PCB Tech - Impedance effect of Printed Circuit Board

a. The following systems shall pay special attention to anti electromagnetic interference:

1-1. The microcontroller clock frequency is particularly high and the bus cycle is particularly fast.

1-2. The system contains high-power and high current driving circuits, such as spark generating relays, high current switches, etc.

1-3. System including weak analog signal circuit and high-precision A / D conversion circuit.

b. The following measures shall be taken to increase the anti electromagnetic interference ability of the system:

1. Select microcontroller with low frequency:

Selecting a microcontroller with low external clock frequency can effectively reduce noise and improve the anti-interference ability of the system. For square wave and sine wave with the same frequency, the high-frequency component of square wave is much more than sine wave. Although the amplitude of the high-frequency component of the square wave is smaller than the fundamental wave, the higher the frequency, the easier it is to be emitted as a 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

The microcontroller is mainly manufactured by high-speed CMOS technology. The static input current at the signal input end is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output end of high-speed CMOS circuit has considerable load capacity, that is, considerable output value. If the output end of a gate is led to the input end with relatively high input impedance through a long line, the reflection problem is very serious, which will cause signal distortion and increase system noise. When TPD > TR, it becomes a transmission line problem. Problems such as signal reflection and impedance matching must be considered.

The delay time of the signal on the printed circuit board is related to the characteristic impedance of the lead, that is, to the dielectric constant of the printed circuit board material. It can be roughly considered that the transmission speed of the signal on the lead of the printed board is about 1 / 3 to 1 / 2 of the speed of light. The tr (standard delay time) of common logic telephone elements in the system composed of microcontroller is between 3 and 18ns.

On the printed circuit board, the signal passes through a 7W resistor and a 25cm long lead, and the online delay time is about 4 ~ 20ns. In other words, the shorter the signal lead on the printed circuit, the better, and the longest should not exceed 25cm. Moreover, the number of vias should be as few as possible, preferably no more than 2. [url href = www.51dz. COM / d.asp? I = topmanazhi] > > > more

When the rising time of the signal is faster than the signal delay time, it should 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, td > TRD should be avoided. The larger the printed circuit board, the faster the system can not be too fast.

Summarize a rule of printed circuit board design with the following conclusions:

When the signal is transmitted on the printed board, its delay time shall not be greater than the nominal delay time of the device used.

3. Reduce cross interference between signal lines:

A step signal with rise time tr at point a is transmitted to end B through lead ab. The delay time of the signal on line AB is TD. At point D, due to the forward transmission of the signal at point a, the signal reflection after reaching point B and the delay of line AB, a page pulse signal with width tr will be induced after TD time. At point C, due to the transmission and reflection of the signal on AB, a positive pulse signal with a width twice the delay time of the signal on AB line, i.e. 2TD, will be induced. This is the cross interference between signals. The intensity of interference signal is related to di / at of point C signal and distance between lines. When the two signal lines are not very long, what you see on AB is actually the superposition of two pulses.

The micro control manufactured by CMOS process has high input impedance, high noise and high noise tolerance. The digital circuit is superimposed with 100 ~ 200mV noise, which does not affect its work. If the first mock exam is a AB signal, the interference becomes intolerable. If the printed circuit board is a four layer board, one of which is a large area of ground, or a double-sided board, and the reverse side of the signal line is a large area of ground, the cross interference between signals will become smaller. The reason is that the characteristic impedance of the signal line is reduced in a large area, and the reflection of the signal at the d end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the medium from the signal line to the ground and directly proportional to the natural logarithm of the medium thickness. If the first mock exam is AB, the interference of CD to AB will be avoided. There is a large area below the AB line. The distance from AB line to CD line is larger than that of AB line to ground. Local shielding ground can be used, and ground wires can be arranged on the left and right sides of the lead on the side with lead connection.

4. Reduce noise from power supply

While the power supply provides energy to the system, it also adds its noise to the power supply. The reset line, interrupt line and other control lines of microcontroller in the circuit are most vulnerable to external noise. The strong interference on the power grid enters the circuit through the power supply. Even in the battery powered system, the battery itself has high-frequency noise. The analog signal in the analog circuit can not withstand the interference from the power supply.

5. Pay attention to the high frequency characteristics of printed wiring boards and components

At high frequency, the lead, via, resistance, capacitance, distribution of connectors, inductance and capacitance on the printed circuit board can not be ignored. The distributed capacitance of capacitance and inductance can not be ignored. The resistance will reflect the high-frequency signal, and the distributed capacitance of the lead will play a role. When the length is greater than 1 / 20 of the corresponding wavelength of the noise frequency, the antenna effect will be generated, and the noise will be transmitted outward through the lead.

The via of the printed circuit board causes a capacitance of about 0.6pf.

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

A connector on a circuit board has a 520nh distributed inductance. A dual in-line 24 pin IC chip base introduces 4 ~ 18nh distributed inductance.

These small distributed parameters are negligible for the microcontroller system at low frequency; Special attention must be paid to high-speed systems.

6. The component layout shall be reasonably divided

The anti electromagnetic interference shall be fully considered for the arrangement of components on the printed circuit board. One of the principles is that the lead between components shall be as short as possible. In the layout, the analog signal part, high-speed digital circuit part and noise source part (such as relay, high current switch, etc.) shall be reasonably separated to minimize the signal coupling between them.

7. Handle the grounding wire

On the printed circuit board, the power cord and ground wire are the most important. The most important means to overcome electromagnetic interference is grounding.

For the double-sided board, the ground wire layout is particularly particular. By adopting the single point grounding method, the power supply and ground are connected to the printed circuit board from both ends of the power supply, one contact for the power supply and one contact for the ground. On the printed circuit board, there should be multiple return ground wires, which will gather on the contact of the return power supply, which is the so-called single point grounding. The so-called opening of analog ground, digital ground and high-power devices means that the wiring is separated and finally collected to this grounding point. Shielded cables are usually used when connecting to signals outside the printed circuit board. For high frequency and digital signals, both ends of the shielded cable are grounded. One end of shielded cable for low-frequency analog signal shall be grounded.

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

8. Use the decoupling capacitor.

A good high-frequency decoupling capacitor can remove high-frequency components as high as 1GHz. Ceramic chip capacitors or multilayer ceramic capacitors have good high frequency characteristics. When designing a printed circuit board, a decoupling capacitor should be added between the power supply and ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, the energy storage capacitor of the integrated circuit provides and absorbs the charge and discharge energy at the moment of opening and closing the door of the integrated circuit; On the other hand, the high-frequency noise of the device is bypassed. The typical decoupling capacitor of 0.1uF in digital circuit has a 5NH distributed inductance, and its parallel resonance frequency is about 7MHz, that is, it has a good decoupling effect on the noise below 10MHz, and has little effect on the noise above 40MHz.

1uF, 10uF capacitors, parallel resonance frequency above 20MHz, the effect of removing high-frequency noise is better. Where the power supply enters the printed board, it is often advantageous to have a high-frequency capacitor of 1uF or 10uF. Even battery powered systems need this capacitor.

One charge discharge capacitor, or storage discharge capacitor, shall be added to every 10 or so integrated circuits. The capacitance can be 10uF. It is better not to use electrolytic capacitor. Electrolytic capacitor is rolled up by two layers of PU film. This rolled up structure is shown as inductance at high frequency. It is best to use bile capacitor or polycarbonate brewing capacitor.

The decoupling capacitance value is not strictly selected and can be calculated as C = 1 / F; That is, 10MHz is taken as 0.1uF, and for the system composed of microcontroller, it can be taken as 0.1 ~ 0.01uF.

Third, Some experience in reducing noise and electromagnetic interference.

If you can use low-speed chips, you don't need high-speed ones. High-speed chips are used in key places.

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

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

Use the lowest frequency clock that meets the system requirements.

The clock generator is as close as possible to the device using the clock. The shell of quartz crystal oscillator shall be grounded.

Circle the clock area with ground wire, and the clock line shall be as short as possible.

The I / O drive circuit shall be as close to the printed board as possible to leave the printed board as soon as possible. The signal entering the printed board shall be filtered, and the signal from the high noise area shall also be filtered. At the same time, the method of string terminal resistance shall be used to reduce the signal reflection.

The useless terminal of MCD shall be connected high, or grounded, or defined as the output terminal. All the terminals connected to the power supply grounding on the integrated circuit shall be connected and shall not be suspended.

The input end of the unused gate circuit shall not be suspended, the positive input end of the unused operational amplifier shall be grounded, and the negative input end shall be connected to the output end. (10) The printed board shall use 45 broken lines instead of 90 broken lines as far as possible to reduce the external transmission and coupling of high-frequency signals.

The printed board is divided according to the frequency and current switching characteristics, and the noise components and non noise components should be farther away.

Single point grounding power supply and single point grounding shall be used for single panel and double-sided board. The power line and ground wire shall be as thick as possible. If it is affordable, multilayer board shall be used to reduce the capacitance inductance of power supply and ground.

Clock, bus and chip selection signals shall be far away from I / O lines and connectors.

Analog voltage input line and reference voltage terminal shall be far away from digital circuit signal line as far as possible, especially clock.

For a / D devices, the digital part and analog part should be unified rather than crossed.

The clock line perpendicular to the I / O line has less interference than the parallel I / O line, and the clock element pin is far away from the I / O cable.

The element pin shall be as short as possible, and the decoupling capacitor pin shall be as short as possible.

The key lines shall be as thick as possible, and protective areas shall be added on both sides. High speed lines should be short and straight.

Lines sensitive to noise shall not be parallel to large current and high-speed switching lines.

Do not route wires under quartz crystals and noise sensitive devices.

Do not form current loop around weak signal circuit and low-frequency circuit.

Do not form a loop for any signal. If it is inevitable, keep the loop area as small as possible.

One decoupling capacitor per IC. A small high-frequency bypass capacitor shall be added to each electrolytic capacitor.

Use high-capacity tantalum capacitor or poly cool capacitor instead of electrolytic capacitor as circuit charge and discharge energy storage capacitor. When using tubular capacitors, the shell shall be grounded.