The principle of the two-wire transmitter is to use the 4~20mA signal to provide power for itself. If the transmitter itself consumes more than 4 mA, it will not be possible to output a lower 4 mA value. Therefore, it is generally required that the two-wire transmitter itself consumes less than 3.5 mA of power (all circuits including the sensor). This is one of the fundamental principles of the design of a two-wire transmitter.
From the overall structure point of view, the two-wire transmitter consists of three parts: the sensor, the conditioning circuit, and the two-wire V/I converter. The sensor converts physical quantities such as temperature and pressure into electrical parameters, and the conditioning circuit amplifies, conditions, and converts the weak or non-linear electrical signals output by the sensor into a linear voltage output. The two-wire V/I conversion circuit controls the overall power consumption current according to the output of the signal conditioning circuit; at the same time, the voltage is obtained from the loop and stabilized for use by the conditioning circuit and the sensor.
In addition to the V/I conversion circuit, each part of the circuit has its own power consumption current. The core design idea of the two-wire transmitter is to include all currents in the feedback loop of the V/I conversion. In a two-wire transmitter, the total power consumption of all circuits cannot be greater than 3.5 mA, so the low power consumption of the circuit becomes a major design difficulty.