PCB design anti-interference measures
In the design of electronic systems, in order to avoid detours and save time, the anti-interference requirements should be fully considered and met, and remedial measures for anti-interference should be avoided after the design is completed. There are three basic elements of interference:
(1) Interference source refers to the component, device or signal that produces interference. It is described in mathematical language as follows: du/dt, where di/dt is large is the interference source. For example, lightning, relays, thyristors, motors, high-frequency clocks, etc. may all become sources of interference.
(2) Propagation path refers to the path or medium through which interference propagates from an interference source to sensitive devices. The typical interference propagation path is conduction through wires and space radiation.
(3) Sensitive devices refer to objects that are easily disturbed. Such as: A/D, D/A converter, single chip microcomputer, digital IC, weak signal amplifier, etc. The basic principle of anti-interference design is to suppress interference sources, cut off interference propagation paths, and improve the anti-interference performance of sensitive devices.
1 Suppression of interference sources
Suppress the interference source is to reduce the du/dt and di/dt of the interference source as much as possible. This is the best first consideration and the most important principle in anti-jamming design, and it often achieves a multiplier effect. The reduction of du/dt of the interference source is mainly achieved by paralleling capacitors at both ends of the interference source. Reducing the di/dt of the interference source is achieved by adding series inductance or resistance and adding a freewheeling diode in the interference source loop.
Common measures to suppress interference sources are as follows:
(1) Add a freewheeling diode to the relay coil to eliminate the back electromotive force interference generated when the coil is disconnected. Only adding a freewheeling diode will lag the off time of the relay, and the relay can act more times per unit time after increasing the voltage stabilizing diode.
(2) Connect a spark suppression circuit in parallel at both ends of the relay contact (usually RC series circuit, the resistance is generally selected from a few K to tens of K, the capacitance is selected 0.01uF), to reduce the impact of electric sparks.
(3) Add a filter circuit to the motor, pay attention to the capacitor and inductor lead wires as short as possible.
(4) Each IC on the circuit board should be connected in parallel with a 0.01μF ~ 0.1μF high-frequency capacitor to reduce the impact of the IC on the power supply. Pay attention to the wiring of high-frequency capacitors. The wiring should be close to the power supply terminal and as thick and short as possible. Otherwise, the equivalent series resistance of the capacitor is increased, which will affect the filtering effect.
(5) Avoid 90-degree broken lines when wiring to reduce high-frequency noise emissions.
(6) Both ends of the thyristor are connected to the RC suppression circuit in parallel to reduce the noise generated by the thyristor (it may break the thyristor when the noise is serious).
According to the propagation path of interference, it can be divided into conducted interference and radiated interference.
The so-called conducted interference refers to the interference transmitted to sensitive devices through wires. The frequency band of high-frequency interference noise and useful signals is different. It can be solved by adding a filter on the wire to cut off the propagation of high-frequency interference noise, and sometimes an isolation optocoupler can be added. Power supply noise is the most harmful, so special attention should be paid to it. The so-called radiation interference refers to the interference transmitted to sensitive devices through space radiation. The general solution is to increase the distance between the interference source and the sensitive device, isolate them with a ground wire and add a shielding cover on the sensitive device.
2 Common measures to cut off the interference propagation path are as follows:
(1) Fully consider the influence of power supply on the microcontroller. The power supply is well done, and the anti-interference of the entire circuit is solved by more than half. Many MCUs are very sensitive to power supply noise. Filter circuits or voltage regulators should be added to the MCU power supply to reduce the interference of the power supply noise on the single chip. For example, you can use magnetic beads and capacitors to form a π-shaped filter circuit. Of course, when the conditions are not high, you can also use 100Ω resistors instead of magnetic beads.
(2) If the I/O port of the microcontroller is used to control noise devices such as motors, isolation should be added between the I/O port and the noise source (addition of a π-shaped filter circuit). To control noise devices such as motors, isolation should be added between the I/O port and the noise source (addition of π-shaped filter circuit).
(3) Pay attention to the crystal wiring. The crystal should be as close as possible to the pins of the microcontroller, isolate the clock area with a ground wire, and the crystal case should be grounded and fixed. This measure can solve many difficult problems.
(4) Reasonable division of circuit boards, such as strong and weak signals, digital and analog signals. As far as possible, keep interference sources (such as motors, relays) away from sensitive components (such as microcontrollers).
(5) Use a ground wire to isolate the digital area from the analog area. The digital ground and analog ground should be separated, and finally connected to the power ground at one point. A/D and D/A chip wiring is also based on this principle, and manufacturers have considered this requirement when assigning A/D and D/A chip pinouts.
(6) The ground wires of the single-chip microcomputer and high-power devices should be grounded separately to reduce mutual interference. Place high-power devices on the edge of the circuit board as much as possible.
(7) The use of anti-interference components such as magnetic beads, magnetic rings, power filters, and shields in key places such as I/O ports, power lines, and circuit board connection lines of single-chip computers can significantly improve the anti-interference performance of the circuit.
3 Improve the anti-interference performance of sensitive devices
Improving the anti-interference performance of sensitive devices refers to the method of reducing the pick-up of interference noise as far as possible from the sensitive devices and recovering from abnormal conditions as soon as possible.
Common measures to improve the anti-interference performance of sensitive devices are as follows:
(1) When wiring, minimize the area of the loop ring to reduce the induced noise.
(2) When wiring, the power and ground wires should be as thick as possible. In addition to reducing the voltage drop, it is more important to reduce the coupling noise.
(3) For the idle I/O port of the single-chip microcomputer, do not leave it unconnected and connect to ground or power. The idle terminals of other ICs are grounded or connected to power without changing the system logic.
(4) The use of power monitoring and watchdog circuits for single-chip, such as: IMP809, IMP706, IMP813, X25043, X25045, etc., can greatly improve the anti-interference performance of the entire circuit.
(5) Under the premise that the speed can meet the requirements, try to reduce the crystal oscillator of the MCU and select the low-speed digital circuit.
(6) IC devices should be directly soldered to the circuit board as much as possible, and less IC holders are used.
In order to achieve a good anti-interference, so we often see the wiring on the PCB divided by the ground. But not all digital circuits and analog circuits must be ground plane segmentation. Because this division is to reduce noise interference.