Model: AD210AN

Brand: AD

Package: DIP

Brand new original spot

AD210AN Integrated Amplifier

AD210AN is an integrated analog isolation amplifier chip of AD Company. In this isolation amplifying circuit, two pins 16 and 17 of AD210 are connected together to realize the signal tracking function. The signal source Vs is connected to the signal source Vs through the resistor Ra between the two pins of 18 and 19, and the 18 pin and the Vs share the ground. Pins 1 and 2 are output pins, and Rb is the output load resistance (Ra=Rb=1kΩ is optional when using). The circuit can realize 1:1 isolation transmission function.

Optical Isolation Circuit Design Scheme (3)

Optical isolators can be composed of various application circuits, such as optical isolation circuits, long transmission line isolators, TTL circuit drivers, CMOS circuit drivers, pulse amplifiers, etc.

Summary of optoelectronic isolation circuit design schemes (detailed explanation of six analog circuit design schematics)

Optical Isolation Circuit Design Scheme (4)

Optical isolation is an important anti-interference measure for data acquisition and control systems. Due to the nonlinearity of optocoupler devices, optoelectronic isolation of analog quantities will bring greater signal distortion. In order to improve the linearity of the optoelectronic isolation circuit, the negative feedback method is used to feed the output current of the optocoupler device back to the input terminal. Carry out the static characteristic test of the optoelectronic isolation circuit.

Deep negative feedback circuit design, the use of deep negative feedback can improve the linearity of the system, according to this principle, the photoelectric isolation circuit of the analog signal is designed here. In this circuit, one LP521 optocoupler and two LM2904 integrated operational amplifiers are used. LP521 has two optocouplers, one of which is used for signal conversion and the other for negative feedback; op amp 2904, 1A is used to form a negative feedback circuit. The op amp 2904 constitutes a voltage follower, which is used to increase the load capacity of the circuit. The analog signal photoelectric isolation circuit is shown in Figure 3.

Summary of optoelectronic isolation circuit design schemes (detailed explanation of six analog circuit design schematics)

The output voltage of the circuit and the input voltage have a linear relationship: the light-emitting diodes of the two optocoupler channels of the LP521 are connected in series, and the current If1=If2 passes through the two diodes. Because the two optocouplers are packaged in one device, the optoelectronic characteristics are basically the same. Ideally, it can be considered that the collector output currents of the two optocouplers are equal, that is, Ice1=Ice2.

According to the properties of the ideal op amp, the following formula can be obtained:

Vi=Ice2R1(1)

Vo=Ice1R2(2)

So Vo=(R2/R1)Vi, the output voltage is proportional to the input voltage, and the proportional coefficient is determined by R1 and R2.

Optical Isolation Circuit Design Scheme (5)

1. Introduction of RS485 bus

The RS485 bus is a common serial bus standard that adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. RS485 bus is the most widely used bus when the communication distance is required to be tens of meters to thousands of meters. And it also has a wide range of applications in multi-node working systems.

2. Introduction of typical circuit of RS485 bus

RS485 circuits can be generally divided into isolated and non-isolated types. The isolated type has better performance in anti-interference and system stability than the non-isolated type, but the non-isolated type can also be used in some occasions. Let's talk about the typical non-isolated circuit first. The non-isolated circuit is very simple. It only needs an RS485 chip to be directly connected to the serial communication port of the MCU and an I/O control port. As shown in Figure 1:

Summary of optoelectronic isolation circuit design schemes (detailed explanation of six analog circuit design schematics)

Figure 1. Typical 485 communication circuit diagram (non-isolated type)

Of course, the above picture is not a complete 485 communication circuit diagram, we also need to add a 4.7K pull-up bias resistor on the A line; add a 4.7K pull-down bias resistor on the B line. The R16 in the middle is a matching resistor, generally 120Ω. Of course, this depends on the cable you use for transmission. (Matching resistance: In the entire communication system of 485, for the transmission stability of the system, we generally add matching resistances to the first node and the last node. Therefore, we generally set a variable resistance at each node when designing. Whether the 120Ω resistance of the jumper is used or not is set by the on-site personnel. Of course, how to distinguish the first node or the last node has to be answered by the experts on site.) For TVS, we generally use 6.8V Yes, we will explain this further later.

Optical Isolation Circuit Design Scheme (6)

Introduction to Optical Isolation

Optical couplers are generally composed of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light emitting diode (LED), so that it emits light of a certain wavelength, which is received by the photodetector to generate a photocurrent, which is further amplified and output.

This completes the electrical-optical-electrical conversion, thus playing the role of input, output and isolation. Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectional, so it has good electrical insulation ability and anti-interference ability. And because the input end of the optocoupler is a low-resistance element that works in current mode, it has a strong common mode rejection capability. Therefore, it can greatly improve the signal-to-noise ratio as a terminal isolation element in long-term transmission of information. As an interface device for signal isolation in computer digital communication and real-time control, it can greatly increase the reliability of computer work.

Summary of optoelectronic isolation circuit design schemes (detailed explanation of six analog circuit design schematics)

Figure 1. Opto-isolator working principle

Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectional, so it has good electrical insulation ability and anti-interference ability. And because the input end of the optocoupler is a low-resistance element that works in current mode, it has a strong common mode rejection capability. Therefore, it can greatly improve the signal-to-noise ratio as a terminal isolation element in long-term transmission of information. As an interface device for signal isolation in computer digital communication and real-time control, it can greatly increase the reliability of computer work.

Key benefits of optocouplers

One-way signal transmission, the input end and output end are completely electrically isolated and isolated, the output signal has no effect on the input end, strong anti-interference ability, stable operation, no contact, long service life and high transmission efficiency. Optocoupler is a new type of device developed in the 1970s, and is now widely used in electrical insulation, level conversion, inter-stage coupling, drive circuit, switching circuit, chopper, multivibrator, signal isolation, inter-stage isolation, Pulse amplification circuit, digital instrument, long-distance signal transmission, pulse amplification, solid state relay (SSR), instrumentation, communication equipment and computer interface. In a single-chip switching power supply, a linear optocoupler can be used to form an optocoupler feedback circuit, and the duty cycle can be changed by adjusting the control terminal current to achieve the purpose of precise voltage regulation.

The role of optoelectronic isolation circuit

In the case of electrical isolation, the light is used as the medium to transmit signals, and the input and output circuits can be isolated, so it can effectively suppress system noise, eliminate the interference of ground loops, and have fast response speed, long life, small size and durability. Impact and other benefits make it widely used in strong-weak current interfaces, especially in the forward and backward channels of microcomputer systems.

Summary of optoelectronic isolation circuit design schemes (detailed explanation of six analog circuit design schematics)

Figure 2. Opto-Isolator Application

Measurement of voltage, current, temperature, pressure, stress and flow is an important part of industrial and process control applications. Such application environments often involve hazardous voltages, transient signals, common-mode voltages, and unstable ground potentials, which may damage the measurement system and reduce measurement accuracy. To overcome the above drawbacks, the design of measurement systems in industrial applications incorporates electrical isolation. This white paper focuses on isolation in analog measurements, answers common questions about isolation applications, and also covers the technical implementation of different isolation methods.

Ways to achieve isolation

Isolation requires that signals travel through an isolation barrier without direct electrical connections. Commonly used non-contact signal transmission devices include light emitting diodes (LEDs), capacitors, inductors, and the like. The basic principle behind such devices is the three most common isolation techniques: optoelectronic, capacitive, and inductive coupling. LEDs emit light when powered. Photoelectric isolation uses LEDs and photoelectric detection equipment to achieve isolation barriers, and transmit signals through light.

Summary of optoelectronic isolation circuit design schemes (detailed explanation of six analog circuit design schematics)

Optical isolation circuit

The photodetection device accepts the light signal from the LED and converts it into the original electrical signal. Optical isolation is the most commonly used isolation method. The advantage of using opto-isolation is the ability to avoid electrical and magnetic field noise. The disadvantage is that the transfer speed is limited by the switching speed of the LED, high power scattering and LED wear.

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