Part 1: PCB design basics （2）

Published:2011/8/23 1:58:00 Author:Li xiao na From:SeekIC

By Karel Walraven

Get cracking

We’ll now gel lo work, keeping the above information in mind. A little bit of searching on the Internet will quickly produce a number of free demo versions of nice printed circuit board layout software. At the end of the article we have included a few links lo get you started.

When designing a PCB layout you start with the fixed constraints. These are the components (or size of the PCB] that for one reason or another have lo be in a fixed location. When a potentiometer has to protrude through a hole in the front panel, you could put it anywhere on the PCB, but in the end it still has to fit behind the front panel, so you may as well start with that. Otherwise, any other work would simply be a waste of time.

Follow this by temporarily placing all the other components on their mast ideal location, it is all right to put them on lop of each other, this is not yet the time lo make compromises (Figure 4).



C2 and R3 are as close as possible to pin 3 of the IC. R4 and R5 as close to pin 2 as possible. You will now notice that you hove to make choices. R3 can be placed close to pin 3 or close to CI. In this case we always chose the shortest trace for the connection with the highest impedance. This is, otter all, the most sensitive point for picking up interference! CI is a relatively low impedance to ground, so that side of R3 is not that critical. The impedance on the other side of R3 is definitely higher. This connection is more sensitive and needs to be kept short. This is also true for R4 and R5 (R5 is connected on one side lo the relatively low impedance of the opamp output). That is why it is better lo make the signal path at the output a little longer so that the signal path at the input is as short as possible.

Power supply decoupling capacitor C5 is o little on its own in the schematic, but has to be placed as close as possible to pins A and 7. It is not important if it is closer to either 4 or 7, only the total length (areal) of the path matters. If all of this is becoming too much, don’t panic! Remember three things:

- place decoupling components close to the load
- keep high impedance traces as short as possible
- make a star-point for ground connections **.

In general the following applies: keep the area of loops as small as is possible.

Wrapping up

Continue by placing the components in their final location (Figure 5). If you ore short of space, check if the following tricks are of any help:



- Use SMD components. These are smaller (and therefore belter at higher frequencies) than leaded components and can be placed on the bottom of the circuit board, so they do not occupy space on the component side. In Figure 2b you can see that with respect to the area an SMD decoupling capacitor is absolutely superior to an ’ordinary’ decoupling {2a) capacitor.

- Split large capacitors into two smaller capacitors. You can quite safely place the decoupling capacitor with the larger value a little further away because it provides far the lower frequencies. Place a small (for example 10 to 100nF| capacitor close lo the component to where the power supply track runs.

Tips

Finally another few lips, don’t give interference an opportunity to slip ’past’ the decoupling components. Figure 6 illustrates what we mean by this.



If you are designing o double-sided PCB, then it is usual lo reserve one side as a ground plane. This makes the layout easier and works very well from an electrical perspective, because ground then becomes a real ground (= reference] as much as possible. When using leaded components each connection to ground is then made automatically. With SMDs you will have to make a manual connection with the aid of an additional via (a connection through the board from the top- lo the bottom side) which must be close to the ground pin of the SMD component. If you don’t do this the length of the ground trace could become too much!

Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/23/Part_1__PCB_design_basics_（2）.html

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