1V~5V signal conversion to 4mA~20mA output circuit

The 4mA to 20mA current loop has long been predicted to disappear, but this analog interface is still the most common method of connecting current loop power and sensing circuits. This interface requires converting a voltage signal (typically 1V to 5V) to a 4mA to 20mA output. The stringent accuracy requirements dictate the use of expensive precision resistors or trimming potentiometers to calibrate out the initial errors of less precise devices to meet design goals. Neither of these techniques is optimal in today's automated test equipment-dominated, surface-mount manufacturing environment. It is difficult to obtain precision resistors in surface-mount packages, and trimming potentiometers require manual intervention, which is incompatible with a production environment. 

Linear Technology's LT5400 four-matched resistor network helps solve these problems with a simple circuit that requires no trimming but achieves an overall error of less than 0.2% (Figure 1). The circuit uses a two-stage amplifier that takes advantage of the unique matching characteristics of the LT5400. The first stage amplifier applies a typical 1V to 5V output (usually 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 voltage minus the input voltage. 

There are three major sources of error in this part of the circuit: the matching of R1 and R2, the offset voltage of IC1A, and the leakage current of Q2. The exact values ​​of R1 and R2 are not important, but they must be accurately matched to each other. The LT5400A-grade version achieves this goal with an error of ±0.01%. The LT1490A has an offset voltage of less than 700μV between 0°C and 70°C. This voltage produces an error of 0.07% at an input voltage of 1V. The leakage current of the NDS7002A is 10nA, although its value is usually much smaller. This leakage current represents an error of 0.001%. 

The second stage maintains the voltage across R3 equal to the voltage across 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 placing a precise 250Ω shunt resistor in parallel with R3, this current will accurately track the input voltage. 

The error sources for the second stage are the value of R3, the offset voltage of IC1R, and the leakage current of Q1. Resistor R3 directly sets the output current, so its value is critical to the accuracy of the circuit. This circuit uses a commonly used 250Ω shunt resistor to complete the current loop. The RiedonSF-2 device in Figure 1 has an initial accuracy of 0.1% and low temperature drift. Similar to the first stage, the offset voltage produces an error of no more than 0.07%. The leakage current of Q1 is less than 100nA, producing a maximum error of 0.0025%. 

Without any trimming, the total output error is better than 0.2%. The current sense resistor R3 is the dominant error source. If a higher quality device (such as a Vishay PLT series device) is used, an accuracy of 0.1% can be achieved. The current loop output is subject to considerable stress in use. Diodes D1 and D2 from the output to the 24V loop supply and ground help protect Q1; R6 provides some isolation. Higher isolation can be achieved by increasing the value of R6 and sacrificing some conditioned voltage at the output. If the maximum output voltage requirement is less than 10V, then the value of R6 can be increased to 100Ω to provide higher isolation from output stress. If the design requires increased protection, a transient voltage suppressor can be added to the output, of course, this will result in some loss of output accuracy due to leakage current. 

This design uses only two of the four matched resistors in the LT5400 package. The other two resistors can be used for other circuit functions, such as a precision inverter, or another 4mA to 20mA converter. In addition, additional resistors can be introduced in parallel with R1 and R2. This approach reduces the statistical error introduced by the resistors by a factor of the square root of two. 

Figure 1: Precision matched resistors provide accurate voltage-to-current conversion