Despite the long-predicted demise of the 4- to 20-mA current loop, this analog interface is still the most common method of connecting current-loop sources to a sensing circuit. This interface requires the conversion of a voltage signal—typically, 1 to 5V—to a 4- to 20-mA output. Stringent accuracy requirements dictate the use of either expensive precision resistors or a trimming potentiometer to calibrate out the initial error of less precise devices to meet the design goals.
Neither technique is optimal in today’s surface-mounted, automatic-test-equipment-driven production environment. It’s difficult to get precise resistors in surface-mount packages, and trimming potentiometers require human intervention, a requirement that is incompatible with production goals.
The Linear Technology LT5400 quad matched resistor network helps to solve these issues in a simple circuit that requires no trim adjustments but achieves a total error of less than 0.2% (Figure 1). The circuit uses two amplifier stages to exploit the unique matching characteristics of the LT5400. The first stage applies a 1 to 5V output—typically, from a DAC—to the noninverting input of op amp IC1A. This voltage sets the current through R1 to exactly VIN/R1 through FET Q2. The same current is pulled down through R2, so the voltage at the bottom of R2 is the 24V loop supply minus the input voltage.
This portion of the circuit has three main error sources: the matching of R1 and R2, IC1A’s offset voltage, and Q2’s leakage. The exact values of R1 and R2 are not critical, but they must exactly match each other. The LT5400A grade achieves this goal with ±0.01% error. The LT1490A has less-than-700-μV offset voltage over 0 to 70°C. This voltage contributes 0.07% error at an input voltage of 1V. The NDS7002A has a leakage current of 10 nA, although it is usually much less. This leakage current represents an error of 0.001%.
For more detail: Convert 1 to 5V signal to 4- to 20-mA output