Precision current-transmitter/source-sink calibrator has wid
based on a precision op amp (max4236), this loop-powered, high-compliance, precision current sink (or source) can be modified for use as a 4–20ma sink transmitter.
a similar version of this article appeared in the march 24, 2009 issue of ed magazine.
many industrial i&c applications use analog current loops as the physical link for measurements and actuator control. current loops in the 4–20ma and 5–50ma ranges are typical, along with others less common. most popular and still surviving is the 4–20ma standard.
loop accuracy depends on the transmitter's ability to transform the sensor signal to a current, and on the stability of the current-transfer function (measured at the receiver) with respect to temperature, physical length of the loop, stability of the loop-supply voltage, and other uncertainties that appear as the hardware deteriorates with age.
besides linearity and stability of its current-transfer function, the transmitter output of a current-loop circuit must provide a large dynamic impedance, to minimize the influence of loop voltage (at the transmitter terminals) on output current. the maximum voltage tolerated at the terminals is also important. the higher the better, because the electromagnetic environment in industrial plants is not always benign. the minimum allowed transmitter-terminal voltage, which determines the maximum loop length for a given wire gauge (or the minimum wire cross-section needed for a given loop run) should be minimized at the same time.
the transmitter circuit of figure 1 provides a very high output impedance (108 to 109 ohms) and a wide compliance range (4v to 90v). the lower limit is set by the amplifier's minimum operating voltage, and the upper limit by the power dissipation allowed in the output device (an n-channel depletion mosfet) at the 20ma high end of the current range. for brief periods of 20ms or so, the maximum voltage can go as high as 200v.
figure 1. this basic loop-powered, high-compliance, precision current sink (or source) (a) can be modified for use as a 4–20ma sink transmitter (b).
the basic circuit (figure 1a) is configured as a loop calibrator. with the components shown, it can operate with 10-bit accuracy from 0°c to 85°c, plus the added uncertainty of resistor r. having only two terminals, it can operate as either a sinking or a sourcing calibrator.
in figure 1b, this circuit is modified for use as an interface to an open-loop current sensor with 0v-to-4v output range. accuracy of the transfer function in this case depends on the reference (max6138) and the precision of the resistors in the interface network. because the reference equipotential (ground) is shared by the instrument and the transmitter, you should exercise care not to create ground loops in the circuit implementation.
the max4236 input characteristics (20µv maximum offset and 2µv/°c offset temperature drift), its high gain (110db), and its ability to operate with inputs down to the negative rail, makes negligible the influence of all other components. to achieve higher accuracy, therefore, you must use standard trimming techniques that employ either mechanical or integrated-circuit adjustment potentiometers.
the max4236 ability to operate with input voltages down to the negative rail is essential for circuit accuracy, because it allows the amplifier's operating current to be forced through the output current-sense resistor, where it becomes a part of the output current under circuit control. further information, including full data sheets for the max4236, max5490, and max6138 is available at www.maxim-ic.com.
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a similar version of this article appeared in the march 24, 2009 issue of ed magazine.
many industrial i&c applications use analog current loops as the physical link for measurements and actuator control. current loops in the 4–20ma and 5–50ma ranges are typical, along with others less common. most popular and still surviving is the 4–20ma standard.
loop accuracy depends on the transmitter's ability to transform the sensor signal to a current, and on the stability of the current-transfer function (measured at the receiver) with respect to temperature, physical length of the loop, stability of the loop-supply voltage, and other uncertainties that appear as the hardware deteriorates with age.
besides linearity and stability of its current-transfer function, the transmitter output of a current-loop circuit must provide a large dynamic impedance, to minimize the influence of loop voltage (at the transmitter terminals) on output current. the maximum voltage tolerated at the terminals is also important. the higher the better, because the electromagnetic environment in industrial plants is not always benign. the minimum allowed transmitter-terminal voltage, which determines the maximum loop length for a given wire gauge (or the minimum wire cross-section needed for a given loop run) should be minimized at the same time.
the transmitter circuit of figure 1 provides a very high output impedance (108 to 109 ohms) and a wide compliance range (4v to 90v). the lower limit is set by the amplifier's minimum operating voltage, and the upper limit by the power dissipation allowed in the output device (an n-channel depletion mosfet) at the 20ma high end of the current range. for brief periods of 20ms or so, the maximum voltage can go as high as 200v.
figure 1. this basic loop-powered, high-compliance, precision current sink (or source) (a) can be modified for use as a 4–20ma sink transmitter (b).
the basic circuit (figure 1a) is configured as a loop calibrator. with the components shown, it can operate with 10-bit accuracy from 0°c to 85°c, plus the added uncertainty of resistor r. having only two terminals, it can operate as either a sinking or a sourcing calibrator.
in figure 1b, this circuit is modified for use as an interface to an open-loop current sensor with 0v-to-4v output range. accuracy of the transfer function in this case depends on the reference (max6138) and the precision of the resistors in the interface network. because the reference equipotential (ground) is shared by the instrument and the transmitter, you should exercise care not to create ground loops in the circuit implementation.
the max4236 input characteristics (20µv maximum offset and 2µv/°c offset temperature drift), its high gain (110db), and its ability to operate with inputs down to the negative rail, makes negligible the influence of all other components. to achieve higher accuracy, therefore, you must use standard trimming techniques that employ either mechanical or integrated-circuit adjustment potentiometers.
the max4236 ability to operate with input voltages down to the negative rail is essential for circuit accuracy, because it allows the amplifier's operating current to be forced through the output current-sense resistor, where it becomes a part of the output current under circuit control. further information, including full data sheets for the max4236, max5490, and max6138 is available at www.maxim-ic.com.
飞利浦照明推出可见光通信LiFi技术
电磁干扰是怎么产生的 电磁干扰屏蔽简易方法
热轧钢厂为去氧化皮而烦恼，有什么解决方案吗
运算放大器输入偏置电流的理解和影响
电源设计的患者安全和舒适
Precision current-transmitter/source-sink calibrator has wid
八种不同类型的抽样技术介绍
开放原子开源基金会TOC（技术监督委员会）第七十五次全体会议
串口调试要注意什么？串口调试工具有哪些？
远程医疗和远程保健服务加速医疗物联网的需求
分析自举电路的作用
现代化大企业中IT与OT的分工
中国超美国成全球最大智能设备市场
敷铜板的种类,PCB classification
物联网IoT技术应用下，智慧社区如何做到安全与智慧并存
LED灯向农产业方向的迈进
自适应实时DSP架构用于监测电网中的谐波成分和各种电能质量因素
可信软件堆栈和可信计算组的DICE架构
区委书记莅临新亚胜光电走访调研
AAL2/AAL5等ATM连接的区别