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A signal representation of 4-20ma is common because it does not require a lot of power - using a typical 24 volt power source, the resistance to deliver 20ma is only 1.2 KOhms and the power is only 0.48 watts - and, by offsetting the low end to 4ma, the receiver can detect an open connection.

Normally, a 4-20ma current loop is connected with a twisted pair cable, with both the current source and the current sink on one end. This way, you get good common mode rejection of noise. Using a differential amplifier with, say, a 10 ohm receiver resistor, you only need a delta voltage of 200 mV, so you can still have a substantial common mode range, typically, + or - 12 volts.

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