Design Description

This design uses discrete components to implement a 4- to 20-mA V-to-I current transmitter designed using space-grade (SP) components for use in space applications. The circuit converts a differential voltage to a current which can be transmitted over a long cable. Linear operation of this current transmitter depends on the op amp input common-mode range, the headroom available for the transistor and LDO, and linear matching of currents through R₃ and R₄ across the input voltage range. Since the inverting input to the op amp is tied to its negative supply, the op amp must support common-mode voltages to its negative rail. It is important that 2-wire loop-powered current transmitters plus any peripherals that are on the same supply loop consume less quiescent current than the minimum output current because the loop return current flows through the output path.