This design example shows a loop-powered, 4-mA to 20-mA field transmitter featuring the AFE88101. The AFE78101 can also be used in this design for lower-resolution applications.

This design example combines several circuit elements to create a subsystem that can support most field sensors in two-wire, current-loop applications. The design accepts bus voltages from 12 V to 36 V, while regulating the loop-current representation of a sensor to a post-calibration accuracy of less than 0.1% full-scale range (FSR) of total error at room temperature. The high integration in the system allows for a compact circuit, making this device an excellent choice for field transmitters where space is a concern. In field-transmitter applications, the current-loop transmitter, microcontroller, sensors, and analog front end are all required to consume less than the minimum bus current of 3 mA. Use an integrated DC/DC converter in the system to extend the current budget and allow more current for sensors and the AFE.

Figure 8-2 shows the schematic diagram for the loop-powered, 4-mA to 20-mA field transmitter.