Part 1: PCB design basics 3

Published:2011/8/24 2:19:00 Author:Amy From:SeekIC

Karol 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 at their mast ideal location, it is all right to put them on top of each other, this is not yet the time to 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 (area!) 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 o star-point for ground connections **.

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

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