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      발표자

      Hello and welcome to the TI Precision Lab series discussing current loop transmitters. In this series, we'll be covering concepts around current loop transmitters focusing on the common 4 to 20 milliamps range. This video will serve as an introduction to the 4 to 20 milliamp concept, as well as provide a few application examples and different approaches to designing a current transmitter into your system. Future videos in this series will discuss the different wiring configurations of current loop transmitters, specifically as 2-wire and 3-wire, discuss design considerations of each configuration, and introduce the integrated solutions that TI offers.

      The integrated solution that we will cover have a number of on-chip features that would provide you a simpler design process and a high level of precision. At the end of this series, you will have a better understanding of how to use TI's 2-wire and 3-wire current loop transmitters in your design. Industrial and building automation are becoming more common to reduce cost and increase efficiency.

      The addition of automation can save energy in a building or increase the speed and quality of work in industrial applications. In an industrial facility, such as a manufacturing site, there are field sensors, actuators, and motors that monitor and carry out process controls. Things such as level sensors, temperature sensors, valves, and heaters may control the level and temperature of liquid in a tank. In a large building or across a campus of several buildings, many monitoring devices control systems in the building, such as lighting, HVAC, or humidity control.

      Process control allows for these automations. The sensors and monitoring devices need to be able to send their process variables to the control stations, sometimes over very long distances. The control stations then transmit control data to the field devices to perform a task.

      This lecture will introduce the advantages of current transmitters in these applications. The 4 to 20 milliamps current loop was designed to emulate the old 3 to 15 PSI pneumatic system that was used up until the 1950s to facilitate industrial controls throughout a building. The typical 4 to 20 milliamp transmission path is shown here starting with a sensor on the left.

      The sensor measures a variable over a specific range, for example, a temperature between 0 and 150 degrees C, and outputs a voltage proportional to where the measurement falls within the set range. The transmitter converts the sensor output to afford a 4 to 20 milliamp signal, which corresponds to the full scale of the sensor. 4 milliamps represent the minimum scale 0 degrees C in our example. And 20 milliamps represents maximum scale, 150 degrees C in our example.

      It then sends this current signal, usually over a long distance, to a receiver that converts the signal back to a voltage. A simple receiver would consist of a resistor that would convert this current back to a voltage by Ohm's law. A typical value for this resistor is 250 ohms resulting in a final voltage of 1 to 5 volts.

      A 4 milliamp 0 level and 20 milliamp full scale level allow for fault detection in the system. 0 amps on the transmitter output would indicate that there is a break in the transmission line. Smart measurement devices can also detect internal faults, such as sensor failure. Then a [INAUDIBLE] 43 signal standard allows for the user to set an under scale or over scale failure mode that will output a 3.6 milliamp or 21 milliamp signal for at least 4 seconds to report a failure to the control station. 3.6 milliamps, 21 milliamps, and 4 seconds are used to avoid false alarms.

      The 4 milliamp 0 level also allows the transmitter to supply up to 4 milliamps of current to power external input circuitry. We will introduce that capability in a future video. These next few slides describe the benefits of using a 4 to 20 milliamp transmitter.

      There are a few types of 4 to 20 milliamp transmitter configurations. We will discuss the differences between these configurations in detail in future videos. The main difference in the configurations is the number of wires used and how the transmitter is powered.

      The 2-wire configuration is loop supplied. The power and supply current are shared in the same loop. The 3-wire configuration has a local power supply. The three wires include the power, ground, and signal wires. The 4-wire configuration is also locally supplied, but the output is isolated from the supply. And the receiver does not share the same ground as the transmitter.

      4 to 20 milliamp current transmitters allow for a robust system that transfers data to and from analog output and input modules. Shown here is a simple block diagram of a current transmission. The 4 to 20 milliamp transmitter is modeled as a current source.

      The receiver, in this case, is a 250 ohm resistor that turns the transmitted current back into a voltage at the control station. This voltage can be translated to a digital signal for processing using an analog to digital converter. Transmission from the field sensors to control station or control station to field devices can be done with as little as two wires. The configuration in this example is a 2-wire system that uses a 24-volt nominal loop supply.

      Sensing control signals must often travel distances of a kilometer or more. A voltage transmission was sent over a kilometer-long wire, the voltage drops due to the impedance of the wire would cause enough attenuation to render the arriving signal unusable. 4 to 20 milliamp current loops are essentially lossless, even at long distances, since the voltage drop caused by the long transmission wires does not impact the accuracy of the current signal.

      Kirchhoff's current law states that the current in the loop is equivalent at any point in the loop. So if a current of 8 milliamps is read at a receiver, the current in the transmitter a kilometer away must also equal 8 milliamps. Ideally, the resistance between these two wires is infinite. But a nonideal resistance between the wires can cause current leakage paths to add some error to the system.

      With respect to electronics, the industrial environment is very harsh. Signal transmissions can occur on a factory floor or between buildings. And multiple unknown noise sources can be present at any given time.

      Thankfully, the inherent low impedance of a current loop system due to the 250 ohm receiver make it much less sensitive to induced noise than, for example, the high impedance input of a voltage amplifier. Typical current noise sources are on the order of microamps, which is often acceptable when compared to the 16 milliamp signal span. Furthermore, using 4 milliamps as a live 0 improves signal to noise ratio at low levels, which allows low signal levels to be accurately interpreted without added noise or interference.

      Now that we know why you would want to use a 4 to 20 milliamp transmitter, let's look into how they are used. Programmable Logic Controllers, also known as PLCs, at a control station feature both analog input and analog output modules. The analog output modules are used to control an element placed in the field such as in actuator, motor, valve, or heater.

      Since analog output modules and PLCs are powered by the PLC backed plane, the systems are almost all 3-wire transmitters that feature power, ground, and voltage or current out. Analog outputs are used in field elements to transmit data back from a remote sensor or process back to the PLC analog input module. Since there are often many types of sensors used in industrial automation and often multiple monitoring elements for each control element, field elements comprise the majority of the market for analog outputs.

      2-wire, loop-powered 4 to 20 milliamp transmitters are the most common field elements due to the minimal number of wires required. The two wires in this system transmit the signal current and the transmitter supply power. Let's go through an example to better understand how a PLC system might operate using analog output current transmitters.

      The goal here is to keep a remotely located liquid storage tank within a certain temperature range. As the liquid's temperature must be measured below the surface, a submersible temperature sensor is required. This severely limits the possibility of providing an onboard power supply to power the sensor.

      Due to the remote location, supply availability, and type of data to be transmitted, a 2-wire approach is chosen. Two wires are used to connect the remotely located transmitter to the central monitoring station. Those two wires provide power for the transmitter and sensor and carry the return signal containing the proportional current based on the sensor's reading.

      The current is converted to a voltage when it passes through a resistor. And the resulting voltage is then read by the monitoring station. This creates a current loop that develops a voltage across the resistor at the receiver that varies as the temperature sensor responds to the temperature in the liquid. Once the monitoring station receives the data back from the remote location, it can then decide what it wants to do with that data.

      In this example, if the temperature of the liquid becomes too low, a 3-wire transmitter can send a command for a valve to open up a line, allowing water that is heated in an external heating chamber to flow through coils placed inside the liquid storage tank. The valve has a higher current draw and cannot be loop supplied like the temperature sensor. Therefore, it uses its own local supply. The transmission to the valve consists of the current signal and a ground reference to tie the valve's local ground to the transmitter's ground. The three wires that give this type of transmitter its name are its dedicated supply and ground along with the current signal.

      There are a few ways you can use 4 to 20 milliamp transmission in your design. For a solution that meets your exact needs, you can choose to build a discrete 4 to 20 milliamp transmitter out of op amps, resistors, and transistors. This solution offers flexibility but also consumes the most bored space.

      There are many integrated solutions on the market. Each integrated solution provides multiple functions all on one IC. TI's XTR product line provides a partially integrated solution for the 4 to 20 milliamp transmission. You can customize the input circuitry to the XTR, depending on the type of sensor or control input you want to use. TI's XTRs are analog input, analog output 4 to 20 milliamp transmitters. Input types can include bridge sensors, RTD sensors, and outputs of digital to analog converters.

      The different XTR products can provide current-to-current transmission or voltage-to-current transmission that allow for a variety of sensor inputs. The partial integration of this solution makes the circuit smaller and also can offer a higher precision for a lower cost than a discrete solution. TI offers fully integrated digital to analog converters that can provide a 4 to 20 milliamp output. The combined functionality of a digital to analog converter and a 4 to 20 milliamps transmitter in one IC makes this the smallest and most precise solution for digital input, analog output transmitters.

      In the next few slides, we will discuss examples of discrete, partially integrated, and fully integrated transmitters. Shown here is a 2-wire, fully discrete 4 to 20 milliamp transmitter with a digital to analog converter driving the input. Building a fully discrete transmitter offers the most customizability for your application.

      You can customize for the added features, precision, and power supply requirements of your design. Higher precision will require parts with higher accuracy. These higher accuracy parts will increase the total cost of your circuit.

      The linked reference designs give more information on building discrete 2-wire and 3-wire 4 to 20 milliamp transmitters. For a 2-wire system, take a look at TI PD 158. And for a 3-wire system, view TI PD 102.

      The schematic on this slide shows a 2-wire, partially integrated 4 to 20 milliamp transmitter using an XTR 116. The input to the XTR 116 comes from a bridge sensor amplified by an INA 333. As you can see, the XTR 116 offers on-chip voltage regulators and voltage references for use with the input circuitry.

      TI's PGA 308 and PGA 309 could also be used to amplify the bridge sensor signal as well as condition the signal by correcting for offset and gain errors. Using a partially integrated solution with an XTR part offers higher precision, match resistors, and more features than the fully discrete version in a much smaller package. We have general purpose 2-wire transmitters like the XTR 116 along with specialized RTD and bridge conditioners. We also have general purpose 3-wire transmitters like the XTR 300 that are easily configured to operate in several input, output ranges.

      XTR products from TI consolidate the 2-stage current output circuitry into one integrated circuit with specified performance. The partially integrated solution is a good compromise between performance, flexibility, board space, and ease of design. TI PD 190 is a reference design for 2-wire transmitter using an XTR 116. And TI PD 155 is a 3-wire reference design using an XTR 300.

      The figure on this slide shows a 3-wire, fully integrated 4 to 20 milliamp transmitter using TI's DAC 8775. This solution allows for direct interface with a microcontroller. TI's fully integrated 4 to 20 milliamp digital to analog converters use SPI communication to obtain a sensor reading or control signal from a digital processor.

      The digital to analog converter translates this data into an analog signal and has internal circuitry that turns the signal into a 3-wire 4 to 20 milliamps analog output. These parts have internal regulators that can provide the digital supply to the rest of the system. Additional features, such as adaptive power management with a single inductor loose converter and an alarm for over temperature, watchdog timeout, or short circuit are also included in the solution.

      Because the digital to analog converter and transmitter are combined into one IC, the fully integrated solution is the best for high performance, size, and ease of design when interfacing with digital inputs. As seen on the DAC 8775 block diagram, there is the option to interface with a HART modulator. This will be explained on the next slide. TI PD 216 and TI PD 119 are examples of 3-wire, fully integrated transmitters using TI's digital to analog converters.

      The HART acronym stands for Highway Addressable Remote Transducer. In a field transmitter, HART uses frequency shift [INAUDIBLE] to represent binary data with two waveforms. HART is based on the bell 202 FSK standard and operates at 1,200 bits per second. This is the same standard adopted by the telecom industry in the 1990s to transmit caller ID data.

      A 1 is represented by a 1,200 Hertz signal called a mark. And a 0 is represented by 2,200 Hertz signal called a space. The signal is AC coupled onto the current waveform produced by the field transmitter. This allows 2-way communication between the transmitter and receiver.

      HART can be used to transmit device identification information, calibration data, or other diagnostic data. A receiver near the sensor would demodulate this data and feed it to the sensor's communication port. This is useful to reduce the number of wires used over long distances when sensors using 4 to 20 milliamps transmission also need to be calibrated or relay diagnostic data.

      That concludes our first video introducing current loop transmitters. In the next video, we will go into more detail on 2, 3, and 4-wire 4 to 20 milliamps transmitters. Please try the quiz to check your understanding of this video's content.

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      Precision labs 시리즈: 특수 기능 증폭기