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Wiring is one of the most basic work skills of PCB designers.It succeed or not can directly influence the performance of whole system,most of the high-speed design theories also need implemented and validated by Layout.Thus, wiring is essential in the high-speed PCB design.Because it requires high skills of designing process, heavy workload and patience.PCB wiring include wiring of single-sided PCB ,double-sided and multilayer PCBs.Some circumstances may be encountered in the actual wiring,here i will share some experiences and methods.We can analyse its rationality and give some strategies to optimize.
One Several Experiences of PCB Traces
1、The lines at the end of input&output should be avoided adjacent parallel to avoid reflection interference.Ground wire isolation when necessary.The wiring of the two adjacent layers need to be perpendicular to each other.
2、With analog to digital has a short circuit, please note that there is only one connection point, and there is no common on the PCB, which is determined by the system design.
3、The distance between standard components is 100mil(2.54mm),so the base of the grid system is generally defined as 100mil (2.54 mm) or less than the integer multiples of 100mil,like:50mil、25mil、20mil,etc. Generally,layout 50mil of grid, 5mil of wiring,distance of holes and device is 25mil (so that the device can wiring between the lines)
4、The PCB board delay is 0.167ns/inch.The delay will be increased if many holes ,device pins and multi-constrained cable setted.
5、The wider wire diameter is, the closer to the power is,or the higher the dielectric constant of the isolation layer is,the characteristic impedance is smaller.
6、Wiring on PCB equivalent as the capacitor in series and parallel.Parallel resistance is usually very high.Any high speed and high power consumption of the device should be placed together to reduce the supply voltage transient overshoot.
7、If you are using CMOS or TTL circuit design,the wiring length should be less than 7 inches when the operating frequency is less than 10MHz,at 50MHz shall be no more than 1.5 inches while if the frequency reach or more than 75MHz the length should be 1 inch.
Two Several Methods of PCB Wiring
1．Right Angled Wiring
This way usually is avoided in wiring,which is also being a standard to measure the quality of wiring.Because it will make the changes on the width of transmission
2.Differential Wiring
Differential signal is widely used in high-speed circuit design.Compared with the ordinary single-ended signal,the differential signal has the obvious advantages, such as strong anti-interference ability, effective suppression of EMI and accurate timing positioning.For PCB engineers, most concern is how to ensure that can fully play the different wirings of these advantages in the actual layout. All these rules are not used to move rigidly, many engineers seem not understand the nature of high speed differential signal transmission.
3．Serpentine Wiring
It is the most common use in wiring layout.The main purpose is to adjust the delay and meet the system timing design requirements.But it will destroy the signal quality, change the transmission delay, the wiring should be avoided as far as possible.
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