Why 4~20mA Analog Output is so Popular?

The most widely used analog signal of pressure transmitters, like our UPB13 model, is 4~20mA current. Do you want to know the reason?

The reason for using the current signal is that it is not easily disturbed, because the amplitude of the noise voltage in the industrial field may reach several V, but the power of the noise is very weak. The noise current is usually less than nA level, and the error it brought to 4-20mA transmission is very small; the internal resistance of the current source tends to be infinite, and the wire resistance in series in the loop does not affect the accuracy, so it can transmit hundreds of meters on the ordinary twisted pair line. Due to the large internal resistance and constant current output of the current source, at the receiving end we only need to Place a 250 ohm resistor to ground to get a voltage of 0-5V. The benefit of a low input impedance receiver is that the nA level of input current noise produces only very weak voltage noise.

The upper limit of 20mA is due to the requirements of explosion-proof: the spark energy caused by the current on and off of 20mA is not enough to ignite the gas. 

The reason why the lower limit is not set to 0mA is to be able to detect disconnection: it will not be lower than 4mA during normal operation. 

When the transmission line is disconnected due to a fault, the loop current will drop to 0. 

For more details, please contact Stella Meng.