The presented circuit is suggested by Texas Instruments (www.ti.com) in its Application Report SBOA107B and uses as base two integrated circuits of the company, the XTR115 and the PGA309. The purpose of the circuit in Figure 1 is to generate a 4 mA to 20 mA signal for interfacing between sensors and processing circuits in current mode. The circuit employs the 4 mA to 20 mA current mode industry standard where the voltage output of the PGA309 can be turned into current mode by the XTR115 current transmitter. Resistors R1 and R2 are responsible for transforming the sensor output from voltage mode to current mode. The purpose of resistor R3 is to provide the circuit offset in order to determine the minimum current of 4 mA at the beginning of the scale. The circuit is also equipped with a low-pass filter in order to cut the noise that may be present, within a limit of 500 Hz. More information about this circuit can be obtained on the company's website.
