PCB News - PCB design differences between analog circuits and digital circuits

Let's share the similarities between PCB design routing strategies for Analog circuits and digital circuits.

1. Bypass or decoupling capacitor

For wiring, simulators, and digital devices, these types of capacitors are required, and each capacitor requires a capacitor connected near its power supply pin. This value is typically 0.1 degrees Fahrenheit. The power side of a system requires another type of capacitor, typically around 10 degrees Fahrenheit. The range of capacitance values is 1/10 to 10 times the recommended value. However, the pins should be short and close to the device (for 0.1 degrees Fahrenheit capacitors) or the power supply (for 10 degrees Fahrenheit capacitors). Adding bypass or decoupling capacitors to a circuit board and placing these capacitors on the circuit board are common sense in both digital and analog design. But interestingly, the reasons vary.

In analog wiring designs, bypass capacitors are typically used to bypass high-frequency signals on power supplies. If bypass capacitors are not added, these signals may enter sensitive analog chips through power supply pins. Typically, the frequency of these high-frequency signals exceeds the simulator's ability to suppress high-frequency signals. If bypass capacitors are not used in analog circuits, noise may be introduced into the signal path and, in worse cases, may cause vibration.

Decoupling capacitors are also required for digital devices such as controllers and processors, but the reasons are different. One function of these capacitors is to act as a "mini" charging bank. In digital circuits, large currents are usually required to switch gate states. Since switching transient currents are generated on the chip, it is advantageous to have an additional "standby" charge when it switches and flows through the circuit board. If the switching action is performed without sufficient charge, the power supply voltage will vary significantly.

If the voltage changes too much, the digital signal level will enter an uncertain state, and the state machine in the digital device may operate incorrectly. Switching current flowing through circuit board wiring can cause voltage changes, and parasitic inductance exists in circuit board wiring. The following formula can be used to calculate the voltage change: V=LdI/dt where five=voltage change; I=circuit board wiring reactance; Di=change in current flowing through the line; Depth is the time that the current changes.

Therefore, it is preferable to apply a bypass (or decoupling) capacitor at the power supply pin of a power supply or active device for various reasons. Power and ground cables should be placed together to reduce the possibility of electromagnetic interference. If the power and ground wires do not match correctly, a system loop will be designed, which is likely to generate noise. On this circuit board, the loop area is 697 square centimeters using Figure 3 With the method shown, radiated noise on or outside the circuit board is unlikely to induce voltage in the circuit.

Differences in routing strategies between analog and digital domains

The basic principles of circuit board wiring apply to analog and digital circuits. A basic rule of thumb is to use a complete ground plane. This common sense reduces the effect of data logging/data transmission (current variation over time) in digital circuits, which can change ground potential and cause noise to enter analog circuits. The wiring technology for digital and analog circuits is basically the same, with only a slight difference. For analog circuits, it is important to keep the loop in the digital signal line and ground plane as far away from the analog circuit as possible. This can be achieved by connecting the analog ground plane individually to the system ground connection, or by placing the analog circuit at the farthest end of the circuit board, that is, at the end of the line. This is done to keep the signal path to a minimum of external interference. This is unnecessary for digital circuits, which can tolerate a large amount of noise in the ground plane without any problem.

As mentioned above, in each PCB design, the noise portion of the circuit is separated from the "quiet" (non noise) portion. Generally speaking, digital circuits are "rich" in noise and insensitive to noise (because digital circuits have a large voltage noise tolerance); On the other hand, analog circuits have a much lower voltage noise tolerance. Of the two, analog circuits are the most sensitive to switching noise. In the wiring of a mixed signal system, the two circuits are separate.

Analog circuit

2. Parasitic components generated by PCB design

There are two basic parasitic components that can easily cause problems in PCB design: parasitic capacitance and parasitic inductance. When designing a circuit board, placing two wires close together can create parasitic capacitance. This can be achieved by placing a line on top of another line on two different floors, or by placing a line next to another line on the same floor. In both wiring configurations, the change in voltage over time (dV/dt) on one wire can generate a current on the other wire. If the other line is a high impedance line, 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 a fast voltage transient occurs in the vicinity of a high impedance analog circuit, this error will seriously affect the accuracy of the analog circuit.

Analog circuits have two disadvantages in this environment: 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 technologies. The most common technology is to change the size of wires based on capacitance equations. The most effective size to change is the distance between two lines.

It should be noted that the variable D in the denominator of the capacitance equation decreases with the addition of D. Another variable that can be changed is the length of the two lines. In this case, as the length L decreases, the capacitive reactance between the two lines also decreases. Another technology is to lay a ground wire between two lines. The ground wire is low impedance, and adding such additional wires will weaken

3. Electric field generating interference

The principle of parasitic inductance in a circuit board is similar to that of parasitic capacitance. Also arrange two lines, with one line placed on the other in two different layers; Or place one line next to another on the same layer, as shown in Figure 6 Is shown in. In these two wiring configurations, due to the inductive reactance of the wiring, the change in current over time (dI/dt) of one wiring will generate a voltage on the same wiring; Due to mutual inductance, the other line will generate a proportional current.

If the voltage variation on the first line is large enough, interference can reduce the voltage tolerance of digital circuits and generate errors. This phenomenon is not unique to digital circuits, but is more common in digital circuits with large instantaneous switching currents. To eliminate potential noise from electromagnetic interference sources, it is best to separate "quiet" analog lines from noisy input/output ports.

In order to achieve low impedance power and ground networks, the inductive reactance of digital circuit conductors should be minimized, and the capacitive coupling of analog circuits should be minimized.

Once the digital and analog ranges have been determined, careful wiring is crucial to achieving a PCB. Cabling strategies are often seen as a rule of thumb because it is difficult to test the ultimate success of a product in a laboratory environment. Therefore, although there are similarities in routing strategies between digital circuits and analog circuits, these differences should be recognized and taken seriously.