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If you want accurate loop current measurement, you need a respectively connected shunt. Shunt resistance value and acceptable voltage drop is a matter of your design parameters. Presently the large load resistor value is the dominant restriction.
Sounds like you didn't actually analyze the circuit behavior. The large shunt (680 ohms) you put on receiver side forms a low pass with the cable capacitance + internal circuit capacitance like C1 in your original "circuit2". Fast current loop is only possible with low load resistance.
Still not clear why you want both fast rise time and precise current.
I'm not sure what you are asking me here. Suffice it to say, I want the fastest possible rise and fall times on the output wave forms without a lot of distortion for driving long cable lengths. Typically that is the reason to use loop current outputs rather than voltage outputs.
Sounds like you didn't actually analyze the circuit behavior. The large shunt (680 ohms) you put on receiver side forms a low pass with the cable capacitance + internal circuit capacitance like C1 in your original "circuit2". Fast current loop is only possible with low load resistance.
Still not clear why you want both fast rise time and precise current.