Convert 1V~5V signal to 4mA~20mA output
Although it has long been predicted that the 4mA to 20mA current loop will disappear, this analog interface is still the most common method of connecting current loop power and detection circuits. This interface requires a voltage signal (typically 1V to 5V) to be converted to an output from 4mA to 20mA. Strict accuracy requirements determine that expensive precision resistors or trimmer potentiometers must be used to calibrate the initial error of less-precision devices to meet design goals. In today's automated test equipment-based and surface mount production environments, neither of these technologies is the best approach. It is difficult to obtain a precision resistor in a surface mount package, and the trimmer potentiometer requires manual intervention, which is incompatible with the production environment.
Linear Technology's LT5400 four-match resistor network helps solve these problems. The network uses a simple circuit that does not require fine-tuning 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 adds a typical 1V to 5V output (usually from the DAC) to the non-inverting input of op amp IC1A. This voltage accurately sets the current through R1 to VIN/R1 through FET Q2. The same current is pulled low through R2, so the voltage at the bottom of R2 is the 24V loop supply voltage minus the input voltage.
This part of the circuit has three main sources of error: the match between 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 match each other exactly. The LT5400A grade achieves this with ±0.01% error. 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 NDS7002A has a leakage current of 10nA, although its value is usually much smaller. This leakage current represents an error of 0.001%.
The second stage pulls the current through Q1, keeping the voltage on R3 equal to the voltage on R2. Because the voltage across R2 is equal to the input voltage, the current through Q1 is exactly equal to 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 source for the second stage is the value of R3, the offset voltage of IC1R, and the leakage current of Q1. Resistor R3 sets the output current directly, so its value is critical to the accuracy of the circuit. This circuit uses a common 250Ω shunt resistor to complete the current loop. The initial accuracy of the Riedon SF-2 device in Figure 1 is 0.1% and the temperature drift is very low. Similar to the case of the first stage, the offset voltage produces an error of no more than 0.07%. The leakage current of Q1 is less than 100nA, and the maximum error generated is 0.0025%.
The total output error is better than 0.2% without any fine tuning. Current sense resistor R3 is the primary source of error. If you use a higher quality device (such as the Vishay PLT series device), you can achieve 0.1% accuracy. The current loop output is subject to considerable stress during 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 at the expense of some qualified voltage at the output. If the maximum output voltage requirement is less than 10V, then the value of R6 can be increased to 100Ω, providing higher isolation for output stress. If the design requires enhanced protection, then a transient voltage suppressor can be added to the output, which of course results in a loss of output accuracy due to leakage current.
This design uses only two of the four matching resistors in the LT5400 package. It is also possible to use two additional resistors for other circuit functions (such as precision inverters) or another 4mA to 20mA converter. In addition, other resistors can be introduced in parallel with R1 and R2. This method reduces the statistical error produced by the resistor by a square root of 2.
Figure 1: Exactly matched resistors provide accurate voltage to current conversion
Telephone receiver cable, 4p4C configuration, RJ9 cable
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