Convert 1 to 5V signal to 4- to 20-mA output
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.
The second stage holds the voltage on R3 equal to the voltage on R2 by pulling current through Q1. Because the voltage across R2 equals the input voltage, the current through Q1 is exactly the input voltage divided by R3. By using a precision 250Ω current shunt for R3, the current accurately tracks the input voltage.
Total output error is better than 0.2% without any trimming. Current-sensing resistor R3 is the dominant source of error. If you use a higher-quality device, such as the Vishay PLT series, you can achieve an accuracy of 0.1%. Current-loop outputs are subject to considerable stresses in use. Diodes D1 and D2 from the output to the 24V loop supply and ground help protect Q1; R6 provides some isolation. You can achieve more isolation by increasing the value of R6, with the trade-off of some compliance voltage at the output. If the maximum output-voltage requirement is less than 10V, you can increase R6’s value to 100Ω, affording even more isolation from output stress. If your design requires increased protection, you can fit a transient-voltage suppressor to the output with some loss of accuracy due to leakage current.
This design uses only two of the four matched resistors in the LT5400 package. You can use the other two for other circuit functions, such as a precision inverter, or another 4- to 20-mA converter. Alternatively, you can place the other resistors in parallel with R1 and R2. This approach lowers the resistor’s statistical error contribution by the square root of two.