The circuit in Figure 1 provides a 4- to 20-mA voltage-to-current conversion for noise-immune data transmission in industrial data-acquisition systems. The circuit supplies two voltages to the inverting input of the control amplifier IC_1A. The first voltage, from the follower IC_1B, introduces an offset in the transfer function of the circuit. The IC_1C-IC_1D differential stage produces the second voltage, which is twice the voltage across the current-sensing resistor, R_CS. The R₁-R₁ divider halves and sums the two voltages at the input of IC_1A.

The control amplifier IC_1A must deliver enough current through R_CS to stay within the linear region. The transfer function of the converter is

With the values of V₂ and R_CS (Figure 1), the circuit produces the standard 4- to 20-mA industrial range when the input voltage V₁ varies from -1 to 1V. The circuit’s linearity is better than 0.02%. The capacitor connected to IC_1A ensures start-up of the system upon power-up; the transistor serves as a current booster.

Two features make implementing this converter easy. First, the circuit relies on resistor ratios rather than on absolute tolerances. The choice of resistor values is not critical; values in the region of 5 k(ohm) give satisfactory results. Second, the circuit eliminates the influence of amplifier offset voltages on the output current. Because the offset voltages and their temperature drift are similar for all the amplifiers on the TL074 quad-op-amp chip, you don’t need to use temperature-compensation techniques. (DI #1819)