电路布线经验

1、Layout/Grounding (Precision, High Speed and Digital)In this chapter, we will discuss five topics. The first topic covers fundamental similarities and differences between analog and digital layout. Then we will talk about the hidden components (resistors, inductors, and capacitors) embedded in your P

2、C board. The next section of this chapter will talk about how to improve your A/D converter accuracy and resolution. Getting another converter will not help here. Focusing on the interaction between the PCB and your converter will improve your results. This will be followed by the fourth section whe

3、re we will discuss two layer layout techniques. Finally, we will end with an example of how to do a poor layout and then how to fix it.The Similarities of Analog and Digital Layout Practices Bypass or Decoupling Capacitors The Power and Ground Should Be Routed TogetherThe Similarities of Analog and

4、Digital Layout PracticesBypass or Decoupling Capacitors In terms of layout, analog devices and digital devices all require these types of capacitors. Both types of devices require that you position one capacitor as close to the power supply pin(s) as possible. A common value for this capacitor is 0.

5、1 F, but it is not unusual to find a 1 F bypass capacitor (for lower frequency circuits) or a 0.01 F capacitor in higher frequency circuits. The selection of the proper capacitor for your circuit is discussed in detail in Chapter 6. A second class of bypass or decoupling capacitor in the system is r

6、equired at the power supply source. The value of this capacitor is usually about 10 F.The Similarities of Analog and Digital Layout PracticesBypass or Decoupling Capacitors Figure 11.1: In analog and digital PCB design, you should place the bypass or decouple capacitors (0.1 F) as close to the devic

7、e as possible. You should also place the power supply decoupling-capacitor (10 F) at the power-source or where the power-bus enters the board. In all cases, these capacitors should have short leads.The Similarities of Analog and Digital Layout Practices Bypass or Decoupling Capacitors The placement

8、of the bypass or decoupling capacitors are just common sense for both types of designs, but interesting enough, for different reasons. In the analog layout design, bypass capacitors generally serve the purpose of redirecting high frequency signals on the power supply trace. This noise would otherwis

9、e enter into the sensitive analog chip, through the power supply pin. Generally, these high frequency signals occur at frequencies beyond rejection capability of the analog device. The possible consequences of not using a bypass capacitor in your analog circuit results in the addition of undue noise

10、 to the signal path or worse yet, oscillationThe Similarities of Analog and Digital Layout PracticesBypass or Decoupling Capacitors For digital devices, such as controllers and processors, the decoupling capacitor on the power supply pin are required, but for a different reason. One of the functions

11、 of these capacitors serves as a “mini” charge reservoir. Frequently in digital circuits, a great deal of current is required to execute the transitions of the changing gate states. Because of the switching transient currents that occur on the chip and throughout the circuit board, having additional

12、 charge “on-call” is advantageous. The consequence of not having enough charge locally to execute this switching action could result in a significant dynamic and static change in the power supply voltage. When the voltage change is too large, it will cause the digital signal level to go into the ind

13、eterminate state.The Similarities of Analog and Digital Layout PracticesBypass or Decoupling Capacitors So for multiple reasons, it is a good idea to bypass (or decouple) the power supply at the power supply and at the power supply pin of all of the active devices.The Similarities of Analog and Digi

14、tal Layout Practices2. The Power and Ground Should Be Routed Together When you match power and ground traces with respect to location, you lesson the opportunities for EMI. If you dont match power and ground, system loops are part of the layout. The possibility of seeing “noisy “ results without exp

15、lanation is real. Figure 11.2 shows an example of a PCB design with the unmatched power and ground traces.The Similarities of Analog and Digital Layout Practices2. The Power and Ground Should Be Routed Together When you match power and ground traces with respect to location, you lesson the opportuni

16、ties for EMI. If you dont match power and ground, system loops are part of the layout. The possibility of seeing “noisy “ results without explanation is real. Figure 11.2 shows an example of a PCB design with the unmatched power and ground traces.The Similarities of Analog and Digital Layout Practic

17、es2. The Power and Ground Should Be Routed TogetherFigure 11.2: The power and ground traces are laid out using different routes to the device on this board. This mismatch opens the opportunity for EMI into the electronics of this board.The loop area in Figure 11.2 is 697 cm2. This loop is a perfect

18、antenna for noise in the area. With this board, you may be able to pick up radio signals. In the 1980s, one of the German engineers that I worked with was able to design boards of this class and “pick-up”radio-free Europe.The Similarities of Analog and Digital Layout Practices2. The Power and Ground

19、 Should Be Routed TogetherFigure 11.3: In this one layer board, the power trace and ground trace arelaid next to each other on their way to the device on this board. Figure 11.2shows a board where the traces are better matched. The opportunity forEMI into the electronics of this board is lessened by

20、 679/12.8 or 54x.In Figure 11.3, the signal and ground line are next to each other. This greatly reduces the loop area. An even better solution would be to have a ground plane, which would be underneath the power supply trace. An even better solution would be to have a ground plane and a separate po

21、wer plane.概貌图注意单片机电源和地线的环路高亮部分为GND注意单片机电源和地线的环路高亮部分为VccThe Similarities of Analog and Digital Layout PracticesThe fundamentals of circuit board layout apply to analog circuits as well as digital circuits. One fundamental rule of thumb is to use uninterrupted ground planes. This common practice reduc

22、es the effects of I/t (change in current with time) in digital circuits. In digital circuits, the change in current with time changes the potential of ground. In analog circuits, injected noise is caused by I/t. But, when comparing digital and analog circuits, you should exercise an added precaution

23、 with analog circuits in order to keep the digital signal lines and return paths in the ground plane as far away from the analog circuitry as possible. .3. Where the Domains Differ Ground Planes Can Be a ProblemThe Similarities of Analog and Digital Layout PracticesThis can be done by connecting the

24、 analog ground plane separately to the system ground or having the analog circuitry at the farthest side of the boardthat is, at the end of the line. This is done so that signal paths have a minimal amount of interference from external sources. The opposite is not true for digital circuitry. The dig

25、ital circuitry can tolerate a great deal of noise on the ground plane before problems start to appear.3. Where the Domains Differ Ground Planes Can Be a ProblemThe Similarities of Analog and Digital Layout PracticesLocation of ComponentsIn every PCB design, you should separate the noisy and quiet po

26、rtions of the circuit, as mentioned above. Generally, the digital circuitry is “rich” with noise. Alternatively, digital circuitry is less sensitive to this type of noise because of the larger voltage noise margins.3. Where the Domains Differ Ground Planes Can Be a ProblemLocation of Components When

27、 you look at analog circuits, you will easily find that they are not as forgiving as the digital circuits. The voltage noise margins of the analog circuitry are much smaller. Of the two domains, the analog domain is most sensitive to switching noise. In the layout of a mixed signal system, you shoul

28、d separate the two domains. Figure 11.4 shows this is graphically.The Similarities of Analog and Digital Layout Practices3. Where the Domains Differ Ground Planes Can Be a ProblemThe Similarities of Analog and Digital Layout Practices3. Where the Domains Differ Ground Planes Can Be a ProblemFigure 1

29、1.4: If possible, (a) the digital and analog portion of circuitsshould be separated in order to separate the digital switching activityfrom the analog circuitry. Additionally, (b) the high frequency shouldbe separated from the low frequency where possible, keeping thehigher frequency components clos

30、er to the board connector.4. Where the Board and Component Parasitics Can Do the Most Damage The major classes of parasitics generated by the PC board layout come in the form of resistors, capacitors, and inductors. For instance, you can build PCB resistors with your traces that span between compone

31、nts. You can build unintentional capacitors into the board with traces, soldering pads, and parallel traces. The Similarities of Analog and Digital Layout Practices4. Where the Board and Component Parasitics Can Do the Most Damage Unintentional inductors come from loop inductance, mutual inductance,

32、 and vias. All of these parasitics stand a chance of interfering with the effectiveness of your circuit as you transition from the circuit diagram to the actual PCB. You will clearly see in this section of the chapter the most troublesome class of board parasitics and see examples of where these par

33、asitics the effect on circuit performance.The Similarities of Analog and Digital Layout Practices4. Where the Board and Component Parasitics Can Do the Most DamageFeeling the Pain of Those Unnecessary CapacitorsThe Similarities of Analog and Digital Layout PracticesFigure 11.5: You can easily place

34、capacitors into a PCB by layingout two traces in close proximity. With this type of capacitor, fastvoltage changes on one trace can initiate a current signal in theother trace. (Chapter 9 also discusses this issue.)4. Where the Board and Component Parasitics Can Do the Most DamageYou can easily mini

35、mize this type of phenomena using one of two techniques(1)(2) Another technique used is to lay a ground trace between the two offending traces. Not only is the ground trace low impedance, but an additional trace like this will break up the E-fields that are causing the disturbance.The Similarities o

36、f Analog and Digital Layout Practices4. Where the Board and Component Parasitics Can Do the Most Damage This type of capacitor can cause problems in mixed signal circuits where sensitive, high impedance, analog traces are in close proximity to digital traces.The Similarities of Analog and Digital La

37、yout PracticesExample:4. Where the Board and Component Parasitics Can Do the Most DamageThe Similarities of Analog and Digital Layout PracticesThis configuration produces inconsistent noise on the analog line because the data input code on that particular digital trace changes. These changes are dep

38、endent on the programming requirements for the digital potentiometer.Figure 11.7: This is the first attempt at the layout for the circuit in Figure 11.6. In this figure, you can see that a critical, high impedance, analog line is very close to a digital trace.4. Where the Board and Component Parasit

39、ics Can Do the Most DamageOn the bench, I measured the digital signal that was coupled into the sensitive, analog wiper, trace. Figure 11.8 shows the scope photo.The Similarities of Analog and Digital Layout Practices4. Where the Board and Component Parasitics Can Do the Most DamageThe Similarities

40、of Analog and Digital Layout PracticesFigure 11.9: With a new layout, the analog lines are separate fromthe digital lines. This distance has eliminated the digital noise thatwas causing interference in the previous layout.4. Where the Board and Component Parasitics Can Do the Most DamageThe Similari

41、ties of Analog and Digital Layout PracticesOnce again, when the digital and analog domains meet, careful layout is critical if you intend to have a successful final PCB implementation. In particular, active digital traces close to high impedance, analog traces will cause serious coupling noise. You

42、can avoid this noise coupling phenomena by putting distance between traces.4. Where the Board and Component Parasitics Can Do the Most DamageThe Similarities of Analog and Digital Layout Practices5. Inductors Designed into the PCB The way that an inductor is designed into a board is similar to the c

43、onstruction of a capacitor. Again this is done by placing two traces, one on top of the other with two layers or by placing them beside each other on the same layer, as shown in Figure 11.11.The Similarities of Analog and Digital Layout Practices In both trace configurations, changes in current with

44、 time (I/t) on one trace could generate a voltage in the same trace due to the inductance on that trace and initiate a proportional current on the second trace due to the mutual inductance5. Inductors Designed into the PCBThe Similarities of Analog and Digital Layout PracticesTo eliminate potential

45、noise for EMI sources it is best to separate quiet analog lines versus noisy I/O ports. Try to implement low impedance power and ground networks, minimize inductance in conductors for digital circuits and minimize capacitive coupling in analog circuits.The Art of Laying Out Two-Layer BoardsPay Now o

46、r Pay Later with the Auto Router and Analog CircuitsGospel :It is tempting to use the auto router when designing printed circuit board (PCB). More often than not, a purely digital board, (especially if the signals are relatively slow, and the circuit density is low) will work just fine. But as you t

47、ry to layout analog, mixed signal or high-speed circuits with the auto routing tool that is available with your layout software, there may be some issues. The probability of creating serious circuit performance problems is very real.The Art of Laying Out Two-Layer BoardsPay Now or Pay Later with the Auto Router and Analog CircuitsWith this layout, there are several areas of concern, but the most troubling issue is the grounding strategy. If you follow the ground traces on the top layer, the traces connect every device on that layer. A second ground connection for