PCB Blog - PCB board signal isolation technology

The PCB board signal isolation technology is to make the digital or analog signal do not have a current connection that crosses the barrier between the transmitting and receiving ends when transmitting. This allows the difference between ground or reference levels outside the transmitter and receiver to be up to several kilovolts, and prevents loop currents between different ground potentials that could damage the signal. Main applications are:
(1) The noise of the system ground is relatively large, which is easy to damage the signal. Isolation can separate the signal into a clean signal subsystem ground and power supply to ensure the reliability of the isolated part of the signal and meet the system design requirements.
(2) The system voltage difference is very large. For example, in a high-current circuit, we usually convert the working voltage into the allowable working range of the IC through isolation.
(3) The electrical connection between the reference levels can create a current path that is unsafe for the operator. Keep current flow within safe limits through isolation.

In isolation technology, it is crucial for designers to select different isolation devices according to the different types of isolated signals and isolation requirements:
(1) isolation devices rely on optical transmitters and receivers to cross the isolation barrier. There are mainly optocouplers (Figure 4-18) and isolated transceiver ICs. The current of the system is interrupted by light, and the capacitor also avoids electrical interference. Such devices are used for digital signals.
(2) Analog transformer, which couples the sending signal and the receiving signal through the electromagnetic induction of the transformer (Figure 4-19). Transformers are difficult to make, and parameters are difficult to control, and it is usually impossible to make ICs, so it is not very convenient to use. But linearization issues force the use of transformers for analog signal isolation.
(3) In order to overcome the inconvenience of using the transformer, the engineer uses a modulated carrier wave to make the analog signal cross this barrier. So came up with a capacitor circuit to couple the modulated signal to cross the barrier. The high slew rate transient voltage acting on the isolation barrier can be used as a signal for the single capacitor barrier device, and a two capacitor differential circuit is developed to make the error. Capacitive barrier technology is now used in digital and analog isolation devices.

1. PCB board signal isolation technology
1.1 Isolating Serial Data Streams
There is a wide range of options for isolating digital signals. If the data stream is bit-serial, options range from simple optocouplers to isolated transceiver ICs. Key design considerations include:
(1) the required data rate;
(2) The power requirements of the isolated end of the system;
(3) Whether the data channel must be bidirectional.
LED-based optocouplers are a technique used to isolate design problems. There are now LED-based ICs available with data rates of 10Mb/s and above. An important design consideration is that the LEC light output decreases over time. So at an early stage, the LEDs must be supplied with excess current so that they can still provide sufficient output light intensity over time. The need to supply excess current is a serious problem because there may be limited supply of power at the isolated side. Because LEDs can require drive currents greater than those available from simple logic output stages, special drive circuits are often required.

1.2 Isolated Parallel Data Bus System
The isolation of parallel digital data bus mainly has six design parameters. On the basis of serial isolation, three more main design parameters will be added:
(1) The bit width of the bus;
(2) allowable deviation;
(3) Clock speed requirements.
An array of optocouplers can accomplish this task, but the support circuitry can be complex. Propagation time mismatch between optocouplers will cause data skew, causing data errors at the receiving end. To minimize this problem, isolated digital couplers are used to support double-buffered data buffering at the input and output.

1.3 Analog Signal Isolation
In many systems, analog signals must be isolated. The circuit parameters considered for analog signals are completely different from digital signals. Analog signals are usually first considered:
(1) isolation accuracy;
(2) Linearity;
(3) Frequency response;
(4) noise considerations;
(5) Isolated power supply.
High requirements for power supply and ground, especially for the input stage, require that the power supply and ground of the input stage are not disturbed 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 an isolation amplifier cannot be improved by the corresponding application circuits, but these circuits can reduce noise and reduce input stage power requirements.

1.4 Multifunctional IC for isolation
Versatile data acquisition ICs with isolation capabilities give designers the opportunity to accomplish multiple tasks across isolation screens. A complete data acquisition device can include multiple analog switches, sample-and-hold circuits, programmable gain instrumentation amplifiers, A/D converters, and one or more digital I/O channels. All these functions are controlled through a serial data port. There are many devices a designer can choose from and use in designs where the ground potential in the system is very different. Each device is designed for unique system requirements. The new device's high level of performance integration enables more complex operations across isolation barriers that were not previously possible.

2. PCB isolation signal layout and wiring
Signal isolation is not only electrically isolated, but also isolated as much as possible in the layout and wiring of the PCB. The following points are mainly noted:
(1) Pay attention to the layout of the isolated part of the circuit, try to layout it in a certain part of the circuit board, and have a certain separation from other circuits. Try to ensure that this part of the circuit is less disturbed by other signals.
(2) Pay attention to the isolation of part of the power supply and ground. Be sure to isolate the power supply and ground wire, especially for analog signals. The cleanliness of the power supply and ground is high. It is necessary to isolate the power supply and ground of this part of the circuit through a transformer. . When routing, also layout and allocate independent power, ground traces or networks.
(3) The layout of the isolation device should be placed reasonably to ensure the independence of the isolation part.
(4) Part of the isolation circuit needs electrostatic protection, high voltage protection, electromagnetic radiation protection, etc., pay attention to the installation position of the outflow protection equipment on PCB board.