What are the similarities and differences between analog and digital wiring?
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 about major developments in electronic products, it still exists, and there will always be some circuit designs that interface with analog or real environments. Wiring strategies in the analog and digital fields have some similarities, but when getting better results, because of their different wiring strategies, simple circuit wiring design is no longer the optimal solution. 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.
Similarities between analog and digital wiring strategies
Bypass or decoupling capacitor
When wiring, both analog devices and digital devices need these types of capacitors, and both need to connect a capacitor close to its power supply pin. This capacitor value is usually 0.1mF. Another type of capacitor is required on the power supply side of the system, usually the value of this capacitor is about 10mF.
The location of these capacitors is shown in Figure 1. The capacitance range is between 1/10 to 10 times the recommended value. But the pin must be short and as close as possible to the device (for 0.1mF capacitor) or power supply (for 10mF capacitor).
Adding bypass or decoupling capacitors on the circuit board and the location of these capacitors on the board are common sense for digital and analog designs. But interestingly, the reasons are different. In analog wiring design, bypass capacitors are usually used to bypass high-frequency signals on the 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 analog devices to suppress high-frequency signals. If bypass capacitors are not used in the analog circuit, noise may be introduced in the signal path, and more serious cases may even cause vibration.
figure 1
In analog and digital PCB design, bypass or decoupling capacitors (1mF) should be placed as close to the device as possible. The power supply decoupling capacitor (10mF) should be placed at the power line entrance of the circuit board. In all cases, the pins of these capacitors should be short.
figure 2
On this circuit board, different routes are used to route the power and ground wires. Because of this improper coordination, the electronic components and circuits on the circuit board are more likely to suffer electromagnetic interference.
image 3
In this single panel, the power and ground wires to the components on the circuit board are close to each other. The matching ratio of the power line and the ground line in this circuit board is appropriate as shown in Figure 2. The probability of electronic components and circuits in the circuit board being subjected to electromagnetic interference (EMI) is reduced by 679/12.8 times or about 54 times.
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" charge bank. In digital circuits, a large amount of current is usually required to perform gate state switching. Because switching transient currents are generated on the chip during switching and flow through the circuit board, it is advantageous to have additional "standby" charges. If there is not enough charge when the switching action is performed, the power supply voltage will change greatly. Too much voltage change will cause the digital signal level to enter an uncertain state, and may cause the state machine in the digital device to operate incorrectly. The switching current flowing through the circuit board trace will cause the voltage to change, and the circuit board trace has parasitic inductance. The voltage change can be calculated by the following formula: V = LdI/dt
Among them, V = voltage change; L = circuit board trace inductance; dI = current change through the trace; dt = current change time.
Therefore, for many reasons, it is better to apply bypass (or decoupling) capacitors at the power supply or at the power supply pins of active devices.
The power cord and ground wire should be routed together
The position of the power cord and the ground wire are well matched to reduce the possibility of electromagnetic interference. If the power line and the ground line are not properly matched, a system loop will be designed and noise will likely be generated. An example of a PCB design where the power line and ground line are not properly matched is shown in Figure 2 above.
On this circuit board, the designed loop area is 697c㎡. Using the method shown in Figure 3 above, the possibility of radiated noise on or off the circuit board inducing voltage in the loop can be greatly reduced.
The difference between analog and digital wiring strategies
Ground plane 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 an uninterrupted ground plane. This common sense reduces the dI/dt (current change 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, there is another point to note, that is, keep the digital signal lines and loops in the ground plane as far away from the analog circuits as possible. This can be achieved by connecting the analog ground plane to the system ground connection separately, or placing the analog circuit at the farthest end of the circuit board, which is the end of the line. This is done to keep the external interference on the signal path to a minimum. There is no need to do this for digital circuits, which can tolerate a lot of noise on the ground plane without problems.
Figure 4
(Left) Isolate the digital switch action from the analog circuit, and separate the digital and analog parts of the circuit. (Right) The high frequency and low frequency should be separated as much as possible, and the high frequency components should be close to the connectors of the circuit board.
Figure 5
Placing two close traces on the PCB can easily form parasitic capacitance. Due to the existence of this kind of capacitance, a rapid voltage change on one trace can generate a current signal on the other trace.
Figure 6
If you do not pay attention to the placement of the traces, the traces in the PCB may produce line inductance and mutual inductance. This parasitic inductance is very harmful to the operation of circuits including digital switching circuits.
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