This design implements the 4mA to 20mA output stage for field transmitters and sensors. AFE881H1 and AFE882H1 integrates digital-to-analog conversion capabilities with high resolution up to 16 bits. Low power consumption allows the system to operate directly from a loop supply. Moreover the AFE has an integrated MODEM to enable HART communication functionality. With AFE881H1, the whole design can be operated on a single 1.8V power supply.

Polarity reversal protection is achieved through the use of a Graetz bridge at the input, enabling operation in either polarity. Overvoltage protection is provided by the TVS3301 TVS diode, safeguarding against excessive voltage levels. High-frequency noise is reduced by the presence of a ferrite bead.

Current regulation is achieved through a pass transistor paired with a Zener diode and the first LDO. The system sinks part of the current and sinks the rest mostly in the Zener diode, allowing power dissipation to be shared among multiple devices and minimizing thermal stress. A dedicated operational amplifier translates the output voltage of the DAC into the loop current.

Two power supplies, 1.8V and 3.3V, are available from LDOs to power the components of the system. A stable 1.25V reference voltage is provided directly on the board circuit. The reference voltage output is also accessible through a pin header.