Two-wire loop requires only 2 wires for power and output connections.
Two-wire loops may be used for transmitting 4-20mA signals. Two-wire loops can also be used for connecting voltage generating sensors like thermocouple, pH and ORP sensors etc. These later sensors usually generate output signals of moderately low level (0-500mV) and are very susceptible to induced noise.
The two-wire current-mode configuration overcomes many of these constraints. It allows local signal conditioning, and because it operates in a manner that is relatively immune to induced noise.
it has become the standard for most critical process-control measurements. This configuration also offers economic advantages because of reduced wiring cost and its compatibility with intrinsically safe installation practices.
However, it does impose constraints on the design of both the transmitter and data acquisition equipment.
First, the transmitter must perform all functions within a limited power budget. Sensor excitation, amplification and signal conditioning functions must be accomplished with 4mA or less at voltages typically ranging from about 12 to 36 volts.
At the receiver, the signal usually must be converted into a voltage and level-shifted to a zero-based range so that subsequent conversion into digitized format may be done using analog-to-digital converter.
Both the current to voltage conversion and level shifting can introduce extra errors in both zero and span if proper care is not taken for circuit design.