Design Goals

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.

Design Notes

1. Use low-tolerance resistors to achieve high current gain matching accuracy.
2. All resistors and capacitors must be verified space-grade for this design.
3. In this design, I_ret is a local ground, separate from the supply ground, and must be allowed to float.
4. The current through R₃ can be considered a quiescent current. That is, it is a current that should be minimized so as to not use up the current budget. However, as I₃ gets smaller, the matching ratio R₃ / R₄gets larger, so in practice I₃ cannot be made arbitrarily small.
5. For linear operation, the ratio of the current through R₃ when V_in = V_{in_max} and V_in = V_{in_min}, I_{3_max} / I_{3_min}, must be equal to the ratio of the maximum output current to the minimum output current, I_{out_max} / I_out-min.
6. This discrete 2-wire current transmitter solution allows the flexibility to set the output current to be different than the standard 4–20 mA or 4–24 mA. This may be necessary because options for space-grade components are relatively limited. For example, if the total quiescent current for the current transmitter plus surrounding peripherals on the same supply loop cannot meet the requirement that they sum to less than I_{out_min} (= 4 mA for this design), the output current range may be shifted to 8–24 mA or something similar.
7. C₁ is a bypass compensation capacitor and is required for the stability of the circuit. Recommended capacitor is 100 nF or greater.
8. R₅ and C₂ are additional components for compensating the circuit. They are not required for stability, but are added for flexibility if additional compensation is required.
9. C₃ is a required output capacitor for the TPS7A4501-SP. Recommended capacitor is 1 µF to 10 µF for stability.
10. R₆ linearizes the relationship between the op-amp output and the current through the transistor and limits the current through the transistor.
11. This design can be implemented with a single LMP2012QML-SP or a similar device. See Design Alternate Op Amp for an alternative device.