How to increase the system's ability to resist electromagnetic interference

Some of the following systems should pay special attention to electromagnetic interference:

1. A system with a particularly high clock frequency and a very fast bus cycle.

2. The system contains high-power, high-current drive circuits, such as spark-generating relays, high-current switches, and so on.

3. A system with a weak analog signal circuit and a high precision A/D conversion circuit.

In order to increase the system's anti-electromagnetic interference capability, the following measures are taken:

1. Select a microcontroller with low frequency:

The use of a microcontroller with a low external clock frequency can effectively reduce noise and improve the system's anti-interference ability. For square waves and sine waves of the same frequency, the high frequency components in the square wave are much more than the sine waves. Although the amplitude of the wave of the high-frequency component of the square wave is smaller than the fundamental wave, the higher the frequency, the easier it is to emit the noise source. The most influential high-frequency noise generated by the microcontroller is about three times the clock frequency.

2. Reduce distortion in signal transmission

Microcontrollers are primarily manufactured using high speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output of the high-speed CMOS circuit has considerable load capacity, that is, a considerable output value, and the output of one gate passes a very long period. The long line leads to the input with a relatively high input impedance, and the reflection problem is very serious, which causes signal distortion and increases system noise. When Tpd>Tr, it becomes a transmission line problem, and signal reflection, impedance matching, etc. must be considered.

The delay time of the signal on the printed board is related to the characteristic impedance of the lead, which is related to the dielectric constant of the printed wiring board material.

It can be roughly assumed that the transmission speed of the signal on the printed circuit board leads is between about 1/3 and 1/2 of the speed of light. The Tr (standard delay time) of a conventional logical telephone component in a system composed of a microcontroller is between 3 and 18 ns.

On a printed circuit board, the signal passes through a 7W resistor and a 25cm long lead. The line delay time is approximately 4~20ns. That is to say, the shorter the lead of the signal on the printed circuit, the better, the longest should not exceed 25cm. Also, the number of vias should be as small as possible, preferably no more than two.

When the rise time of the signal is faster than the signal delay time, it must be processed according to fast electronics. At this time, the impedance matching of the transmission line should be considered. For the signal transmission between the integrated blocks on a printed circuit board, the case of Td>Trd should be avoided, and the larger the printed circuit board, the less the speed of the system should be too fast.

Use the following conclusions to summarize a rule for printed circuit board design:

The signal is transmitted on the printed board and the delay time should not be greater than the nominal delay time of the device used.

3. Reduce cross interference between signal lines:

At step A, a step signal with a rise time of Tr is transmitted through the lead AB to the B terminal. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the A-point signal, the signal reflection after reaching point B and the delay of the AB line, a page pulse signal of width Tr is induced after Td time. At point C, due to the transmission and reflection of the signal on the AB, a width of twice the delay time of the signal on the AB line, that is, a positive pulse signal of 2Td, is induced. This is the cross interference between the signals. The strength of the interfering signal is related to the di/at of the C-point signal and to the distance between the lines. When the two signal lines are not very long, what is seen on AB is actually the superposition of two pulses.

The micro-control made by CMOS technology has high input impedance, high noise and high noise tolerance. The digital circuit is superimposed with 100~200mv noise and does not affect its operation. If the AB line in the figure is an analog signal, this interference becomes intolerable. For example, if the printed circuit board is a four-layer board, one of which is a large-area ground or a double-panel, and the reverse side of the signal line is a large-area ground, the cross-interference between such signals becomes small. The reason is that the large area reduces the characteristic impedance of the signal line, the signal

The reflection at the D end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the medium between the signal line and the ground, and is proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, to avoid the interference of the digital circuit signal line CD to AB, there should be a large area below the AB line, and the distance from the AB line to the CD line is greater than 2 to 3 times the distance between the AB line and the ground. A local shield can be used to ground the left and right sides of the leads on the lead side.

4, reduce the noise from the power supply

The power supply, while providing energy to the system, also adds its noise to the power supply that is being supplied. The reset lines, interrupt lines, and other control lines of the microcontroller in the circuit are most susceptible to external noise. Strong interference on the electrical network enters the circuit through the power supply. Even in battery-powered systems, the battery itself has high frequency noise. Analog signals in analog circuits are more immune to interference from the power supply.

5, pay attention to the high frequency characteristics of printed wiring boards and components

At high frequencies, the leads, vias, resistors, capacitors, and the distributed inductance and capacitance of the printed circuit board are not negligible. The distributed inductance of the capacitor is not negligible, and the distributed capacitance of the inductor cannot be ignored. The resistor produces a reflection of the high frequency signal, and the distributed capacitance of the lead acts. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect is generated and the noise is emitted outward through the lead.

The via of the printed wiring board causes a capacitance of approximately 0.6 pf.

The packaging material of an integrated circuit itself introduces a 2~6pf capacitor.

A connector on a board with a distributed inductance of 520nH. A two-column, 24-pin integrated circuit pedestal that introduces a distributed inductor of 4 to 18 nH.

These small distribution parameters are negligible for this line of microcontroller systems at lower frequencies; special attention must be paid to high speed systems.

6, the component layout should be rationally partitioned

The position of the components on the printed circuit board should be fully considered against electromagnetic interference. One of the principles is that the leads between the components should be as short as possible. In the layout, the analog signal part, the high-speed digital circuit part, the noise source part (such as a relay, a large current switch, etc.) are reasonably separated, so that the mutual signal coupling is minimized.

7, handle the ground wire

Power lines and ground lines are the most important on printed circuit boards. The main means of overcoming electromagnetic interference is grounding.

For the double-panel, the grounding wire arrangement is particularly stressful. By adopting the single-point grounding method, the power supply and the ground are connected from the two ends of the power supply to the printed circuit board, and the power supply has one contact and one ground. On the printed circuit board, there must be multiple return ground lines, which will be gathered back to the contact point of the power supply, which is called single point grounding. The so-called analog ground, digital ground, high power device opening points, the wiring is separated, and finally gathered to this grounding point. Shielded cables are commonly used when connected to signals other than printed circuit boards. For high frequency and digital signals, both ends of the shielded cable are grounded. A shielded cable for low-frequency analog signals is grounded at one end.

Circuits that are very sensitive to noise and interference or circuits with particularly high frequency noise should be shielded with a metal cover.

8, use decoupling capacitors.

Good high frequency decoupling capacitors can remove high frequency components up to 1 GHz. Ceramic chip capacitors or multilayer ceramic capacitors have good high frequency characteristics. When designing a printed circuit board, a decoupling capacitor is added between the power supply of each integrated circuit and the ground. The decoupling capacitor has two functions: one is the storage capacitor of the integrated circuit, which provides and absorbs the charge and discharge energy of the integrated circuit when the door is opened and closed; on the other hand, the high frequency noise of the device is bypassed. The decoupling capacitor with a typical decoupling capacitor of 0.1uf in digital circuit has a 5nH distributed inductor. Its parallel resonant frequency is about 7MHz, which means better decoupling for noise below 10MHz. For 40MHz or more. The noise hardly works.

1uf, 10uf capacitor, parallel resonance frequency above 20MHz, the effect of removing high frequency noise is better. It is often advantageous to have a 1uf or 10uf de-HF capacitor where the power supply enters the printed board, even for battery-powered systems.

For every 10 or so ICs, add a charge and discharge capacitor, or a storage capacitor. The capacitor size is 10uf. It is best not to use an electrolytic capacitor. The electrolytic capacitor is rolled up by two layers of tantalum film. The rolled structure is an inductor at high frequencies, and it is preferable to use a bile capacitor or a polycarbonate capacitor.

The selection of the decoupling capacitor value is not strict, and can be calculated according to C=1/f; that is, 10 Hz is taken as 0.1 uf, and the system composed of the microcontroller can be between 0.1 and 0.01 uf.

Second, some experience in reducing noise and electromagnetic interference.

1. High-speed chips can be used in key places without the use of low-speed chips.

2. The series of resistors can be used to reduce the jump rate of the upper and lower edges of the control circuit.

3. Try to provide some form of damping for relays, etc.

4. Use the lowest frequency clock that meets the system requirements.

5. The clock generator is as close as possible to the device that uses the clock. The quartz crystal oscillator housing should be grounded.

6. Circle the clock area with a ground wire and keep the clock line as short as possible.

7. Place the I/O driver circuit as close as possible to the side of the board so that it leaves the board as quickly as possible. Filter the signal entering the printed board, from

8. The signal from the high noise area should also be filtered, and the signal reflection can be reduced by using the string termination resistor.

9. The MCD useless terminal should be connected high, or grounded, or defined as the output terminal. The end of the integrated circuit should be connected to the power supply ground. Do not hang it.

10. Do not leave the input terminal of the unused circuit. The unused input terminal is grounded and the negative input is connected to the output.

11. The printed board should use 45-fold lines instead of 90-fold lines to reduce the external transmission and coupling of high-frequency signals.

12. The printed board is divided by frequency and current switching characteristics, and the noise components are farther away from the non-noise components.

13. Single-panel and double-panel single-point power supply and single-point grounding, power supply line and grounding wire are as thick as possible. If the economy is affordable, use a multi-layer board to reduce the power supply and grounding inductance.

14. Keep clock, bus, and chip select signals away from I/O lines and connectors.

15. The analog voltage input line and reference voltage should be as far as possible from the digital circuit signal line, especially the clock.

16. For A/D type devices, the digital part and the analog part are more uniform and do not cross.

17. The clock line is perpendicular to the I/O line and has less interference than the parallel I/O line. The clock component pins are far from the I/O cable.

18. The component pins are as short as possible and the decoupling capacitor pins are as short as possible.

19. The key lines should be as thick as possible and with protective ground on both sides. The high speed line should be short and straight.

20. Noise sensitive lines should not be parallel to high current, high speed switching lines.

21. Do not route underneath the quartz crystal and below the noise sensitive device.

22. Weak signal circuit, do not form a current loop around the low frequency circuit.

23. Do not form a loop for any signal. If it is unavoidable, make the loop area as small as possible.

24. One decoupling capacitor per integrated circuit. A small high frequency bypass capacitor is added to each electrolytic capacitor.

25. Use a large-capacity tantalum capacitor or a condenser capacitor instead of an electrolytic capacitor for the circuit to charge and discharge the storage capacitor. When using a tubular capacitor, the housing should be grounded.