Some commonly used PCB layout methods

Mainly interline crosstalk, influencing factors:

Right Angle routing

Does shielded wire

Impedance matching

Long line drive

Reduction of output noise

The reason is diode reverse current abrupt change and loop distributed inductance. Diode junction capacitors form high-frequency attenuation oscillations, and the equivalent series inductance of filter capacitors weakens the role of filtering, so the solution to the peak interference in the output waveform modification is to add small inductors and high frequency capacitors.

For diodes, the maximum response voltage, maximum forward current, reverse current, forward voltage drop and operating frequency should be considered.

The basic methods of power anti-interference are:

The ac voltage regulator and AC power filter are used to screen and isolate the power transformer, and the varistor is used to absorb the surge voltage. In the special case that the power supply quality is very high, the generator set or inverter can be used for power supply, such as the online UPS uninterrupted power supply. Adopt separate power supply and classification power supply. A decoupling capacitor is connected between the power supply of each PCB and the ground. Shielding measures should be taken for power transformers. Transient voltage suppressor TVS was used. TVS is a widely used high-efficiency circuit protection device that can absorb surge power up to several kilowatts. TVS is particularly effective against static electricity, overvoltage, grid interference, lightning strike, switch ignition, power reverse and motor/power noise and vibration.

Multichannel analog switch: In the measurement and control system, the controlled quantity and the measured loop is often several or dozens of paths. Common A/D and D/A conversion circuits are often used for A/D and D/A conversion of multichannel parameters. Therefore, multi-channel analog switch is often used to switch the path between each controlled or tested circuit and A/D and D/A conversion circuit in turn, so as to achieve the purpose of time-sharing control and itinerant detection. Multiple input signals are connected to the amplifier or A/D converter through the multiplexer by the method of single-terminal and differential connection, which has strong anti-interference ability.

Transients occur when a multiplexer switches from one channel to another, causing a transient spike in voltage at the output. In order to eliminate the error introduced by this phenomenon, a samplehold circuit between the output of the multiplexer and the amplifier can be used, or the method of software delay sampling.

The input of multiplex converter is often polluted by various environmental noises, especially common mode noises. A common mode choke is connected to the input end of the multiplex converter to suppress the high frequency common mode noise introduced by external sensors. The high frequency noise generated during the converter’s high frequency sampling not only affects the measurement accuracy, but also may cause the microcontroller to lose control. At the same time, because of the high speed of SCM, it is also a huge noise source for multiplex converter. Therefore, the photoelectric coupler should be used between the microcontroller and A/D isolation.

Amplifier: The selection of amplifier generally uses different performance integrated amplifier. In the complex and harsh sensor working environment, the measurement amplifier should be selected. It has the characteristics of high input impedance, low output impedance, strong resistance to common mode interference, low temperature drift, low offset voltage and high stable gain, so that it is widely used as a preamplifier in weak signal monitoring system. Isolation amplifiers can be used to prevent common-mode noise from entering the system. Isolation amplifier has the characteristics of good linearity and stability, high common mode rejection ratio, simple application circuit and variable amplification gain. The module 2B30/2B31 with amplification, filtering and excitation functions can be selected when using resistance sensor. It is a resistance signal adapter with high precision, low noise and complete functions.

High impedance introduces noise: The high impedance input is sensitive to the input current. This occurs if the lead from the high impedance input is close to a lead with a rapidly changing voltage (such as a digital or clock signal line), where the charge is coupled to the high impedance lead by parasitic capacitance.

The relationship between the two cables is shown in Figure 7. In the figure, the value of parasitic capacitance between two cables mainly depends on the distance between cables (d) and the length of the two cables remaining parallel (L). Using this model, the current generated in high-impedance wiring is equal to: I=C dV/dt

Where: I is the current of high-impedance wiring, C is the capacitance value between two PCB wiring, dV is the voltage change of wiring with switching action, dt is the time it takes for voltage to change from one level to the next level

In the RESET foot string into a 20K resistance, greatly improve the anti-interference performance, the resistance must depend on the CPU reset foot.