Comparative Analysis of the Similarities and Differences between Analog and Digital Wiring Strategies

Bypass or decoupling capacitors

When routing, both analog and digital devices require these types of capacitors, and both need to connect a capacitor close to their power pins, which is usually 0.1mF. Another type of capacitor is required on the system power supply side, which is usually about 10mF.

The location of these capacitors is shown in Figure 1. The capacitance range is between 1/10 and 10 times the recommended value. However, the leads must be short and as close to the device (for a 0.1mF capacitor) or the power supply (for a 10mF capacitor) as possible.

Adding bypass or decoupling capacitors to a circuit board, and the placement of these capacitors on the board, is common sense for both digital and analog design. But interestingly, the reasons are different. In analog wiring design, bypass capacitors are often 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 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 analog circuits, noise may be introduced into the signal path, and in more serious cases, even vibration.

Figure 1 In analog and digital PCB designs, bypass or decoupling capacitors (1mF) should be placed as close to the device as possible. Power supply decoupling capacitors (10mF) should be placed where the power lines enter the board. In all cases, the leads of these capacitors should be short.

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

Figure 3 In this single-sided board, the power and ground lines to the components on the board are close to each other. The matching ratio of the power and ground lines in this board is appropriate as shown in Figure 2. The possibility of electromagnetic interference (EMI) of electronic components and lines in the 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 act as a "mini" charge reservoir. In digital circuits, large currents are often required to switch gate states. Since switching transients are generated on the chip and through the board, it is beneficial to have extra "spare" charge. If there is not enough charge to perform the switching action, it will cause a large change in the power supply voltage. If the voltage change is too large, the digital signal level will enter an undefined state and may cause the state machine in the digital device to operate incorrectly. The switching current flowing through the board trace will cause a voltage change. The board trace has parasitic inductance, which can be calculated as follows: V = LdI/dt

Where V = change in voltage; L = inductance of the board trace; dI = change in current through the trace; dt = time over which the current changes.

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

The power line and ground line should be laid together

A good match between the power and ground lines can reduce the possibility of electromagnetic interference. If the power and ground lines are not matched properly, a system loop will be designed and noise is likely to be generated. An example of a PCB design with improper power and ground line matching is shown in Figure 2.

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