The Difference between Analog And Digital Circuits in PCB Design

The difference between analog and digital circuits in PCB design

The number of digital designers and digital circuit board design experts in the engineering field is constantly increasing, which reflects the development trend of the industry. Although the emphasis on digital design has brought significant development to electronic products, there still exists, and will continue to be, some circuit designs that interface with analog or real-world environments. The wiring strategies in the analog and digital fields have some similarities, but to achieve better results, simple circuit wiring design is no longer the optimal solution due to their different wiring strategies.

This article discusses the basic similarities and differences between analog and digital wiring in terms of bypass capacitors, power supply, ground wire design, voltage errors, and electromagnetic interference (EMI) caused by PCB wiring.

01 Similarities between analog and digital cabling strategies

Bypass or decoupling capacitor

When wiring, both analog devices and digital devices require these types of capacitors, which need to be connected to a capacitor near its power pin. This capacitance value is usually 0.1uF. The power supply side of the system requires another type of capacitor, usually with a value of approximately 10uF.

The positions of these capacitors are shown in Figure 1. The capacitance range is between 1/10 and 10 times the recommended value. But the pins must be short and as close as possible to the device (for 0.1uF capacitors) or power supply (for 10uF capacitors).

Adding bypass or decoupling capacitors on a circuit board, as well as the location of these capacitors on the board, is common knowledge for both digital and analog designs. But interestingly, the reasons are different.

In analog wiring design, bypass capacitors are usually used for high-frequency signals on the bypass power supply. If bypass capacitors are not added, these high-frequency signals may enter sensitive analog chips through the power supply pins. Generally speaking, the frequency of these high-frequency signals exceeds the ability of the simulator to suppress high-frequency signals. If bypass capacitors are not used in analog circuits, noise may be introduced in the signal path, and in more severe cases, it may even cause vibration.

In analog and digital PCB design, bypass or decoupling capacitors (0.1uF) should be placed as close as possible to the device. The decoupling capacitor (10uF) of the power supply should be placed at the power line inlet of the circuit board. In all cases, the pins of these capacitors should be relatively short.

On the circuit board shown in Figure 2, different routes are used to route the power and ground wires. Due to this inappropriate combination, the electronic components and circuits on the circuit board are more likely to be affected by electromagnetic interference.

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

For digital devices such as controllers and processors, decoupling capacitors are also required, but the reasons are different. One function of these capacitors is to serve as a "miniature" charge bank.

In digital circuits, switching gate states typically requires a significant amount of current. Due to the transient current generated on the chip during switching and flowing through the circuit board, it is advantageous to have additional "backup" charges. If there is not enough charge during the switch action, it will cause a significant change in the power supply voltage. If the voltage changes too much, it can cause the digital signal level to enter an uncertain state, and it is likely to cause the state machine in the digital device to operate incorrectly.

The switch current flowing through the circuit board wiring will cause a change in voltage, and there is parasitic inductance in the circuit board wiring. The voltage change can be calculated using the following formula: V=LdI/dt. Among them: V=change in voltage, L=impedance of circuit board wiring, dI=change in current flowing through the wiring, dt=time of current change.

Therefore, for various reasons, it is a good practice to apply bypass (or decoupling) capacitors at the power supply or power pins of active devices.

The power cord and ground wire should be laid together

Good coordination between the power and ground wires can reduce the possibility of electromagnetic interference. If the power cord and ground wire are not properly matched, a system loop will be designed and it is likely to produce noise.

An example of PCB design with improper matching of power and ground wires is shown in Figure 2. On this circuit board, the designed loop area is 697cm ². By using the method shown in Figure 3, the likelihood of radiated noise on or outside the circuit board inducing voltage in the loop can be greatly reduced.

02 Differences in wiring strategies between analog and digital domains

Ground level is a challenge

The basic knowledge of circuit board wiring is applicable to both analog and digital circuits. A basic rule of thumb is to use an uninterrupted ground plane, which reduces the dI/dt (current over time) effect in digital circuits, which changes the ground potential and causes noise to enter analog circuits.

The wiring techniques for digital and analog circuits are basically the same, with one exception. For analog circuits, another point to note is to keep the digital signal line and the loop in the ground plane as far away from the analog circuit as possible. This can be achieved by connecting the analog ground plane separately to the system ground connection end, or by placing the analog circuit at the farthest end of the circuit board, which is the end of the circuit. This is done to minimize external interference on the signal path.

For digital circuits, this is not necessary. Digital circuits can tolerate a large amount of noise on the ground plane without any problems.

Figure 4 (left) isolates the digital switch action from the analog circuit, separating the digital and analog parts of the circuit. (Right) Try to separate high and low frequencies as much as possible, and the high-frequency components should be close to the connectors on the circuit board.   

Figure 5 shows two adjacent wires on the PCB, which can easily form parasitic capacitance. Due to the presence of this capacitor, rapid voltage changes on one line can generate current signals on another line.

If the placement of the wiring in Figure 6 is not taken seriously, 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.

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 are not sensitive to noise (because digital circuits have a large voltage noise tolerance); On the contrary, the voltage noise tolerance of analog circuits is much smaller.

Among the two, analog circuits are the most sensitive to switch noise. In the wiring of mixed signal systems, these two types of circuits need to be separated, as shown in Figure 4.

Parasitic components generated by PCB design

It is easy to form two basic parasitic components that may cause problems in PCB design: parasitic capacitance and parasitic inductance.

When designing a circuit board, placing two wires close to each other will generate parasitic capacitance. You can do this: place one routing line above the other on different layers; Alternatively, on the same layer, place one routing line next to another routing line, as shown in Figure 5.

In these two wiring configurations, the variation of voltage over time (dV/dt) on one wiring may generate current on the other wiring. If the other line has high impedance, the current generated by the electric field will be converted into voltage.

Fast voltage transients most commonly occur on the digital side of analog signal design. If a fast voltage transient occurs in the wiring near a high impedance analog wiring, this error will seriously affect the accuracy of the analog circuit. In this environment, analog circuits have two disadvantages: their noise tolerance is much lower than that of digital circuits; High impedance wiring is quite common.

Using one of the following two techniques can reduce this phenomenon. The most commonly used technique is to change the size between the wires based on the capacitance equation. The most effective size to change is the distance between two routing lines. It should be noted that the variable d in the denominator of the capacitance equation will decrease as d increases. Another variable that can be changed is the length of the two wires. In this case, as the length L decreases, the capacitance between the two wires will also decrease.

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

The principle of parasitic inductance in circuit boards is similar to the principle of parasitic capacitance formation. It is also to lay two wires, placing one wire above the other on different layers; Alternatively, on the same layer, place one routing line next to another, as shown in Figure 6.

In these two wiring configurations, the variation of current over time (dI/dt) on one wiring will generate voltage on the same wiring due to the inductance of this wiring; And due to the presence of mutual inductance, a proportional current will be generated on another routing line. If the voltage change on the first line is significant enough, 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.

To eliminate potential noise from electromagnetic interference sources, it is best to separate quiet analog circuits from noisy I/O ports. To achieve a low impedance power and ground network, efforts should be made to minimize the inductance of digital circuit wires and minimize the capacitance coupling of analog circuits.

03 Conclusion

After determining the digital and analog range, careful wiring is crucial for achieving a successful PCB. Cabling strategies are usually introduced as empirical guidelines because it is difficult to test the final success of a product in a laboratory environment. Therefore, although there are similarities in the wiring strategies of digital and analog circuits, it is still important to recognize and take seriously the differences in their wiring strategies.