Part 1: PCB design basics 1

Published:2011/8/24 2:07:00 Author:Amy From:SeekIC

Karol Walraven

What do most electronics engineers enjoy a lot? Designing printed circuit boards, of course! In an irregular series of articles we would like to familiarize you with this subject. We start as simple as possible and it will become more difficult as we progress. Feedback and requests to cover certain topics are encouraged and welcomed!

When you’re just designing a small PCB, it often is not that important where the actual components ore placed. However, there are a few rules of thumb that, when fol­lowed, quickly and easily result in a much better PCB. Belter in this context means lower noise in the desired output signal and loss interfer­ence with other equipment. The last sentence means, translated into practice: "your amplifier provides a better sound, makes no additional noises when the fridge turns on and does not interfere with your mobile phone".

The following statement is per­haps surprising, but is actually true: "The optimum position of a component on o PCB is for nearly 100% determined by its function in the schematic diagram." That means you cannot avoid thinking about the func­tion of each and every compo­nent in the circuit.

Example

We use an amplifier stage as an example. What are the dif­ferent functionalities that can be distinguished?

In Figure 1, C5 is a decou­pling capacitor, which decouples the power supply voltage. The current drawn by ICI will change during operation and C5 ensures that the peak current demand can be delivered.



CI is a decoupling capacitor as well, but in this case it serves the purpose of prevent­ing any hum and interference from the power supply from reaching the signal.

C3 is a decoupling capacitor too, but this lime not for the power supply voltage, but for the signal instead. Its function is lo ensure that the inverting input of ICI is connected lo ground, via R4, for AC volt­ages.

Rule 1: Decoupling compo­nents must be placed as dose as is practicable to the nodes that need to be decoupled.

Long traces hove a higher impedance (= resistance for high frequencies) and as a consequence the decoupling will not work as well. The higher the frequency, the more important it becomes to keep the traces shod. Pay close attention! To clarify this a little more, lake a look al Fig­ure 2a .The traces towards and away a/ways form a loop (also when one side of the component is connected lo ground). In the figure we have shaded the area between the traces for clarity. We cannot overemphasize that it is very important lo place the traces in such a way that the shaded area is us small as is possible. If you have to choose between long tracks with a smaller enclosed area or shorter faces with larger enclosed area, then choose the layout with the smallest area" Why that is so we will explain at a later time. It is a little too advanced for the present discussion.

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