The minimum output current of my 2-wire 4-20mA transmitter is greater than 4mA.
Why is that? How can I lower it?
The current consumption of a 2-wire transmitter, sensor, and its driving circuitry needs to be below 4mA. As seen in the current flow diagram below for the XTR116, the output current, IO, is a summation of IIN, IRET, and IBJT. IIN and IBJT are always positive, so IO can never be smaller than IRET. Be sure to pay attention to the active currents of the components used, not just quiescent current, especially with any digital devices. Often, a ROS resistor is added between the reference and input of the XTR to add a 40uV offset to the transmitter. This causes the output to be 4mA when the voltage input is 0V. Without a ROS resistor, when the input to the XTR is 0V the current output will be the minimum output the transmitter can produce. This will be roughly equal to the supply current of the sensor transmitter circuitry being powered from the loop.
If your driving circuitry must consume more than the 4mA, it is possible to supply the driving circuitry with its own supply. (See: Can I use a 2-wire transmitter with a sensor or microcontroller that uses its own supply?) Or if possible, you can switch to a 3 or 4-wire transmitter.
The attached article goes into more detail on 2-wire transmitter current consumption limitations.
2-Wire 4-20mA Sensor Transmitters: Sensor and Transmitter Current Consumption Limitations
