Signal conditioning solution for 4-20mA current transmitter and its application in industrial control

 

 

Working principle of 4~20ma current transducer

In industrial sites, using an instrument amplifier to complete signal conditioning and long-distance transmission will cause the following problems: First, since the transmitted signal is a voltage signal, the transmission line will be interfered by noise; second, the distributed resistance of the transmission line will cause voltage drop; third, how to provide the operating voltage of the instrument amplifier on site is also a problem.

 

 

In order to solve the above problems and avoid the influence of related noise, we use current to transmit signals because current is not sensitive to noise. The 4-20ma current loop uses 4ma to represent the zero signal and 20ma to represent the full scale of the signal, while signals below 4ma and above 20ma are used for various fault alarms.

 

 

There are two types of 4-20ma current transmitters: two-wire and three-wire. When the monitoring system needs to drive the on-site drive devices such as valves through long lines, a three-wire transmitter is generally used. Here, the XTR is located at the monitored system end, and the system directly supplies power to the XTR. The power supply is the third line other than the two current transmission lines. The two-wire system is that the XTR and the sensor are located at the on-site end. Due to the existence of on-site power supply problems, the receiving end generally uses a 4-20ma current loop to supply power to the remote XTR, and reflects the size of the signal through 4-20ma.

 

 

4-20 mA products are sensing and measurement applications, as shown in Figure 1. There are many types of sensors in industrial sites that can be converted into 4-20 mA current signals. TI has some very convenient transmitter chips for RTD and bridge . Since TI’s transmitter chips contain common functional circuits such as voltage excitation source, current excitation flow, voltage regulator circuit, instrument amplifier, etc., it is very convenient to convert the signals of many sensors into 4-20 mA signals.

Most applications of bridge sensors are used to measure pressure. In a practical circuit, if the resistance on each arm of the Wheatstone bridge is 2k, then its equivalent resistance is 2k whether viewed from the excitation voltage end or the differential output end. When there is no pressure, its bridge is balanced and the output voltage is 0. When pressure is applied, a differential voltage will be generated due to the imbalance of the bridge, and the differential voltage will reflect the magnitude of the pressure.

 

Full scale and hue are two main technical indicators of pressure sensors. The sensors used in the real world have certain nonlinearity, and their output voltage will change with the change of temperature. The change of output voltage with temperature is not linear, and both full scale and hue have this property.

 

4～20ma sensor signal conditioning solution

The 4-20 mA current loop is structurally composed of two parts, namely the transmitter and the receiver . The transmitter is generally located at the field end, sensor end or module end, while the receiver is generally at the PLC and computer end, and it is generally inside the controller .

 

In the two-wire 4-20ma circuit application, the working power and signal share one wire, and the working power is provided by the receiving end. In order to avoid 50/60hz power frequency interference, current is used to transmit the signal. The main issues that need to be considered in the two-wire solution are: determine the number of receivers used. When there are multiple receivers, it will require the transmitter to have a lower working power supply voltage. Another consideration is to reduce the voltage drop of the loop current at the receiving end.

 

The design of the two-wire solution needs to consider:

(1) The number of receivers in the circuit loop: more receivers will require the transmitter to have a lower operating voltage; (2) The required operating voltage of the transmitter must have a certain margin;

(3) Determine whether the sensor’s excitation method is voltage or current.

converter for 4-20 mA signals . It contains a 5 V voltage regulator circuit that can power external circuits. A precision on-chip reference voltage can be used for voltage bias or sensor excitation.

 

The three-wire 4-20ma circuit is designed to provide working power from the transmitter end. To avoid 50/60hz power frequency interference, current is used to transmit signals. The xtr regulator and the load on site share a ground connection. The design of the scheme needs to consider:

(1) The number of receivers in the current loop;

(2) More receivers require the transmitter to have a higher operating voltage;

(3) Ensure the required working voltage of the transmitter and have a certain margin.

 

The three-wire transmitter application provided by TI is shown in Figure 3. In the figure, XTR110 is a precision voltage-current converter for analog signal transmission. It can directly convert the input voltage of 0-5V or 0-10V to the output signal of 4-20mA, 0-20mA, and 5-25mA. XTR110 contains a precision resistor network to adapt to different input and output requirements. A 10V voltage reference can be used to drive external circuits.

 

4～20ma calibration

Traditional 4-20ma calibration requires special fixtures, special laser or manual resistor adjustment, and the adjustments are mutually influential, requiring a process of testing, adjusting, retesting, and re-adjusting, with limited adjustment times and range. Electronic devices and sensors are not easy to adjust.

Modern digital 4-20ma correction allows electronic devices and sensors to be adjusted after packaging; the correction coefficient can be calculated by computer to simplify numerical adjustment; there can be an unlimited number of adjustments, with good resolution and a wide adjustment range; there is no mutual influence during the adjustment process; electronic devices and sensors can be easily adjusted.

 

XTR108 is a solution provided by TI for correcting 4-20mA. It has a current reference of 480A, provides nonlinear correction of RTD, and does not require an external adjustable resistor. The features of XTR108 are:

(1) A linearization circuit having a sensor;

(2) Digital calibration. XTR108 can be set directly through the SPI interface , and EEP ROM can be programmed directly through the SPI interface ;

 

 

amplifier with automatic zero-stabilization programmable gain is 6.26 to 400 times;

(4) The resolution of the programmable current of RTD excitation is 1.54 A;

(5) The correction parameters are stored in the external eeprom;

(6) Programmable over-range and under-range outputs.

 

 

In addition, TI also provides a digital correction solution for bridge sensors – PGA309, which is a programmable analog signal conditioner designed for pressure bridge sensors. It simulates the amplifier sensing signal and provides digital correction of the tone voltage and full-scale voltage, avoiding manual adjustment and achieving long-term stability, and converting the output voltage signal into 4-20mA output.

 

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