It is easy to form parasitic capacitance by placing two close wires on the PCB. Because of this capacitance, a rapid voltage change on one line produces a current signal on the other
The digital switch action is separated from the analog circuit, and the digital and analog parts of the circuit are separated. (right) as far as possible to separate the high frequency from the low frequency, the high frequency components should be close to the circuit board connector. The number of digital designers and digital circuit board design experts in the field of engineering is increasing, which reflects the development trend of the industry. Although the emphasis on digital design has led to significant developments in electronic products, it still exists and will continue to exist in part in the design of circuits that interface with analog or real-world environments. There are some similarities between analog and digital cabling strategies, but for better results, simple circuit cabling is no longer optimal because of the different cabling strategies. This paper discusses the basic similarities and differences between analog and digital cabling in terms of bypass capacitance, power supply, ground design, voltage errors, and electromagnetic interference (EMI) caused by PCB cabling.
Similarities between analog and digital cabling strategies
Bypass or decoupling capacitance
In wiring, both simulator and digital devices require these types of capacitors, which are connected to a capacitor close to the power pin, usually of a value of 0.1uf. A different type of capacitance is required at the supply side of the system, which is usually about 10uF.
The positions of these capacitors are shown in figure 1. Capacitance ranges from 1/10 to 10 times the recommended value. But the pins must be short and as close as possible to the device (for 0.1uf capacitance) or to the power supply (for 10uF capacitance).
The addition of by-pass or decoupling capacitors to the circuit board, and the position of these capacitors on the board, are common sense to both digital and analog design. But interestingly, the reasons are different. In analog cabling design, bypass capacitance is often used to bypass the high frequency signals on the power supply, if the bypass capacitance is not added, these high frequency signals may enter the sensitive analog chip through the power pin. In general, the frequency of these high frequency signals exceeds the simulator’s ability to suppress high frequency signals. If bypass capacitors are not used in an analog circuit, it is possible to introduce noise in the signal path and, in more serious cases, vibration.
In analog and digital PCB designs, bypass or decoupling capacitors (0.1uF) should be placed as close to the device as possible. The power supply decoupling capacitor (10uF) shall be placed at the power line inlet of the circuit board. In all cases, these capacitance pins should be short
In this circuit board, the use of different routes to the power line and ground wire, due to this inappropriate fit, the circuit board’s electronic components and circuit by electromagnetic interference is more likely
In this single panel, the power and ground wires of the device on the circuit board are close to each other. The mix ratio of power and ground wires in this circuit board is shown in figure 2. Electronic components and circuits in circuit boards are 679/12.8 times or about 54 times less likely to suffer from electromagnetic interference (EMI)
For digital devices such as controllers and processors, decoupling capacitors are also required, but for different reasons. One function of these capacitors is to act as a “miniature” library of charges. In digital circuits, switching between gate states usually requires a large current. Since switching occurs when a switching transient current is generated on the chip and flows through the circuit board, it is advantageous to have an additional “standby” charge. If there is not enough charge for the switching operation, the power supply voltage will change greatly. Too large a voltage change can cause the digital signal level to enter an uncertain state and possibly cause an incorrect operation of the state machine in the digital device. The switching current flowing through the circuit board wiring will cause the voltage to change, and the circuit board wiring has parasitic inductance. The following formula can be used to calculate the voltage change: V = LdI/dt
Where, V = voltage change; L = circuit board routing reactance; DI = change of current flowing through the wire; Dt is equal to the change in current.
Therefore, it is a good practice to apply a by-pass (or decoupling) capacitance at the power supply or at the power pins of active devices for a variety of reasons.
The power and ground wires should be laid together
Good alignment of the power cord and ground wire reduces the possibility of emi. If the power and ground wires are mismatched, a system loop is designed and noise is likely to be generated. PCB design examples with improper matching of power line and ground wire are shown in figure 2.
On this circuit board, the designed loop area is 697cm2. By using the method shown in figure 3, the probability of the radiated noise on or outside the circuit board to induce voltage in the loop can be greatly reduced.
The difference between analog and digital cabling strategies
The ground plane is a problem
The basic knowledge of circuit board wiring applies to both analog and digital circuits. A basic rule of thumb is to use an uninterrupted ground plane. This knowledge reduces the dI/dt effect in digital circuits, which changes the ground potential and causes noise to enter the analog circuit. The wiring techniques of digital and analog circuits are basically the same, except for one thing. Another important thing to note about analog circuits is to keep the digital signal lines and circuits in the ground plane as far away from the analog circuit as possible. This can be achieved by either connecting the analog ground plane separately to the system ground connection or placing the analog circuit at the farthest end of the circuit board, the end of the line. This is done to keep external interference to the signal path to a minimum. This is not required for digital circuits, which can tolerate large amounts of noise in the ground plane without problems.
If the wiring is not carefully placed, the wiring in the PCB may produce line inductance and mutual inductance. This parasitic inductance is very harmful to the operation of circuits containing digital switching circuits
As mentioned above, in each PCB design, the noisy part of the circuit is separated from the “quiet” (non-noisy) part. In general, digital circuits are “noise-rich” and insensitive to noise (because digital circuits have a large voltage noise tolerance); In contrast, analog circuits have a much smaller voltage noise tolerance. Of the two, analog circuits are most sensitive to switching noise. In the wiring of a mixed signal system, the two circuits are separated, as shown in figure 4.
PCB design produces parasitic components
In PCB design, it is easy to form two basic parasitic components that may cause problems: parasitic capacitance and parasitic inductance. When designing a circuit board, placing two wires close to each other creates parasitic capacitance. Here’s how to do it: on two different levels, place one cable above the other. Or on the same floor, place one cable next to another cable, as shown in figure 5. In both configurations, a change in voltage over time (dV/dt) on one line may generate a current on the other line. If the other wire is of high impedance, the current generated by the electric field will be converted into voltage.
Fast voltage transients most often occur on the digital side of analog signal design. If the fast voltage transient routing is close to the high impedance simulation routing, this error will seriously affect the accuracy of the simulation circuit. In this environment, analog circuits have two disadvantages: their noise tolerance is much lower than that of digital circuits; High impedance wiring is common.
This can be reduced by using one of two techniques. The most common technique is to change the size between the lines according to the equation of the capacitance. The most effective size to change is the distance between the lines. It should be noted that the variable d in the denominator of the capacitance equation will decrease as d increases. The other variable that can be changed is the length of the two wires. In this case, as the length L decreases, the reactance between the two wires will also decrease.
Another technique is to ground the wires between the two wires. The ground wire is low impedance, and the addition of such another wire would weaken the electric field producing the interference, as shown in figure 5.
The principle of parasitic inductance in circuit board is similar to that of parasitic capacitance. Also is the cloth two lines, in different two layers, put one line on top of the other line; Or on the same level, place one wire next to the other, as shown in figure 6. In the two wiring configurations, the change of the current on one line with time (dI/dt), due to the inductance of this line, will generate voltage on the same line; And due to the presence of mutual inductance, a proportional current will be generated on the other line. If the voltage change on the first line is large enough, the interference may reduce the voltage tolerance of the digital circuit and produce errors. This phenomenon is not unique to digital circuits, but it is more common in digital circuits where there is a large transient switching current.
To eliminate potential noise from electromagnetic sources, it is best to separate “quiet” analog lines from noisy I/O ports. In order to realize low impedance power supply and ground network, the inductive reactance of digital circuit wires should be minimized and the capacitance coupling of analog circuit should be minimized.
After the digital and analog ranges are determined, careful wiring is critical to a successful PCB. Wiring strategies are often introduced as a rule of thumb because it is difficult to test the ultimate success of a product in a laboratory environment. Therefore, while the wiring strategies of digital and analog circuits are similar, it is important to recognize and take seriously the differences in the wiring strategies.