This paper discusses the similarities and differences between analog and digital wiring in the aspects of bypass capacitor, power supply, ground wire design, voltage error and electromagnetic interference (EMI) caused by PCB wiring.

The number of digital designers and digital circuit board design experts in the engineering field is increasing, which reflects the development trend of the industry. Although the emphasis on digital design has brought great development to electronic products, there are still some circuit designs that interface with analog or real environment. There are some similarities between the routing strategies in analog and digital fields, but in order to get better results, the simple circuit routing design is no longer the optimal solution because of its different routing strategies. This paper discusses the similarities and differences between analog and digital wiring in the aspects of bypass capacitor, power supply, ground wire design, voltage error and electromagnetic interference (EMI) caused by PCB wiring.

Similarities between analog and digital cabling strategies

Bypass or decoupling capacitor

When wiring, both analog devices and digital devices need these types of capacitors, and they need to connect a capacitor close to its power supply pin, which is usually 0.1uF. The power supply side of the system needs another kind of capacitor, which is usually about 10uF.

The locations of these capacitors are shown in Figure 1. The capacitance value ranges from 1 / 10 to 10 times of the recommended value. However, the pin should be short and close to the device (for 0.1uF capacitor) or power supply (for 10uF capacitor).

It is common sense for digital and analog design to add bypass or decoupling capacitors on the circuit board and the position of these capacitors on the board. But interestingly, the reasons are different. In analog wiring design, the bypass capacitor is usually used for the high-frequency signals on the bypass power supply. If the bypass capacitor is not added, these high-frequency signals may enter the sensitive analog chip through the power supply pin. Generally speaking, the frequency of these high-frequency signals exceeds the ability of analog devices to suppress high-frequency signals. If bypass capacitors are not used in analog circuits, noise may be introduced into the signal path, and even vibration may be caused in more serious cases.

Similarities and differences between analog and digital PCB wiring

Figure 1 in analog and digital PCB designs, the bypass or decoupling capacitor (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 entrance of the circuit board. In all cases, the pins of these capacitors should be short

Similarities and differences between analog and digital PCB wiring

Fig. 2 on this circuit board, different routes are used to lay power lines and ground wires. Due to this improper coordination, the electronic components and circuits of the circuit board are more likely to be affected by electromagnetic interference

Similarities and differences between analog and digital PCB wiring

Figure 3 in this single panel, the power and ground wires to the devices on the circuit board are close to each other. The matching ratio of power line and ground wire in this circuit board is appropriate in Figure 2. The possibility of electromagnetic interference (EMI) of electronic components and circuits in circuit board is reduced by 679 / 12.8 times or about 54 times

For digital devices such as controllers and processors, decoupling capacitors are also needed, but for different reasons. One function of these capacitors is to serve as a “mini” charge bank. In digital circuits, it usually takes a lot of current to switch the gate state. It is advantageous to have extra “standby” charge because of the transient current generated on the chip during switching and flowing through the circuit board. If there is not enough charge when switching, the power supply voltage will change greatly. If the voltage changes too much, the digital signal level will enter the uncertain state, and the state machine in the digital device may run incorrectly. The switching current flowing through the circuit board will cause the voltage to change, and there is parasitic inductance in the circuit board. The following formula can be used to calculate the voltage change: v = LDI / dt

Where V = voltage change; L = inductive reactance of circuit board wiring; Di = current change through wiring; DT = time of current change.

Therefore, for many reasons, it is better to apply a bypass (or decoupling) capacitor at the power supply or the power supply pin of the active device.

The power cord and ground wire should be laid together

The position of power line and ground wire is well matched, which can reduce the possibility of electromagnetic interference. If the power line and ground wire are not matched properly, the system loop will be designed, and noise may be generated. An example of PCB design with improper matching of power line and ground wire is shown in Figure 2.

On this circuit board, the designed loop area is 697cm2. Using the method shown in Fig. 3, the possibility of the radiated noise on or outside the circuit board inducing voltage in the loop can be greatly reduced.

The difference of cabling strategy between analog and digital domain

The ground level is a problem

The basic knowledge of circuit board wiring is applicable to both analog and digital circuits. A basic rule of thumb is to use uninterrupted ground plane. This common sense reduces the di / dt effect in digital circuits, which changes the ground potential and causes noise to enter analog circuits. The wiring skills of digital and analog circuits are basically the same, except for one point.

For analog circuits, there is another point to pay attention to, that is, the digital signal line and the loop in the ground plane should be as far away from the analog circuit as possible. This can be achieved by connecting the analog ground plane to the system ground connection, or placing the analog circuit at the farthest end of the circuit board, that is, the end of the circuit. This is to keep the signal path to the minimum of external interference. This is not necessary for digital circuits, which can tolerate a large amount of noise on the ground plane without problems.

Similarities and differences between analog and digital PCB wiring

Figure 4 (left) isolates the digital switch action from the analog circuit, separating the digital and analog parts of the circuit. (right) separate high frequency and low frequency as far as possible. High frequency components should be close to the connector of the circuit board

Similarities and differences between analog and digital PCB wiring

Fig. 5 parasitic capacitance can be easily formed when two adjacent wires are laid on PCB. Due to the existence of this capacitor, the rapid voltage change in one line can generate a current signal in another line

Similarities and differences between analog and digital PCB wiring

Fig. 6 if we don’t pay attention to the placement of wiring, the wiring in PCB may produce line inductance and mutual inductance. This kind of parasitic inductance is very harmful to the operation of circuits containing digital switching circuits

Location of components

As mentioned above, in each PCB design, the noise part and the “quiet” part (non noise part) of the circuit should be separated. Generally speaking, digital circuits are “rich” in noise and insensitive to noise (because digital circuits have large voltage noise tolerance); on the contrary, analog circuits have much smaller voltage noise tolerance. Among them, analog circuit is the most sensitive to switching noise. In the wiring of mixed signal system, the two circuits should be separated, as shown in Figure 4.

Parasitic components produced by PCB design

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, parasitic capacitance will be generated when two wires are placed close to each other. You can do this: in two different layers, place one routing line above the other; or in the same layer, place one routing line next to the other, as shown in Figure 5.

In these two routing configurations, the voltage variation with time (DV / DT) of one routing may generate current on the other. If the other line is high impedance, the current generated by the electric field will be converted into voltage.

Fast voltage transients most often occur in the digital side of analog signal design. If the fast voltage transient line is close to the high impedance analog line, this error will seriously affect the accuracy of the analog circuit. In this environment, analog circuit has two disadvantages: its noise tolerance is much lower than that of digital circuit; high impedance wiring is more common.

This phenomenon can be reduced by using one of the following two techniques. The most common technique is to change the size between the wires according to the capacitance equation. The most effective dimension to change is the distance between the two routing lines. It should be noted that the variable D is in the denominator of the capacitance equation. As D increases, the capacitive reactance decreases. Another variable that can be changed is the length of the two routes. In this case, when the length L is reduced, the capacitive reactance between the two wires will also be reduced.

Another technique is to lay a ground wire between the two wires. The ground wire has a low impedance, and adding such another wire will weaken the electric field that produces interference, as shown in Figure 5.

The principle of parasitic inductance in circuit board is similar to that of parasitic capacitance. It is also to lay two routing lines. In two different layers, place one routing line above the other; or in the same layer, place one routing line next to the other, as shown in Figure 6. In these two kinds of routing configurations, the current of one routing changes with time (di / DT). Due to the inductive reactance of this routing, the voltage will be generated on the same routing; and due to the existence of mutual inductance, the proportional current will be generated on the other routing. If the voltage variation on the first line is large enough, the interference may reduce the voltage tolerance of the digital circuit and cause errors. This phenomenon does not only occur in digital circuits, but it is more common in digital circuits because there is a large instantaneous switching current in digital circuits.

In order to eliminate the potential noise of EMI source, it is better to separate the “quiet” analog circuit from the noise I / O port. In order to realize low impedance power supply and ground network, the inductive reactance of digital circuit wires and the capacitive coupling of analog circuits should be reduced as much as possible.

epilogue

After the digital and analog ranges are determined, careful routing is critical to a successful PCB. The routing strategy is usually introduced as a rule of thumb, because it is difficult to test the final success of the product in the laboratory environment. Therefore, although there are similarities between the routing strategies of digital and analog circuits, we should recognize and take seriously the differences between them.

Source: sensor technology

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