PCB copy board signal isolation technology

In isolation technology, it is critical for designers to select different isolation devices based on the different types of isolated signals and isolation requirements:

(1) The first type of isolation device relies on optical transmitters and receivers to cross the isolation barrier. The main ones are optocouplers and isolation transceiver ICs. Light is used to isolate the system's current, and capacitors also prevent electrical interference. This type of device is used for digital signals.

(2) Analog transformers use the electromagnetic inductance of the transformer to couple the sending and receiving signals. Transformers are difficult to make, their parameters are difficult to control accurately, and they are usually impossible to make into ICs, so they are not very convenient to use. However, linearization problems force the use of transformers for analog signal isolation.

(3) To overcome the inconvenience of using transformers, engineers used modulated carriers to allow analog signals to cross the barrier. So they came up with the idea of ​​using capacitor circuits to couple modulated signals across the barrier. The high-conversion-rate transient voltage acting on the isolation barrier can be used as a signal for a single-capacitor barrier device, and a dual-capacitor differential circuit was developed to minimize the error. Capacitor barrier technology has now been applied to digital and analog isolation devices.

1. PCB signal isolation technology

1 Isolating the serial data stream

There are a wide range of options for isolating digital signals. If the data stream is bit serial, the options range from simple optical coupling to

The main design considerations include:

(1) Required data rate;

(2) Power requirements at the system isolation end;

(3) Whether the data channel must be bidirectional.

LED-based optocouplers were the first technology used to isolate design problems. LED-based ICs are now available with data rates of 10Mb/s and above. An important design consideration is that the LED light output decreases over time. So in the early days, excess current had to be provided to the LED to provide sufficient output light intensity over time. Because there may be limited power available on the isolation side, the need to provide excess current is a serious problem. Because the drive current required by the LED can be greater than the current available from a simple logic output stage, special drive circuits are often required.

2 Isolating Parallel Data Bus Systems

There are six main design parameters for isolating parallel digital data buses. Three more main parameters are added to the isolation of serial buses.

Required design parameters:

(1) The bit width of the bus;

(2) Allowable deviation;

(3) Clock speed requirements.

This task can be accomplished using an array of optocouplers, but the supporting circuitry can be cumbersome. Propagation time mismatches between optocouplers will cause data skew, which can lead to data errors at the receiving end. To minimize this problem, isolated digital couplers are used to support double-buffered data buffers at the input and output. 3 Analog Signal Isolation

In many systems, analog signals must be isolated. The circuit parameters considered by analog signals are completely different from those of digital signals.

Analog signals usually first consider:

(1) Isolation accuracy;

(2) Linearity;

(3) Frequency response;

(4) Noise considerations;

(5)Isolate the power supply.

The power supply and ground requirements are high, especially for the input stage, which requires that the power supply and ground of the input stage are not interfered by other parts of the circuit, that is, an isolated power supply is used. It should also be noted that the basic accuracy or linearity of the isolation amplifier cannot be improved by the corresponding application circuit, but these circuits can reduce noise and reduce the input stage power supply requirements.

4 Multifunctional IC for Isolation

Multifunction data acquisition ICs with isolation capabilities give designers the opportunity to accomplish multiple tasks across an isolation screen. A complete data acquisition device may include multiple analog switches, sample-and-hold circuits, programmable gain instrumentation amplifiers, A/D converters, and one or more digital I/O channels. All of these functions are controlled through a serial data port.

There are many devices available to designers to choose from and use in designs where the ground potentials in the system vary widely. Each device is designed for unique system requirements. The high level of performance integration of new devices enables more complex operations across the isolation barrier that were not possible before.

2. PCB Isolation Signal Layout and Wiring

Signal isolation is not only done electrically, but also as far as possible in PCB layout and wiring. Pay attention to the following points:

(1) Pay attention to the layout of the isolated circuit, and try to place it in a certain part of the circuit board, which is separated from other circuits. Try to ensure that this part of the circuit is less affected by other signals.

(2) Pay attention to the isolation of the power supply and ground of the isolated part. The power supply and ground must be isolated well, especially for analog signals, which have high requirements for the cleanliness of the power supply and ground. Transformers are needed to isolate the power supply and ground of this part of the circuit. When wiring, independent power supply and ground routing or networks must also be arranged.

(3) The layout of isolation devices should be placed reasonably to ensure the independence of the isolation part.

(4) Some isolation circuits require electrostatic protection, high voltage protection, electromagnetic radiation protection, etc. Pay attention to the installation location of the outflow protection equipment.

PCB signal isolation technology is to make digital or analog signals have no current connection across the barrier between the sending and receiving ends when they are sent. This allows the difference between the ground or reference level outside the sending and receiving ends to be as high as several thousand volts, and prevents loop currents between different ground potentials that may damage the signal. It is mainly used in:

(1) The noise of the system ground is relatively large, which can easily damage the signal. Isolation can separate the signal into a clean signal subsystem ground and power supply, ensuring the reliability of the isolated part of the signal and meeting the system design requirements.

(2) The system voltage difference is very large. For example, in a high-voltage circuit, we usually use isolation to convert the operating voltage to within the operating range allowed by the IC.

(3) The electrical connection between the reference levels can create a current path that is unsafe for operators. Isolation is used to control the current within a safe range.