Figure 8-3 shows a block diagram of a loop-powered, 4-mA to 20-mA current transmitter.

The terminals connected to the loop are shown on the right side of the block diagram. This connection to the loop powers the entire transmitter. A bridge rectifier at the input protects against reverse connection to the loop. The rectified loop voltage powers a start-up circuit that provides power to an LDO, that in turn powers the AFE88101. The LDO powers a flyback converter acting as a boost and supplies power across an isolation barrier. On the other side of the isolation barrier, another LDO powers the MCU and any sensor connected to the transmitter. The LDOs also power the digital signal isolation on each side of the barrier.

The AFE88101 controls the loop current through the voltage-to-current (V-to-I) converter block. The DAC voltage sets the output from 0.3 V to 2.5 V. The output is sent through a V-to-I converter block using an OPA333 and an NPN bipolar junction transistor (BJT).