and is given by the formula Uout=Rd/Rnt*Rs*Il where Rd is the drain resistor, Rnt is the resistor on the negative terminal of the op amp, Rs is the shunt resistance, and Il is the load current. You can recalculate for different gain ratios of the output, but basically to keep a 1:1 ratio, just make the Rnt resistor 10000 times higher than Rs, and keep everything else the same in the circuit. The MOSFET is biased to minimize the op amp requirements, because without biasing the op amp needs to be rail to rail, and even then, accuracy is not guaranteed. This biasing forces the output of the op amp to be in the most linear region for current measurements of 1mA to around 12A. High Side Current Sensing usually causes less problems with equipment than low side sensing, but is more complex to read reliably across a wide range of currents.
Low Side Sensing - Usually low side current sensing is pretty straightforward, as the voltage drop given by the shunt resistance is referenced directly to ground and doesn't need re-referencing. The voltage to current ratio is directly proportional to the shunt resistance as given by Ohm's law. 1V/1A/1Ohm. To minimise the voltage drop across the shunt resistance, I've used a 100mOhm resistor which gives a voltage/current ratio of 100mV/A. To give a 1V/A ratio I've set up a simple voltage amplifier with a gain of 10. Anyway, low side current sensing can cause problems with sensitive circuits, because the ground potential is no longer fixed, but is floating. This can cause equipment to misbehave.
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