SIGNAL CONDITIONING FEATURE


IN THE LOOP: Converting 1-5V signal


to 4-20mA output T


he 4mA to 20mA current loop is still the most common method of


connecting current-loop sources to a sensing circuit, despite its long-predicted demise. This analogue interface requires the conversion of a voltage signal – typically 1V to 5V – to a 4mA to 20mA output. However, while accuracy requirements dictate the use of either expensive precision resistors or a trimming potentiometer to calibrate out the initial error of less precise devices, neither technique is optimal in today’s surface-mounted, automatic test equipment-driven production environment. It is 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% (see image). The circuit uses two amplifier stages to exploit the matching characteristics of the LT5400. The first stage applies a 1V to 5V output – typically from a DAC – to the non-inverting input of op amp IC1A voltage sets the current through R1 exactly VIN


/R1 through FET Q2 . The same


current is pulled down through R2 voltage at the bottom of R2


, so the 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 leakage. The exact values of R1 ’s and R2


are not critical, but they must match each other exactly. The LT5400A grade achieves this goal with ±0.01% error. In addition, it has less than 700µV offset voltage over 0 to 70˚C, voltage which contributes 0.07% error at an input voltage of 1V. The NDS7002A has a


Q1 ’s leakage current. Resistor R3 directly


sets the output current, so its value is crucial to the precision of the circuit. This circuit takes advantage of the commonly used 250Ω current-loop-completion shunt resistor. The Riedon SF-2 part in the figure has 0.1% initial accuracy and low temperature drift. As in the first stage, offset voltage contributes no more than 0.07% error. Q1


has less than


“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%”


leakage current of 10nA, although it is usually much less. This leakage current represents an error of 0.001%.


STAGE TWO


The second stage holds the voltage on R3


equal to the voltage on R2


pulling current through Q1 the voltage across R2


. This to


voltage, the current through Q1 by . Because


equals the input , is


exactly the input voltage divided by R3


By using a precision 250Ω current shunt for R3


are R3 ’s value; IC1R , the current accurately


tracks the input voltage. The error sources for the second stage ’s offset voltage, and


.


Precision matched resistors provide accurate voltage-to- current conversion


100nA leakage, yielding a maximum error of 0.0025%. 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


, with the provides some


isolation. You can achieve more isolation by increasing the value of R6


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 4mA to 20mA 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.


Linear Technology Corporation www.linear.com


 INSTRUMENTATION | MARCH 2016 23


Thomas Mosteller, field applications engineer at Linear Technology Corporation, looks into how the company’s LT5400 quad matched


resistor network overcomes the issues faced when using the 4mA to


2mA analogue interface to connect current-loop sources to a sensing circuit


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