Comparative Analysis of Similarities and Differences between Analog and Digital Routing Strategies

Analog and digital wiring similarities

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.

Differences in routing strategies between analog and digital domains

The ground plane is a problem

The basics of circuit board layout apply to both analog and digital circuits. A basic rule of thumb is to use an uninterrupted ground plane, which reduces the dI/dt (current change over time) effect in digital circuits, which changes the ground potential and allows noise to enter the analog circuits. The layout techniques for digital and analog circuits are basically the same, with one exception. For analog circuits, another point to pay attention to is to keep digital signal lines and loops 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 or placing the analog circuit at the farthest end of the board, that is, the end of the line. This is done to keep the signal path to the minimum external interference. This is not necessary for digital circuits, which can tolerate a lot of noise on the ground plane without problems.

Figure 4 (Left) Isolate the digital switching action from the analog circuit and separate the digital and analog parts of the circuit. (Right) Separate high and low frequencies as much as possible, and keep high-frequency components close to the connectors on the circuit board.

Figure 5 When two traces are placed close together on a PCB, parasitic capacitance is easily formed. Due to the presence of this capacitance, a rapid voltage change on one trace can generate a current signal on the other trace.

Figure 6 If you don’t pay attention to the placement of traces, traces in the PCB may generate line inductance and mutual inductance. This parasitic inductance is very detrimental to the operation of circuits containing digital switching circuits.

Component location

As mentioned above, in every PCB design, the noisy 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 larger voltage noise tolerance); on the contrary, the voltage noise tolerance of analog circuits is much smaller. Of the two, analog circuits are the most sensitive to switching noise. In the wiring of mixed-signal systems, these two circuits should be separated.