Pressure, temperature, velocity, angle, etc., are all non-electrophysical quantities that are commonly measured in industry. These physical quantities need to be converted into analog electrical signals before they can be transmitted to the control room or data display equipment. And this device that converts physical quantities into electrical signals is collectively known as a transmitter. At present, the most widely used in the industry is the 4~20mA two-wire current output analog transmitter. The principle of the two-wire transmitter is to use the 4~20mA signal to provide electrical energy for itself. If the power consumption of the transmitter itself is greater than 4mA, the lower limit value of 4mA will not be output. Therefore, the power consumption of the two-wire transmitter itself is not more than 3.5mA. This is one of the fundamental principles of two-wire transmitter design. Structurally, a two-wire transmitter is composed of a sensor, a conditioning circuit and a two-wire V/I converter. The sensor converts physical quantities such as pressure and temperature into electrical parameters, and the conditioning circuit amplifies, conditions, and converts the weak or nonlinear electrical signals output by the sensor into linear transmission. The two-wire V/I conversion circuit controls the overall current consumption based on the output of the signal conditioning circuit, and at the same time obtains the voltage from the loop and regulates it for use by the conditioning circuit and sensors. The core design idea of a two-wire transmitter is that all current is contained within the feedback loop of the V/I conversion.

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