If you've ever wondered why factories still use a 1950s-era analog signaling standard in the age of Ethernet, the answer is simple: current loops are nearly indestructible. A 4-20mA loop transmits a sensor reading as a regulated DC current rather than a voltage, and that single design choice solves a remarkable list of problems.

Why current beats voltage over long wires:

• Wire resistance doesn't matter. A current source pushes the same current through 10 ohms or 500 ohms. Voltage drop across the cable simply shifts the compliance requirement, not the signal.
• Noise pickup is rejected. Induced EMI creates voltage noise, but the loop's low impedance shunts it. A 1 kohm receiver sees orders of magnitude less noise than a high-impedance voltage input.
• Broken wires are detectable. "Zero signal" is 4 mA, not 0 mA. If the receiver reads 0 mA, the cable is severed or the transmitter is dead — you get free diagnostics.
• Two-wire power. A "loop-powered" transmitter pulls its own operating current from the same loop, so a single twisted pair carries both power and signal.

The basic topology: a regulated current source (the transmitter) modulates loop current between 4 mA (sensor minimum) and 20 mA (sensor maximum). The receiver is just a precision shunt resistor — typically 250 ohms — that converts the current back to a 1-5 V signal for an ADC.

Concrete example: a pressure transducer measuring 0-100 psi outputs 4 mA at 0 psi and 20 mA at 100 psi. With a 250 ohm receive resistor, the ADC sees 1.000 V to 5.000 V. To compute pressure: P = (V_ADC - 1.0) × 25 psi/V. If V_ADC = 3.2 V, pressure is 55 psi.

Compliance voltage rule of thumb: the transmitter needs enough headroom to push 20 mA through every series element. Sum them up: receive resistor (250 ohm × 20 mA = 5 V) + cable resistance (say 50 ohm round-trip = 1 V) + transmitter internal drop (typically 3-7 V for loop-powered devices) + safety margin. A 24 V supply is the universal industrial standard because it leaves ample compliance for 1000+ meter runs.

Practical design tips: use a precision shunt with 0.1% tolerance — your accuracy floor is set here. Add a small input cap (10-100 nF) across the shunt to filter switching noise, and a TVS diode for surge protection. For loop-powered transmitters, the XTR series from TI (XTR115/116/117) handles the regulation chore in a single chip.