Before you start worrying about the Ground System, all the components should be already placed on the grid to start the PCB layout.
Figure 1: Components Placed on the Grid
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For better understanding, Figure 1 shows the components placed on the grid without traces, a ground system, or a power system. The objective is to place the components as closely as possible to eliminate long traces, and thus reduce electromagnetic noise and interference. Eliminating long traces minimizes internal self inductance, which as already mentioned, is noise. Figure 1 shows one section of the circuitry that will be laid out for the manufacturing purposes for a particular system. Once the components have been laid out, the ground system and power system must be taken into consideration. The power supply trace should run parallel to the ground trace in order to maximize the reduction of electromagnetic noise and interference.
Figure 2: Components with Ground and Power Trace
The above screen shot, Figure 2, shows the power trace (red trace) and the ground trace (green trace). Like I already mentioned, the power and ground traces should run parallel and close together to prevent noise. In my case, I placed the power trace on the top layer (thus red in color) and placed the ground trace on the bottom layer (green denoting the bottom layer).
There are additional measures that can be taken to reduce the noise generated due to the power system and the ground system in a PCB Assembly. One common way to eliminate the noise that is produced due to the ground and power supply traces is to simply use a decoupling capacitor. Basically, all you have to do is place a capacitor between ground and the power supply. It is also important that the capacitor used does not have high impedance. Another important thing to take into account are the pins of the ICs that are unused. It is important to connect the unused pins to a voltage supply with a series resistor, or for a cheaper way (not needing a resistor), just connect the unused input to ground. This will prevent a noise signal to couple to one of the unused inputs and cause a gate to switch when it is not desired. (2)
V. EMI: Placing of Components
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We have discussed some of the techniques that I used to prevent electromagnetic interference (EMI) and noise concerning the ground and power systems when designing a PCB. Additionally, there it is important to take into consideration the noise and EMI from the connections between some components. The connection between oscillators and ICs that they serve should be as short as possible. (2) Also, “to minimize loop area, signal and return paths should be parallel and close together.” (2) The following screen shot is an example of my design taking into account these important notes.
Figure 3: Oscillator and PIC (IC) Connection
In Figure 3, the connections between the PIC and the Oscillator have been minimized and the return paths are close together and in parallel, therefore minimizing the noise in this case. In this example of component placement, mutual and external self inductances have been minimized (to refresh your memory on what these inductances are, please see the section on electromagnetic noise and interference). These inductances were reduced by minimizing the loop area enclosed by current flow. Another technique that should be considered is to make the address, data, and I/O connections as short as possible, with signal and return paths as close as possible, which will again minimize loop area and in turn, minimize mutual and external self inductances. (2) Lastly, the fastest components should be placed on the PCB at a location that is farthest away from external connections as possible. (2) Interestingly, by doing this, the natural loss on the board material will attenuate the noise signals; I find it ironic that this natural loss, in terms of electromagnetic noise and interference, can actually be advantageous.
VI. Conclusion
Electromagnetic interference and noise is an important issue when creating a printed circuit board (PCB). Printed circuit boards are not cheap, so it is imperative that the PCB you design is designed to maximize its capability. To do this, EMI and noise need to be taken into consideration. This document addressed the issues of EMI and noise and in correspondence to PCB development and inductances. In summary, it is
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important to carefully design a PCB having in mind EMI in the following areas: the ground system, the power system, and the placement of components (with respect to each other). I recommend that EMI and noise design techniques be taken into consideration at the start of the PCB development. I learned a lot about EMI after already laying out a lot of my circuitry in Express PCB. To save time, money (so you don’t have to spend additional money, throwing out a PCB that doesn’t work properly because of EMI reasons), and to simply maximize a design, take EMI and noise into consideration while laying out your electronic assembly in the software designing stage.
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