In the design of the circuit and PCB layout , the key is to choose the appropriate EMC requirements of components , such as switches logic components , sockets, clock components on the PCB , and various passive components ( resistors, capacitors and inductors , etc. ) . These components will directly cause EMI problems circuit , so in the beginning stages of the project and design the right choice of active and passive components will help to get the most effective EMC effect.
Most digital IC manufacturers currently offer a low radiation logic products , and some elements of the I / O pins have immunity to electrostatic discharge .
Since most digital circuits with a square wave signal , a square wave signal having a plurality of such high harmonic component, the rise time and fall time of the faster component , the more sharp edges changes , and the frequency of the higher harmonic radiation . This requires that the product design meets the requirements under the circumstances , choose the low edge of the speed of change . For example , when the HC logic elements may achieve the design goal, not using AC logic elements . When CMOS4000 can complete the job , do not select the HC to be realized. In general, you want to select elements have good signal integrity and EMC characteristics as possible.
There are many other general principles of selection elements , the following lists a few common principles.
· Low-E : Most digital IC manufacturers to provide products with low radiation glue logic ( glue logic refers to a simple logic circuit connection is not compatible with the complexity of the circuit ) . For example, two common Philips 80C51 80C51 processor than other products on the lower 50 dB radiation.
· Lowland bomb : IC with lowland bombs usually have better EMC performance.
· Transmission line matching I / O: IC output pin must match the high-speed signal transmission lines . For example, when driving a 25Ω load parallel terminal , you can use the bus driver.
Low input capacitance : low input capacitance device helps reduce peak current status changes logic devices , so you can reduce magnetic radiation and ground return currents .
Select all of the requirements listed above elements should ensure that all elements specified in the data sheet minimum or maximum specification. The same types of components provided by different manufacturers , and its parameter specifications may vary, and EMC characteristics may be different , so the choice of when to pay attention .
Although the element manufacturing phase will determine the characteristics of the element of the package , but the PCB EMC design, the package must be a factor in the design of reference , since the pin element will lead inductance and pin capacitance exists , since these capacitors and inductance presence of PCB EMC will have a significant impact , such as generating RF radiation and ringing , and " ground bounce " component pin will bring RF radiation and signal distortion problems.
