 approximately between 4 and 20ns. In other words, the shorter lead of signal on the printed circuit is better, and the longest should not exceed 25cm. Besides, the number of vias should be as small as possible, preferably less than two.
When the rise time of the signal is faster than the delay time, it is necessary to follow the rules from fast electronics. At this moment, you should consider the impedance matching of transmission line. For the signal transmission between integrated block on a printed circuit board, you should avoid the emergence of Td>>Trd. Generally, the speed of the printed circuit board system cannot be too fast if the board is greater. In conclusion, when the signal is transmitted on the printed board, the delay time should be less than the nominal delay time of the device used.
Reduce the interference between signal lines:
The rise time of point A is Tr step signal which is passed through the lead AB to B side, while the delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the point A signal, the signal reflection after reaching point B and the delay of the AB line, a page pulse signal with a width of Tr will be induced after the Td time. At point C, as the transmission and reflection of the signal on AB line, it will sense a 2Td positive pulse signal that is twice the width of the delay time of the signal on the AB line. This is the interaction between the signal interference. The intensity of the interfering signal is related to the di/at of the C-point signal, which is related to line spacing. When the two signal lines are not very long, the two pulse superposition can be seen on line AB.
The micro-control of CMOS technology is of high input impedance, noise and noise margin, while the digital circuit is superimposed 100 ~ 200mv noise which does not affect its work. If the AB line is an analog signal, then this interference becomes intolerable. If a four-layer board has
large area of ground or the opposite of signal lines is a large area of the ground in double-sided board, the interference between the signals will become smaller. The reason is that the large area of ground reduces the characteristic impedance of the signal line and the signal reflection at the D-side. The characteristic impedance is inversely proportional to the square of the dielectric constant of medium from the signal line to ground, which is proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, for avoiding the interference of digital circuit signal line CD on line AB, the distance between them should be 2 to 3 times greater than the one from line AB and ground. Partially shielding the ground, the lead with junctions will be wired on the both sides.
Reduce the noise from the power supply
The power supply offers energy to the system and adds noise to the powered power supply. In the circuit, microcontroller reset line, interrupt line and some other control lines are most susceptible to external noise interference. The Strong interference on the power grid passes through power supply into the circuit, even if the battery itself also has high-frequency noise in the battery-powered system. Analog signal in circuits cannot withstand the interference from the power supply.
Notice the high-frequency characteristics of PCB and components
In high frequency, the lead, through-hole, resistance, capacitance, distributed inductance and capacitance of connectors cannot be ignored. Besides, distributed inductance of capacitance and distributed capacitance of inductance cannot be neglected. The resistance produces a reflection of the high frequency signal, and the distributed capacitance of the lead will work. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect is generated and the noise is spreading out through the lead. The through-hole in printed circuit board brings about 0.6pf of capacitance, while the encapsulated material in an integrated circuit causes 2 ~ 6pf.A connector on a cir