Principles of anti-interference design for printed circuit boards

Principles of anti-interference design for printed circuit boards

Layout of power cord:

1. According to the current size, try to widen the wire routing as much as possible.

2. The direction of power and ground wires should be consistent with the direction of data transmission.

3. A decoupling capacitor of 10-100 μ F should be connected to the power input terminal of the printed circuit board.

Layout of ground wire:

1. Separate digital from analog.

2. The grounding wire should be thickened as much as possible, and at least 3 times the allowable current on the printed board should be passed, generally up to 2-3mm.

3. The grounding wire should form a dead loop as much as possible, which can reduce the potential difference of the grounding wire.

Decoupling capacitor configuration:

1. The input end of the printed circuit board power supply is connected to an electrolytic capacitor with a temperature of 10-100 μ F. It would be even better if it could be greater than 100 μ F.

2. A 0.01~0.1 μ F ceramic capacitor is connected across the VCC and GND of each integrated chip. If space does not allow, a 1-10 μ F tantalum capacitor can be configured for every 4-10 chips.

3. Devices with weak anti noise capabilities and large changes in turn off current, as well as ROM and RAM, should have capacitors indirectly decoupled at VCC and GND.

4. Install a 0.01 μ F decoupling capacitor on the reset terminal "RESET" of the microcontroller.

5. The lead wires of decoupling capacitors should not be too long, especially for high-frequency bypass capacitors that cannot have leads.

Component configuration:

1. The clock input terminals of the clock generator, crystal oscillator, and CPU should be as close and far away from other low-frequency devices as possible.

2. Try to keep low current circuits and high current circuits as far away from logic circuits as possible.

3. The position and orientation of the printed circuit board in the chassis should ensure that the components with high heat generation are located above.

Separate the wiring of five power lines, AC lines, and signal lines

Power lines and AC lines should be arranged on boards different from signal lines as much as possible, otherwise they should be routed separately from signal lines.

Other principles:

1. Adding a pull-up resistor of around 10K to the bus is beneficial for anti-interference.

2. When wiring, try to have all address lines of the same length and as short as possible.

3. The lines on both sides of the PCB board should be arranged vertically as much as possible to prevent mutual interference.

4. The size of the decoupling capacitor is generally taken as C=1/F, where F is the data transmission frequency.

5. Unused pins can be connected to VCC through pull-up resistors (around 10K) or connected in parallel with the used pins.

6. Heating components (such as high-power resistors) should avoid devices that are easily affected by temperature (such as electrolytic capacitors).

7. Using full decoding has stronger anti-interference ability than line decoding.  

To suppress the interference of high-power devices on the digital element circuits of microcontrollers and the interference of digital circuits on analog circuits, a high-frequency choke ring is used when connecting the digital ground to the common ground point. This is a cylindrical ferrite magnetic material with several holes in the axial direction. A thicker copper wire is passed through the holes and wound one or two times. This device can be regarded as having zero impedance for low-frequency signals and as an inductor for high-frequency signal interference Due to the high DC resistance of inductors, they cannot be used as high-frequency chokes

When signal lines outside the printed circuit board are connected, shielded cables are usually used. For high-frequency and digital signals, both ends of the shielded cable should be grounded. For low-frequency analog signals, it is better to ground one end of the shielded cable.

Circuits that are highly sensitive to noise and interference, or circuits with particularly severe high-frequency noise, should be shielded with a metal cover. The effect of ferromagnetic shielding on high-frequency noise at 500KHz is not significant, while the shielding effect of thin copper skin is better. When fixing the shielding cover with screws, attention should be paid to the corrosion caused by the potential difference when different materials come into contact

Decoupling capacitors

The decoupling capacitor between the power supply and ground of an integrated circuit has two functions: on the one hand, it serves as the energy storage capacitor of the integrated circuit, and on the other hand, it bypasses the high-frequency noise of the device. The typical decoupling capacitance value in digital circuits is 0.1 μ F. The typical value of the distributed inductance of this capacitor is 5 μ H. A decoupling capacitor with 0.1 μ F has a distributed inductance of 5 μ H, and its parallel resonance frequency is approximately 7MHz. This means that it has a good decoupling effect on noise below 10MHz and almost no effect on noise above 40MHz.

Capacitors with 1 μ F and 10 μ F have a parallel resonance frequency above 20MHz, which results in better removal of high-frequency noise.

Every 10 or so integrated circuits require the addition of one charging and discharging capacitor, or one energy storage capacitor, with an optional range of around 10 μ F. It is best not to use electrolytic capacitors. Electrolytic capacitors are made by rolling two layers of thin film together, and this rolled up structure appears as inductance at high frequencies. Use tantalum capacitors or polycarbonate capacitors.

The selection of decoupling capacitors is not strict, and can be based on C=1/F, that is, 0.1 μ F for 10MHz and 0.01 μ F for 100MHz.

When welding, the pins of the decoupling capacitor should be as short as possible, as long pins can cause the decoupling capacitor to self resonate. For example, when the pin length of a 1000pF ceramic capacitor is 6.3mm, the self resonant frequency is about 35MHz, and when the pin length is 12.6mm, it is 32MHz.

Experiences in reducing noise and electromagnetic interference

Principles of anti-interference design for printed circuit boards

1. The method of connecting resistors in series can be used to reduce the jumping rate of the upper and lower edges of the control circuit.

2. Try to make the potential around the clock signal circuit approach zero, circle the clock area with a ground wire, and keep the clock line as short as possible.

3. The I/O driver circuit should be located as close as possible to the edge of the printed board.

4. Do not hang the output terminal of the unused gate circuit, and the positive input terminal of the unused operational amplifier should be grounded, and the negative input terminal should be connected to the output terminal.

5. Try to use 45 ° polylines instead of 90 ° polylines for wiring to reduce the transmission and coupling of high-frequency signals to the outside world.

6. The clock line perpendicular to the I/O line has less interference than parallel to the I/O line.

6. The pins of the components should be as short as possible.

8. Do not trace wires under the quartz crystal oscillator and under components that are particularly sensitive to noise.

9. Do not form a current loop around the ground wire of weak signal circuits and low-frequency circuits.

10. When necessary, add ferrite high-frequency choke coils to the circuit to separate signals, noise, power, and ground.

A via on the printed circuit board causes a capacitance of approximately 0.6pF; The packaging material of an integrated circuit itself causes a distributed capacitance of 2pF~10pF; A connector on a circuit board with a distributed inductance of 520 μ H; A dual in-line 24 pin integrated circuit socket with a distributed inductance of 4 μ H~18 μ H.