4.2 Use Smart Analog Combo in Current Loop

4- to 20-mA current loop is a standard for transmitting remote sensor information to programmable logic controllers over long distances in industrial process monitoring, control, and automation applications. Using the capabilities of the SAC, current loop application designs can have flexible analog configuration while saving BOM cost and PCB size. Figure 15 shows the SAC use case in a 4- to 20-mA current loop application. The first-stage SAC is configured in DAC mode and works as a voltage generator. The second-stage SAC is configured in general-purpose mode and connected with external resistor and transistor. The output of the first-stage SAC is connected to the positive input of second-stage SAC. The second-stage SAC controls the gate voltage of the external transistor and converts the voltage input into a current output. The current value I_Loop is determined by the 12-bit DAC output voltage and the resister values. Setting R₁ = 99 kΩ, R₂ = 100 Ω, R_IN = 100 kΩ, then I_Loop is calculated by Equation 1.

The 4- to 20-mA Loop-Powered RTD Temperature-Transmitter Reference Design Using Smart Analog Combo of MSP430™ MCUs is based on the MSP430FR2355 MCU and demonstrates how to use the SAC in a current loop application.