1). It’s based on pulse-width modulation to implement a cheap but accurate current-loop transmitter. A dual-channel isolator (E1-E2-Q1-Q2) provides galvanic isolation of the PWM input.

The accuracy of a 4-20 mA transmitter depends on the integrity of its current-loop path, which must be free of shunt paths and ground loops, both within the transmitter and external to it. External to ...

The current-loop transmitter in the figure employs an unusual trick to inexpensively implement an optically isolated 4- to 20-mA transmitter: operation of the quad-channel LED/transistor ...