Current signal 4 20 mA circuit. Current loop. How a current loop signal is converted to voltage

And from the previous sections it is clear that unprocessed signals They are very diverse and the range of their changes extends from several millivolts (for a thermocouple) to more than hundreds of volts for a tachogenerator. In addition, they can be caused by changes in DC voltage, AC or even resistance. Therefore, it is quite obvious that if the analog input cards are only in the signal range, then it is necessary to use some.

The origin of the input signal can be represented as shown in Fig. 4.13. The primary signal from the sensor on site is converted into standard signal, and the combination of the sensor and this device is called a transmitter or. After this, the standardized signal information-carrying about the measured variable, can be fed to a conventional analog input board.

A natural question arises: what should this standardized signal be? Analog signals are signals low level and are therefore susceptible to interference (or noise as it is most commonly called). A signal represented by an electric current is less affected by noise than a signal represented by a voltage, so a current loop is usually chosen. The converter and the receiving device are connected according to the diagram shown in Fig. 4.14, and the current signal on the receiving side is converted into voltage using a ballast resistor. A current loop can be used with multiple receiving devices (this could be a meter, chart recorder, or PLC input, for example) connected in series.

The most common standard represents an analog signal in the form of a current with a range of 4-20 mA, where 4 mA is the minimum signal level, and 20 mA is the maximum. If, for example, a pressure transducer produces a 4-20 mA signal representing a pressure in the range of 0-10 bar, then a pressure of 8 bar would correspond to a current of 8 x (20 - 4)/10 + 4 = 16.8 mA. The 4-20 mA signal is often converted to a 1-5 V signal using a 250 ohm ballast resistor.

The 4 mA "null" signal (called offset) serves two purposes. Firstly, it is used against damage to the inverter or cable cord. If the converter or cord breaks or there is a problem in the communication line short circuit, then the current through the ballast resistor will be zero, which corresponds to a “negative” 0 V signal on the receiving side. This can be very easily detected and used as a "converter failure" alarm.

The 4 mA bias current also simplifies the layout. In Fig. 4.14 it was assumed that the converter had local use

Rice. 4.15. Two-wire 4-20 mA converter

power source and provided a current signal. A similar arrangement is possible, but the more common (and simpler) arrangement is the one shown in Fig. 4.15. Here, the power supply (typically 24-30 VDC) is placed on the receiver side, and the signal lines serve to both power the converter and transmit current. The converter draws current from the power source in the range of 4-20 mA in accordance with the measured signal. This current, as before, is converted into voltage using a ballast resistor.

The 4 mA offset provides the current required by the converter to operate. normal operation. Obviously, this cannot be achieved if the signal range is 0-20 mA. Converters connected according to the diagram in Fig. 4.15 are usually called two-wire.

Current sensors (converters) are designed for non-contact monitoring of current in electrical circuits with nominal voltage up to 660 V. The sensor converts the AC input signal into a 4-20mA or 0-20mA or 0-10V DC output signal, which can be sent to universal measuring instruments or control controllers.

The sensors are sealed and can be installed anywhere, including hidden and hard-to-reach places. They cannot be repaired or require maintenance, they contain a built-in current transformer and a universal Ayumi platform, designed specifically for use with the instrument transformers we produce and consisting of a precision op-amp rectifier, an integrating circuit (time constant 0.6-0.8 sec) and an output analog signal shaper .

The rated supply voltage of the sensors is 24V(DC), operability is fully maintained in the voltage range of 20-28V. The sensors are insensitive to pulsations and instability of supply voltages. Operating temperature range -40...+85 degrees C. Currently the following sensors are available for order:

TP03S (photo 2) for rated currents from 1 to 90A with hole. 11mm TTP60 (photo 5) - for currents from 10 to 500A with hole. 37mm TP60 - for currents from 0.05 to 300A with hole. 37mm TP102S (photo 4) - for currents from 0.05 to 40A with a 14 mm hole.

Any currents within the specified ranges are available for ordering. Linearity and stability are extremely high in the range of 1-100% of the rated current. The reduced conversion error is less than 2% without calibration and less than 1% with additional calibration during manufacture. The sensors are produced according to TU 27.11.50.120-001-11976052-2017

When ordering it is possible to specify undervoltage supply 9(12)V with a corresponding decrease in max. output values signal up to 3(5)v.

Name of current sensor for ordering: TP03C-xx/yy-zz(mm), where

• xx - rated current (A)
• yy- output signal: 0-1V/0-10V/0-20mA/4-20mA
• zz-00-rigid output
• mm - note, for example (terminal block) - the outputs are made in the form of a terminal block. Attention! The option is available in full for TPP60 and TP60. For TP03 and TP102 only in relation to the 4-20mA option

For example: TP03S-30A/(4-20mA)-00, i.e. sensor TP03S with nom. input current 30A, output 4-20mA, hard pins for printed circuit mounting.

Please note again: When ordering, the rated current and output signal parameters can be specified at any value within the specified limits, i.e. for TP03S - 1...90A; TP102S - 0.1...40A; TP60 - 0.05...300A TTP60 - 10...500A for input current, and 0...20mA; 1...20mA; 0...10v. for the output signal! The sensitivity of the sensors is no worse than 0.1% of the nominal value. current This is not reflected in the price.

Attention: The input impedance of the meter on the receiving side should be:

• not lower than 50 kOhm for modifications 0-1V;
• not lower than 100 kOhm for 0-10V;
• no higher than 500 ohms for 0-20mA (including resisting conductors)
• not higher than 500 ohms for 4-20mA (including contact conductors) at 24V. current loop supply

The sensor housing provides excellent galvanic isolation from the monitored circuit, which is sufficient for any application.

The TP03S sensor has a hole with a diameter of 11mm, TP102S - 14mm, TTP60 and TP60 - 37mm for controlled lines. If necessary, it is possible to use any current transformers of our production to increase the hole or measured currents. An example of such an implementation is shown in Photo 1. This design allows you to control circuits in a non-contact way, without removing their insulation, which significantly increases the reliability and safety of electrical networks. The small rated measured current and a decent hole TP102S and TP60 also allow it to be used as a differential current transformer for measuring leakage currents in lines (zero-sequence current transformer), for example, for the 100mA version, the input current measurement range is from 1 to 100mA with good linearity.

Design and principle of operation

When current flows in the external circuit, the built-in current transformer provides galvanic isolation and transforms this current into a lower one, which is amplified by a current-voltage amplifier-converter. The resulting voltage is rectified by a precision rectifier and fed to an RC circuit, which allows one to isolate an average voltage proportional to the input. current A voltage-current driver is installed at the output of the RC circuit, which additionally acts as a buffer and brings the output signal to 0. The output voltage is formed when the driver current flows through Rn. Thanks to this, the output voltage can vary within a wide range (0-1V; 0-2V, etc.) for a given input value. current, which allows you to adjust the coefficient. conversion by adjusting the load resistor. This adjustment can also be carried out if it is necessary to reduce the coefficient. transmission or adjustment of the ADC to the existing reference. At the same time, the value of output. voltage and internal resistance(no more than 49.9 ohms for 0-1v and 499 ohms for the 0-10v option) of the analog output allows you to easily interface it with ADC microcontrollers or standard measuring instruments that have a 0-1v or 0-10v input. If necessary, at the manufacturing stage, it is possible to reduce or increase the time constant of the RC circuit or adjust the required output. voltage or current.

The modification of the sensor with a 0-20mA output does not have a built-in resistor. Max. the voltage at output 4 can reach 10V. which limits the input. meter resistance taking into account wire resistance of 500 ohms. In the 4-20mA modification, a built-in 0...10 ohm resistor is installed and a 2-wire connection is used, which limits the input. The resistance of the meter is already up to 800 ohms with a 24V power supply.

Own consumption of Ayumi sensors in the absence of input. current does not exceed 0.8-1mA in the voltage range 20-28V. If the input is exceeded. current above the rated one, the built-in protection circuit is activated, limiting the output current from 20 to 35 mA according to the logarithmic law (24-39 mA for 4-20), while the output voltage cannot exceed 11 V, and the maximum current consumption is 38 mA, which allows its use with low-power power supplies. Please note: the maximum permissible input current for TP03 and TP102 should not exceed 200A to avoid damage to the built-in transformer or electronic circuit. For TTP60 this limit is set at 500A for a long time and 1000A for up to 2 seconds, for TP60 with a range of 0.05-150A at a rate of 300A, for 150-300A at a rate of 500A

Typical sensor connection diagrams are shown in Fig. 3.

• In Fig. 3a shows the connection diagram of TP03S-xx/(0-1v) to the universal 0-1v meter and has no special features; the T03S-xx/(0-10v) to the universal 0-10v meter has a similar connection.
• In Fig. Figure 3b shows a circuit for pairing TP03S-xx/(0-10v) with the ADC of a microcontroller with built-in ION=5v. To reduce the output voltage from 10 to 5V. An additional 510 ohm resistor is installed. For other ION voltages, the value of the additional resistor can be calculated using the formula: Rx=510*Ux/(10-Ux).
• In Fig. 3c shows a diagram of connecting TP03S-xx/(4-20mA) to a universal 4-20mA meter and has no special features.
• In Fig. Figure 3d shows a diagram of connecting TP03S-xx/(0-20mA) to a universal 0-20mA meter.

Current loop is a method of transmitting information using measured values ​​of electric current. Typically, a current loop system includes a sensor (pressure, temperature, gases, etc.), a transmitter, a receiver, and an analog-to-digital converter (ADC) or microcontroller (Figure 1).

Rice. 1.

A voltage proportional to the measured parameter is generated at the sensor output. Transmitter (current amplifier, voltage controlled) converts the voltage from the sensor into a corresponding current of 4 to 20 mA. At the other end of the line, a receiver (current controlled voltage amplifier) ​​converts the 4...20 mA current back into voltage. An analog-to-digital converter digitizes the receiver's output voltage for subsequent processing by a processor or microcontroller.

In systems with a current loop interface, information is transmitted using a signal-modulated current. In a 4...20 mA current loop, the smallest signal value corresponds to a current of 4 mA, and the largest - 20 mA. Thus, the entire range of permissible values ​​is 16 mA. The loop current is maintained at 4 mA at all times, so lower current levels will detect an open line and make this situation easy to diagnose.

As a rule, in industrial automation systems, sensors are located long distances from the central control unit, so the current loop has still not lost its relevance, since it is the most noise-resistant analog interface, especially compared to voltage data transmission methods. A more complete system, including a second current loop (for example, to control a drive), is demonstrated in Figure 2.

Rice. 2.

Based on this scheme, we will consider the solutions that Maxim offers for its implementation.

Operational amplifier
as a voltage-current converter

Figure 3 shows a simple implementation of a voltage-to-current converter using an operational amplifier (op-amp). MAX9943. This op-amp provides an output current of more than ±20 mA at a supply voltage of ±15 V, and is also stable with a capacitive load of up to 1 nF, making it very suitable for use in long transmission lines. To operate in the 0...20 mA output current range, single-supply amplifier power is possible, since the MAX9943 provides an output voltage swing equal to the supply voltage ( rail-to-rail output).

Rice. 3.

In this circuit, the relationship between the input voltage and the load current is described by the expression: V IN = (R2/R1) ґ R SENSE ґ I LOAD + V REF. A typical load resistance value can be several kOhms. In this example: R1 = 1 kOhm; R2 = 10 kOhm; R SENSE = 12.5 ohms; R LOAD = 600 Ohm.

To convert a ±2.5 V input voltage to ±20 mA current, the reference voltage V REF must be set to 0 V. To obtain a 4...20 mA current output from a 0...2.5 V input voltage, an offset must be set to remain constant on the line current 4 mA. With V REF = -0.25 V, the input voltage 0...2.5 V is converted into an output current 2...22 mA. Typically, developers choose a slightly wider dynamic range to allow for later software calibration. The dependences of the input voltage and output current are shown in Figures 4 and 5.

Rice. 4. Dependence of I LOAD on V IN for ±20 mA output

Rice. 5. Dependence of I LOAD on V IN for 4-20mA output

MAX15500 and MAX15501 - Current Loop Conditioners

The circuit in Figure 3 using operational amplifiers- This is a simple implementation of a current loop, which causes difficulties during calibration, as well as a large error in transmitting signals under real operating conditions. In practice, to implement a voltage-to-current converter, it is advisable to use single-chip solutions, technical parameters which are strictly described in the documentation.

Rice. 6.

An example of such a solution is MAX15500/15501, generators of analog current output or voltage output programmable via the SPI interface. The input voltage for these converters is typically taken from the output of an external DAC. For the MAX15500, the input voltage range is 0...4.096 V, and for the MAX15501 - 0...2.5 V. Six operating modes of the output stage are available in software: ±10 V; 0…5 V; 0…10 V; ±20 mA; 0…20 mA; 4…20 mA. Microcircuits provide short circuit protection; detection of a break in the transmission line; protection against overheating and detection of supply voltage drop below the threshold.

MAX5661 - Current Output DAC

The most integrated version of the current output converter is MAX5661. This is a single-channel 16-bit DAC with a precision high-voltage amplifier that provides a complete solution for converting the digital signal from the processor into a programmable current output (0...20 mA or 4...20 mA) or industry standard voltage ±10 V.

Rice. 7.

Control and data transmission to the DAC is carried out via a four-wire SPI interface. The microcircuit has a #FAULT output, which can be used to diagnose an open circuit in the current loop or a short circuit in the voltage output. It should be noted that the MAX5661 requires the use of external source reference voltage 4.096 V. The documentation provides a list of recommended ultra-precision ionizers, e.g. MAX6341, MAX6133 or MAX6033. To quickly master all the functionality of the MAX5661, a development kit is offered MAX5661EVCMAXQU+ with interface to a PC for controlling the DAC using a graphical interface (GUI).

MAX1452 - sensor signal converter
into a current loop

So far we have looked at solutions suitable for converting a signal from a microcontroller or DAC, i.e. for transmitting control signals. To receive a current signal from the sensor, Maxim offers a microcircuit MAX1452, combining the analog part with an op-amp to generate an information signal and digital circuit, providing temperature drift compensation, zero offset adjustment, and PGA-programmable gain. All adjustment coefficients are stored in the built-in EEPROM memory with a capacity of 768 bytes.

Figure 8 shows the circuit diagram for the MAX1452 with 4...20 mA current output and loop power. A transistor is used to generate current in the loop 2N2222A.

Rice. 8.

HART modem DS8500

HART ( Highway Addressable Remote Transducer Protocol) is a digital industrial data transfer protocol that, as a rule, allows you to configure a sensor or obtain information about its condition using a line on which an analog current loop is organized. To transmit digital data, an FSK-modulated signal (frequency switching modulation) is used over a 4...20 mA current loop (Figure 9). This implementation method allows the use of the HART protocol in already existing systems with analog current loop.

Rice. 9.

To organize the HART physical layer (modulation and demodulation), Maxim offers a HART modem chip DS8500, which allows for half-duplex reception and transmission mode, with “1” modulated at a frequency of 1.2 kHz, “0” - 2.2 kHz. Functionally, the DS8500 consists of a demodulator, digital filter, ADC, modulator and DAC (Figure 10).

Rice. 10.

This architecture (with digital filtering and a DAC that generates a pure sinusoidal signal with phase-continuous switching between frequencies) ensures reliable signal reception in noisy environments.

Conclusion

Maxim offers a full range of solutions for organizing data transmission using a current loop, both from sensors to the central control unit, and from this unit to the executive units. In addition, to expand the functionality of such industrial system the Maxim line contains more than 300 different interface chips RS-485/RS-232, CAN, LIN.

Literature

1. "How to use highvoltage and highcurrent-drive opamps in 4-20 mA current-loop systems", Maurizio Gavardoni, Maxim Engineering Journal No. 68

2. “Analog current loop - solutions from Maxim”, Anatoly Andrusevich, “Components and Technologies” No. 8 2009

Nizhny Novgorod

This article is a continuation of a series of publications in the ISUP journal devoted to standardization *, **, *** ****. The article “Converting like into like in measurement and control systems” (ISUP. 2012. No. 1) was devoted to standardization, which converts unified input signals into unified output signals.

Why exactly the 4...20 mA signal?

The wide distribution of the 4...20 mA current unified signal is explained by the following reasons:
- the transmission of current signals is not affected by the resistance of the connecting wires, therefore the requirements for the diameter and length of the connecting wires, and hence the cost, are reduced;
- the current signal operates on a low-resistance (compared to the resistance of the signal source) load, so the induced electromagnetic interference in current circuits is small compared to similar circuits that use voltage signals;
- a break in the transmission line of the 4...20 mA current signal is clearly and easily determined by measuring systems by the zero current level in the circuit (under normal conditions it should be at least 4 mA);
- a current signal of 4…20 mA allows not only to transmit a useful information signal, but also to provide power supply to the normalizing converter itself: minimal permissible level 4 mA is sufficient to power modern electronic devices.

Characteristics of 4…20 mA current loop converters

Let's look at the main characteristics and features that need to be taken into account when choosing. As an example, let us take the NSSI-GRTP normalizing converters produced by the research and production company “KontrAvt” (Fig. 2).

Rice. 2. Appearance of NPSI-GRTP - converters produced by NPF "KontrAvt" with galvanic separation of 1, 2, 4 channels of the current loop

Designed to perform only two main functions:
- measurement of an active current signal of 4...20 mA and its conversion into the same active current signal of 4...20 mA with a conversion coefficient of 1 and with high speed;
- galvanic separation of input and output signals of the current loop.

The main error of the NPSI-GRTP conversion is 0.1%, temperature stability is 0.005% / °C. Operating temperature range - from -40 to +70 °C. Insulation voltage - 1500 V. Performance - 5 ms.

Options for connecting to sources of active and passive signals are shown in Fig. 3 and 4. In the latter case, an additional power source is required.

Rice. 3. Connecting NSSI-GRTP converters to an active source

Rice. 4. Connecting NSSI-GRTP converters to a passive source using an additional power supply unit BP

In measurement systems where it is necessary to separate input signals, the source of the input signal is, as a rule, measuring sensors (MS), and the receivers are secondary measuring devices (MI) (regulators, controllers, recorders, etc.).

In control systems where separation of output signals is required, the sources are control devices (CU) (regulators, controllers, recorders, etc.), and the receivers are actuators (CD) with current control(membrane actuators (MIM), thyristor regulators, frequency converters, etc.).

It is noteworthy that the NPSI-GRTP converter, produced by , does not require separate power. It is powered from an active 4…20 mA input current source. In this case, an active 4...20 mA signal is also generated at the output, and no additional source is required in the output circuits. Therefore, the solution based on current loop separators, which is used in NPSI-GRTP, is very economical.

Three modifications of the converter are available: . They differ in the number of channels (1, 2, 4, respectively) and design (Fig. 2). The single-channel converter is housed in a small, narrow case with a width of only 8.5 mm (dimensions 91.5 × 62.5 × 8.5 mm), two-channel and four-channel in a case with a width of 22.5 mm (dimensions 115 × 105 × 22.5 mm ). Converters with galvanic isolation are used in systems with tens and hundreds of signals; for these systems, placing such a number of converters in structural shells (cabinets) becomes the most important problem. The key factor here is the width of one conversion channel along the DIN rail. in 1-, 2- and 4-channel versions they have an extremely small “channel width”: 8.5, 11.25 and 5.63 mm, respectively.

It should be noted that in the multi-channel modifications NPSI-GRPT2 and NPSI-GRTP4, all channels are completely unconnected. From this point of view, the performance of one of the channels does not in any way affect the operation of other channels. That is why one of the arguments against multi-channel converters - “one channel burns out, and the entire multi-channel device stops working, and this sharply reduces the safety and stability of the system” - does not work. But such an important positive property of multi-channel systems as lower “channel price” is fully manifested. Two- and four-channel modifications of the converters are equipped with screw detachable connectors, which facilitate their installation, maintenance and repair (replacement).

In a number of tasks, it is necessary to supply a 4...20 mA signal to several galvanically isolated receivers. For this, you can use both single-channel converters NSSI-GRTP1, and multi-channel converters NSSI-GRTP2 and NPSI-GRTP4. Connection diagrams are shown in Fig. 5.

Rice. 5. The use of single-channel and dual-channel converters for signal multiplication “1 to 2”

For ease of installation and maintenance, external connections are connected in the single-channel version using spring terminal connectors, and in two- and four-channel versions - with detachable screw connectors.

Rice. 6. Connecting external lines using detachable terminal connectors

Thus, the new line of converters for separating the 4...20 mA current loop, presented by NPF "KontrAvt", can quite reasonably be called a compact and economical solution, capable of competing in terms of characteristics with the corresponding imported analogues. Converters are provided for trial operation, so the user has the opportunity to test the devices in operation, evaluate their characteristics and make an informed decision about the advisability of their use.
____________________________

Since the 1950s, current loops have been used to transmit data from transmitters in monitoring and control applications. With low implementation costs, high noise immunity and the ability to transmit signals over long distances, the current loop has proven to be especially convenient for operation in industrial environments. This material is devoted to a description of the basic principles of operation of a current loop, the basics of design, and configuration.

Using current to transfer data from the converter

Industrial sensors often use a current signal to transmit data, unlike most other transducers, such as thermocouples or strain gauges, which use a voltage signal. Although converters that use voltage as a parameter for transmitting information are indeed effective in many industrial applications, there are a number of applications where the use of current characteristics is preferable. A significant drawback when using voltage to transmit signals in industrial environments is the weakening of the signal when it is transmitted over long distances due to the presence of resistance of wired communication lines. You can, of course, use high input impedance devices to get around signal loss. However, such devices will be very sensitive to noise generated by nearby motors, drive belts or radio broadcast transmitters.

According to Kirchhoff's first law, the sum of currents flowing into a node is equal to the sum of currents flowing out of the node.
In theory, the current flowing at the beginning of the circuit should reach its end in full,
as shown in Fig.1. 1.

Fig.1. In accordance with Kirchhoff's first law, the current at the beginning of the circuit is equal to the current at its end.

This is the basic principle on which the measurement loop operates. Measuring current anywhere in the current loop (measuring loop) gives the same result. By using current signals and data acquisition receivers with low input impedance, industrial applications can benefit greatly from improved noise immunity and increased link length.

Current loop components
The main components of a current loop include a DC source, a sensor, a data acquisition device, and wires connecting them in a row, as shown in Figure 2.

Fig.2. Functional diagram current loop.

A DC source provides power to the system. The converter regulates the current in the wires from 4 to 20 mA, where 4 mA represents the live zero and 20 mA represents the maximum signal.
0 mA (no current) means an open circuit. The data acquisition device measures the value adjustable current. An effective and accurate method for measuring current is to install a precision shunt resistor at the input of the instrumentation amplifier of the data acquisition device (in Fig. 2) to convert the current into a measurement voltage, ultimately obtaining a result that clearly reflects the signal at the output of the converter.

To help better understand the operating principle of a current loop, consider, for example, a system design with a converter that has the following technical characteristics:

The transducer is used to measure pressure
The transducer is located 2000 feet from the measuring device
The current measured by the data acquisition device provides the operator with information about the amount of pressure applied to the transducer

Let's start looking at the example by selecting a suitable converter.

Current System Design

Converter selection

The first step in designing a current system is selecting a converter. Regardless of the type of variable being measured (flow, pressure, temperature, etc.), an important factor in choosing a converter is its operating voltage. Only connecting a power source to the converter allows you to regulate the amount of current in the communication line. The power supply voltage must be within acceptable limits: greater than the minimum required and less than the maximum value that could damage the converter.

For the current system in the example, the selected transducer measures pressure and has an operating voltage of 12 to 30 V. Once the transducer is selected, the current signal must be correctly measured to provide an accurate representation of the pressure being applied to the transducer.

Selecting a Data Acquisition Device for Current Measurement

An important aspect that you should pay attention to when building a current system is to prevent the appearance of a current loop in the ground circuit. A common technique in such cases is isolation. By using insulation, you can avoid the influence of the ground loop, the occurrence of which is explained in Fig. 3.

Fig.3. Ground loop

Ground loops are formed when two connected terminals in a circuit are at different potentials. This difference introduces additional current into the communication line, which can lead to measurement errors.
Data acquisition device isolation refers to the electrical separation of the signal source ground from the measurement device's input amplifier ground, as shown in Figure 4.

Since current cannot flow through the insulation barrier, the ground points of the amplifier and the signal source are at the same potential. This eliminates the possibility of inadvertently creating a ground loop.

Fig.4. Common Mode Voltage and Signal Voltage in an Isolated Circuit

Isolation also prevents damage to the data acquisition device when high common mode voltages are present. Common-mode voltage is a voltage of the same polarity that is present at both inputs of an instrumentation amplifier. For example, in Fig. 4. Both the positive (+) and negative (-) inputs of the amplifier have +14 V common mode voltage. Many data acquisition devices have a maximum input range of ±10 V. If the data acquisition device does not have insulation and the common mode voltage is outside the maximum input range, you can damage the device. Although the normal (signal) voltage at the input of the amplifier in Fig. 4 is only +2 V, adding +14 V can result in a voltage of +16 V
(Signal voltage is the voltage between the “+” and “-” of the amplifier, the operating voltage is the sum of the normal and common mode voltage), which represents a dangerous voltage level for collection devices with lower operating voltage.

In isolation, the common point of the amplifier is electrically separated from ground zero. In the circuit in Figure 4, the potential at the common point of the amplifier is “raised” to the level of +14 V. This technique causes the input voltage to drop from 16 to 2 V. Now that data is collected, the device is no longer at risk of overvoltage damage. (Note that isolators have a maximum common-mode voltage they can reject.)

Once the data acquisition device is isolated and protected, the final step in constructing the current loop is to select the appropriate power supply.

Selecting a Power Source

Determining which power supply best suits your needs is easy. When operating in a current loop, the power supply must produce a voltage equal to or greater than the sum of the voltage drops across all elements of the system.

The data acquisition device in our example uses a precision shunt to measure current.
It is necessary to calculate the voltage drop across this resistor. A typical shunt resistor is 249 Ω. Basic calculations for a current loop current range of 4 .. 20 mA
show the following:

I*R=U
0.004A*249Ω= 0.996 V
0.02A*249Ω= 4.98 V

From a 249 Ω shunt, we can remove a voltage in the range from 1 to 5 V by relating the voltage value at the input of the data acquisition device to the value of the output signal of the pressure transducer.
As mentioned, the pressure transmitter requires a minimum operating voltage of 12 V with a maximum of 30 V. By adding the voltage drop across the precision shunt resistor to the operating voltage of the transmitter, we get the following:

12 V+ 5 V=17 V

At first glance, a voltage of 17V is sufficient. However, it is necessary to take into account the additional load on the power supply that is created by wires that have electrical resistance.
In cases where the sensor is located far from measuring instruments, you must consider the wire resistance factor when calculating the current loop. Copper wires have resistance DC, which is directly proportional to their length. With the example pressure sensor, you need to account for 2000 feet of communication line length when determining the operating voltage of the power supply. The linear resistance of the single-core copper cable is 2.62 Ω/100 feet. Taking this resistance into account gives the following:

The resistance of one core 2000 feet long will be 2000 * 2.62 / 100 = 52.4 m.
The voltage drop across one core will be 0.02 * 52.4 = 1.048 V.
To complete the circuit, two wires are needed, then the length of the communication line doubles, and
The total voltage drop will be 2.096 V. This results in about 2.1 V due to the distance from the converter to the secondary device being 2000 feet. Summing up the voltage drops across all elements of the circuit, we get:
2.096 V + 12 V + 5 V = 19.096 V

If you used 17 V to power the circuit in question, then the voltage supplied to the pressure transducer will be below the minimum operating voltage due to the drop in the resistance of the wires and the shunt resistor. Selecting a typical 24V power supply will satisfy the power requirements of the inverter. Additionally, there is a voltage reserve in order to place the pressure sensor at a greater distance.

With the correct transducer, data acquisition device, cable length, and power supply selected, the design of a simple current loop is complete. For more complex applications, you can include additional measurement channels in the system.