Part 1: PCB design basics 2

Published:2011/8/24 2:14:00 Author:Amy From:SeekIC

Karol Walraven

C12 and C4 are capacitors in the signal path. In this case it is also true that short traces and small area provide less opportunity lo pick up trouble or cause trouble. We repeat the above once more because it is very important that you think about this.

How does a signal connection have an area? In figure 3 you can see that the signal does not only go towards the capacitor, but also has a return path via ground! The ground face has a certain length as well! Signal path arid ground path together form a loop! This should not come as a surprise to you. Your very first acquain­tance with electronics probably involved o small lamp and a battery. And when did the lamp light up? Just when the loop is closed... So, pay close attention to Rule 1!



We can make a few addi­tional comments with regards lo the signal path. The older youths among us, those who grew up with record players and valves, will remember how easy it was to lest an amplifier from days gone by. A finger in the vicinity of the input was enough to generate a loud hum from the speaker. This worked so well because the input to a valve amplifier is high impedance. High impedance connections pick up interference much easier. Always try to make on esti­mate of how high the impedance of a node is and pay extra attention lo make sure that the connection is as short as possible and - if possible -run a parallel trace connected to ground as a shield (= keep­ing the area small*).

Rule 2 is therefore: Keep high-impedance connections shorter than low-impedance ones.

R6 ensures that the output of the circuit in the idle state is exactly zero. This prevents spu­rious noises from the loud* speaker when the amplifier is switched on or off.

R1 and R2 provide for the cor­rect DC bias of the non-invert­ing input of the opamp. This node is decoupled with capac­itor C and is therefore not that critical. After all, the capacitor creates a low-impedance con­nection for interference sig­nals, making the entire net­work less sensitive. The resis­tors may be placed a little further away from the opamp, provided the decoupling capacitor is placed as close to the opamp as possible.

R4 together with R5 determine the amplification. The input of the opamp is the node with the highest impedance, the output is much lower impedance and ground is low impedance as well. The input, therefore, must hove the shortest connection.

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