Application of 4～20mA current loop isolation interface chip IDC3516

Introduction

In industrial sites, when signals are transmitted between electrical equipment and instruments at a long distance, interference often occurs, causing system instability or even erroneous operation. In addition to the influence of internal and external interference in the system, another very important reason is the grounding treatment of various instruments and equipment. In general, the equipment casing needs to be connected to the ground, and the circuit system must also have a common reference ground. However, due to the potential difference between the reference points of each instrument and equipment, a ground loop is formed. Since the ground wire loop current will bring common-mode and differential-mode noise and interference, the system often cannot work properly.

An ideal solution is to electrically isolate the equipment, so that the originally interconnected ground wire network becomes an independent unit, and the interference between them will be greatly reduced.

In industrial automation control systems, instrumentation, and sensor applications, 4~20mA current is widely used to transmit control and detection signals. Because the 4~20mA current loop has strong anti-interference ability and simple circuits, it can be used to transmit analog signals of tens or even hundreds of meters. In general, if the transmission distance exceeds 10 meters, the current signal needs to be isolated.

IDC3516 current loop isolation interface chip

IDC3516 is a monolithic two-wire isolation interface chip. The chip contains a current signal modulation circuit, a magnetoelectric coupling isolation conversion circuit and a demodulation circuit. Through magnetic circuit coupling, energy is transferred from the input line to the output side, and no direct electrical connection is required. The typical value of IDC3516 nonlinearity is 0.2%, and the temperature drift coefficient is 100ppm. It is suitable for most industrial control and measurement occasions. The input signal of the chip is 4~20mA, and the output signal is 4~20mA after isolation. The input equivalent resistance of the chip is small, the linearity is high, and it has a wide input/output creepage distance. Figure 1 is the internal schematic diagram of IDC3516. Among them, pin 24 is the input current signal inflow end, pin 23 is the input current signal outflow end, pins 22 and 21 are the zero point and full scale adjustment potentiometer access ends, and pins 11 and 10 are the output current outflow and inflow ends.

IDC3516 application circuit

Since all functions are integrated inside the chip, IDC3516 is very convenient to use. In practical applications, only a few protection and adjustment components are needed. Figure 2 shows the typical wiring schematic of the IDC3516 series chip. The potentiometer RADJ1 connected between pins 22 and 23 is used to adjust the ratio of input current Ii to output current Io. The variable resistor RADJ2 connected between pins 21 and 22 is the bias adjustment resistor of the output current Io. D1 and D2 are limiter voltage regulators used for line surge voltage and lightning strong interference protection. F1 and F2 are self-recovery fuses used for overcurrent protection in the line. C1 and C2 are filter capacitors. Generally, 1mF ceramic capacitors can be selected to filter out high-frequency differential mode interference and reduce the pulsation components of input and output signals.

Since the chip consumes some energy in signal transmission, the output signal power of the chip is always less than the input power, which is reflected in the voltage drop at the input end. Figure 3 is the input characteristic curve of the chip. The load on the output side is 250Ω. It can be seen that in this case, the signal source needs to have sufficient driving capability (>14V).

Conclusion

The 4~20mA current loop isolation interface chip IDC3516 has a simple wiring circuit and is easy to use. It can be widely used in industrial sites with long-line transmission such as automatic control, instruments and meters, and sensors.