# Analysis of common measures to suppress interference sources in circuit system design

In the design of circuit systems, we often encounter such things. A circuit whose program is clearly copied from the book, but the test results are not correct. Why is this? The reason is interference. We are in In the process of electronic circuit and program design, anti-interference measures must be taken.

There are three basic elements that cause interference:

(1) Interference source refers to the component, equipment or signal that generates interference. It is described in mathematical language as follows: du / dt, where di / dt is large, is the source of interference. Such as: lightning, relays, thyristors, motors, high-frequency clocks, etc. may become interference sources.

(2) Propagation path refers to the path or medium of interference from the source of interference to sensitive devices. Typical interference propagation paths are conducted through wires and radiation in space.

(3) Sensitive devices refer to objects that are easily interfered. Such as: A / D, D / A converter, microcontroller, digital IC, weak signal, etc.

The basic principles of anti-interference design are: suppress interference sources, cut off interference propagation paths, and improve the anti-interference performance of sensitive devices. (Similar to the prevention of infectious diseases)

1. Suppress interference sources

Suppressing the interference source is to reduce du / dt and di / dt of the interference source as much as possible. This is the highest priority and most important principle in anti-interference design, and often it will do more with less. Reducing the du / dt of the interference source is mainly achieved by connecting capacitors in parallel across the interference source. Reducing the di / dt of the interference source is achieved by series inductance or resistance in the loop of the interference source and adding a freewheeling diode.

Common measures to suppress interference sources are as follows:

(1) The freewheeling diode is added to the relay coil to eliminate the interference when the coil is disconnected. Only adding a freewheeling diode will lag the off-time of the relay. After increasing the zener diode, the relay can operate more times in a unit time.

(2) Connect spark suppression circuits in parallel at both ends of the relay contacts (generally RC, resistance is generally selected from several K to tens of K, and capacitance is selected from 0.01uF) to reduce the impact of electric spark.

(3) Add a filter circuit to the motor. Pay attention to the short lead of the capacitor and inductor.

(4) Each IC on the circuit board should be connected with a 0.01μF ～ 0.1μF high-frequency capacitor in parallel to reduce the influence 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 be as short as possible. Otherwise, the equivalent series resistance of the capacitor is increased, which will affect the filtering effect.

(5) Avoid 90-degree polylines during wiring to reduce high-frequency noise.

(6) Both ends of the thyristor are connected in parallel with the RC suppression circuit to reduce the noise generated by the thyristor (when the noise is severe, it may break down the thyristor).

According to the propagation path of interference, it can be divided into two categories: conducted interference and radiated interference.

The so-called conducted interference refers to the interference transmitted to the sensitive devices through the wire. The high-frequency interference noise and the frequency band of the useful signal are different. You can cut off the propagation of high-frequency interference noise by adding a filter on the wire, and sometimes it can be solved by adding an optical isolation. The harm of power supply noise is the biggest, so special attention should be paid to it. The so-called radiated 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 place a shield on the sensitive device.

2.Common measures to cut off the interference propagation path

(1) Fully consider the impact of power supply on the microcontroller. If the power supply is done well, the anti-interference of the entire circuit is more than half solved. Many microcontrollers are very sensitive to power supply noise. Filter circuits or voltage regulators should be added to the microcontroller power supply to reduce the interference of power supply noise on the microcontroller. For example, you can use a magnetic bead and a capacitor to form a π-shaped filter circuit. Of course, you can use a 100Ω resistor instead of the magnetic bead when the conditions are not high.

(2) If the I / O port of the single-chip microcomputer is used to control noise devices such as motors, isolation should be added between the I / O port and the noise source (add a π-shaped filter circuit).

(3) Pay attention to the crystal wiring. The crystal oscillator and the microcontroller pins are as close as possible, the ground area is used to isolate the clock area, and the crystal case is 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. Keep interference sources (such as motors, relays) away from sensitive components (such as microcontrollers) as much as possible.

(5) Use the ground wire to isolate the digital area from the analog area, separate it from the analog ground, and finally connect it to the power ground at one point. A / D and D / A chip wiring is also based on this principle. Manufacturers have considered this requirement when assigning A / D and D / A chip pin arrangements.

(6) The single chip microcomputer and the large ground wire should be grounded separately to reduce mutual interference. Place high-power devices on the edge of the board as much as possible.

(7) The use of anti-jamming components such as magnetic beads, magnetic rings, power filters, and shields in critical places such as I / O ports, power lines, and circuit board connection lines 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 minimizing the pickup of interference noise from the side of sensitive devices, and recovering from the abnormal state as soon as possible.

The common measures to improve the anti-interference performance of sensitive devices are as follows:

(1) Minimize the area of ​​the loop loop when wiring 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 I / O port of the one-chip computer idle, do not suspend, connect to ground or connect the power. The idle ends of other ICs are grounded or connected to power without changing the system logic.

(4) Use single-chip power supply monitoring and watchdog circuits, such as: IMP809, IMP706, IMP813, X25043, X25045, etc., can greatly improve the anti-interference performance of the entire circuit.

(5) On the premise that the speed can meet the requirements, try to reduce the crystal oscillator of the microcontroller and use low-speed digital circuits as much as possible.

(6) IC devices should be directly soldered to the circuit board as much as possible, and less IC sockets should be used.

Let me talk about my experience in this area.

Software

1, often unused code space is completely cleared to "0", because this is equivalent to NOP, can be homed when the program is running;

2. Add a few NOPs before the jump instruction, the purpose is the same as 1.

3. When there is no hardware WatchDog, software can be used to simulate WatchDog to monitor the operation of the program;

4. When dealing with the adjustment or setting of external device parameters, in order to prevent the external device from being mistaken due to interference, the parameters can be resent periodically, so that the external device can be restored to the correct speed as soon as possible;

5, anti-interference in communication, data check digits can be added, can take 3 out of 2 or 5 out of 3 strategy;

6. When there are communication lines, such as I2C, three-wire system, etc., in practice, we found that setting the Data line, CLK line, and INH line to high normally has better anti-interference effect than low.

Hardware

1. The wiring of the ground and power cables must be important!

2.Decoupling of lines;

3.Separation of numbers and modes;

4.Each digital component needs 104 capacitors between ground and power;

5, in the application of the relay, especially at high currents, to prevent the relay contact sparks from interfering with the circuit, a 104 and a diode can be connected between the relay coils, and 472 capacitors indirectly between the contacts and the normally open end, the effect is good!

6. In order to prevent crosstalk of I / O ports, I / O ports can be isolated by methods such as diode isolation, gate circuit isolation, photocoupler isolation, electromagnetic isolation, etc .;

7. Of course, the anti-interference of the multilayer board is definitely better than the single-layer board, but the cost is several times higher.

8. It is most important to choose a device with strong anti-interference ability than any other method.