The 4-20-mA current loop transmitter is responsible for powering all bridge and INA circuitry as well as providing accurate current scaling. The device requires some external circuitry to function properly as shown in Figure 2-5.

Figure 2-5 Surrounding Circuitry for Current Loop Transmitter

The XTR 5-V on chip voltage regulator (V_REG) powers the 2-amp INA while the 4.096-V precision reference (V_REF) powers the bridge and INA reference voltage divider. V_REG can shift hundreds of mV depending on the amount of current it is sinking or sourcing, so it is important to use V_REF when a precise voltage is required (see XTR116 data sheet, Figure 6-5). Current drawn from V_REG or V_REF must be returned to the local ground pin (I_RET).

Resistors R_IN1 and R_IN2 convert the output voltage of the INA into a current that can be fed into the I_IN pin. The offset current resistor (R_OS) is used to add an additional 20-μA of current to I_IN to boost the input current to the desired 40-200-μA. The XTR provides a gain of 100 which is seen at the output current pin (I_OUT = 100*I_IN). Capacitor C_IN and R_IN1 form a low pass filter at the output of the INA to limit noise. Calculations for these passive components are shown in the following equations.

A 10-nF decoupling capacitor (C_S1) between the loop supply voltage (V+) and the output current pin (I_OUT) is recommended. The capacitor and output load resistor form a low pass filter that limits the bandwidth of the system. An external transistor (Q₁) is required to conduct the majority of the output current to avoid on-chip, thermal-induced errors. Power dissipation in this transistor can approach 1-W with maximum loop voltage and output current. For additional protection, a diode bridge and clamping diodes can be considered for reverse voltage and overvoltage surge protection respectively.