Brief description of PCB layout

Brief description of PCB layout

The use of an auto-router to design a printed circuit board (PCB) is appealing. In most cases, automatic routing does not pose a problem for purely digital circuits (especially low-frequency signals and low-density circuits). However, when trying to use the automatic routing tools provided by the cabling software to perform simulation, mixed-signal or high-speed circuit wiring, some problems may occur, and it may cause extremely serious circuit performance problems.

There are many considerations regarding wiring, but the more problematic issue is grounding. If the grounding path starts from the upper layer, the grounding of each device is connected to the ground via a pull wire on the layer. For each device in the lower layer, a ground loop is formed by connecting the through-hole on the right side of the circuit board to the upper layer.

The immediate red flag that the user sees when checking the wiring pattern indicates that there are multiple ground loops. In addition, the underlying ground loop is blocked by a horizontal signal line. The merit of this grounding structure is that the analog device (MCP3202; 12-bit AD converter and MCP4125; 2.5V reference voltage) is centered on the right side of the board. This arrangement ensures that digital ground signals will not pass under these analog chips.

With manual wiring, follow the following design guidelines to ensure good results:

• Design ground as a ground plane as a current return path.
• Separate the analog ground plane from the digital ground plane.
• If it is unavoidable that the signal traces are placed on the same layer as the ground plane, the signal line and the ground line should be designed to be perpendicular to each other to reduce the interference of the signal line with the ground current loop.
• Place the analog circuit next to the circuit board and place the digital circuit system closest to the power supply. The effect of digital switching δi/δt on analog circuits can be reduced.

However, it should be noted that these two dual-layer boards have a ground plane on the lower layer of the circuit board. This is designed to allow engineers to quickly see the wiring when troubleshooting, and this method often appears on the device manufacturer's demonstration and evaluation board. However, a more typical approach is to place a ground plane on top of the board to reduce electromagnetic interference (EMI).

The presence or absence of the current return path of the ground plane

The basic issues that should be considered when dealing with current return paths are:

• (1) If you use only the local cable, try to widen the cable; if you consider only using the cable as the ground cable of the circuit board, the cable should be as wide as possible. Thumb height is a good standard, but it must also be understood that the minimum width of the grounding wire refers to the effective copper foil width from the point to the end of the wire. The definition of “end” here refers to the point farthest from the power connection.
• (2) Avoid forming a closed ground loop.
• (3) If there is no ground plane, star connection can be used.

If the ground plane cannot be designed, the current return path can be handled with a "star" layout

In this way, the ground current of each device is returned individually to the power supply. Users will find that not all devices in Figure 5 have their own return path. U1 and U2 share the return path, and the prerequisite for doing so is to meet the following points of attention for the design.

Do not pass digital circuits through analog devices.

The digital circuit creates a considerable current on the ground loop during switching but its time is short. This phenomenon is caused by the equivalent inductance and resistance of the ground loop.

The inductance of the ground plane or ground will generate a voltage drop of V = Lδi/δt, where L is the equivalent inductance of the ground plane or ground, δi is the change in current from the digital device and δt is the time of the current change.

The voltage change caused by calculating the equivalent resistance part of the ground plane or the ground is V = RI, R is the equivalent resistance of the ground plane or ground, I is the current change of the digital device. This change in the voltage of the ground plane or ground will affect the normal signal between the analog device input and the ground.

Do not pass high-speed current through the low speed device

The change of the ground return signal of the high-speed circuit on the ground plane has similar effects as described above. The formula for determining this interference effect is: V=L[delta]i/[delta]t for ground plane or ground equivalent inductance, and for ground plane or ground line.

The equivalent resistance is V=RI. When the ground plane or ground line of the digital circuit or high-speed circuit passes through the pull wire of the analog device, the signal between the input terminal of the analog device and the ground is changed. Regardless of the technology used, it must be designed to minimize the equivalent resistance and inductance of the ground return path. If a ground plane is used, cutting off the ground plane may increase or decrease the performance of the circuit and must be used with caution.

Sometimes the effect of the continuous ground plane is worse than the separated ground plane. The grounding wiring shown in (a) in this figure is worse than that shown in (b).

The exact analog is closer to the connector, but it is isolated from the digital circuitry and the switching current from the power supply circuit. This is a way to effectively separate the ground return path. This technique is also used in the wiring discussed earlier in Figure 3 and Figure 4.

In conclusion

When discussing the technology related to wiring, two kinds of problems will be discussed: First, what if the management class cannot use the double-layer board or the ground plane, but still need to reduce the noise in the circuit? And how do you design a circuit that meets the ground plane requirements?

In general, the solution is to inform the management hierarchy that a ground plane is necessary if reliable circuit performance is to be achieved.

The main reason for using ground planes is low ground impedance and can reduce a certain degree of EMI. However, if the user is unable to meet the requirements due to cost constraints, some of the suggestions provided in this paper, such as the star network and the correct current return path, can also slightly reduce circuit noise.