PCB Blog - The difference between analog circuit and digital circuit PCB board design

This article discusses the basic similarities and differences between analog and digital routing in terms of bypass capacitors, power supply, ground design, voltage errors, and electromagnetic interference (EMI) caused by PCB board routing. The increasing number of digital designers and digital circuit board designs in the engineering field reflects the trends in the industry. Although the emphasis on digital design has brought about major developments in electronics, there is, and will always be, a portion of circuit design that interfaces with analog or real-world environments. There are some similarities between routing strategies in the analog and digital domains, but when it comes to better results, simple circuit routing design is no longer the solution due to their different routing strategies.

1. Similarities of Analog and Digital Routing Strategies

1.1 Bypass or decoupling capacitors
Both analog and digital devices require these types of capacitors when wiring, and both require a capacitor close to its power pin, typically 0.1uF. Another type of capacitor is required on the power supply side of the system, usually the value of this capacitor is about 10uF. The capacitor value range is between 1/10 and 10 times the recommended value. However, the pins must be short and as close as possible to the device (for 0.1uF capacitors) or the power supply (for 10uF capacitors). Adding bypass or decoupling capacitors on the board, and the placement of those capacitors on the board, is common sense for both 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 no bypass capacitors are added, these high-frequency signals may enter sensitive analog chips through the power supply pins. Generally, the frequencies of these high-frequency signals exceed the ability of the analog device to reject high-frequency signals. If bypass capacitors are not used in analog circuits, it can introduce noise and, in more severe cases, vibrations in the signal path.

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, switching gate states usually requires a large amount of current. Having an extra "spare" charge is beneficial due to switching transients on the chip and through the board when switching. If there is not enough charge to perform the switching action, it will cause a large change in the supply voltage. Voltage variations that are too large can cause digital signal levels to go into indeterminate states and likely cause state machines in digital devices to behave incorrectly. The switching current flowing through the circuit board traces will cause the voltage to change. There is parasitic inductance in the circuit board traces. The following formula can be used to calculate the voltage change: V = LdI/dt. where V = change in voltage; L = inductive reactance of board traces; dI = change in current flowing through the trace; dt = time for current to change. Therefore, it is good practice to apply bypass (or decoupling) capacitors at the power supply or at the power supply pins of active devices for a number of reasons. The power cable and the ground cable should be laid together. The position of the power cable and the ground cable is well matched, which can reduce the possibility of electromagnetic interference. If the power and ground wires are not properly matched, loops in the system are designed and noise is likely to be generated. On this board, the designed loop area is 697cm2. Radiated noise on or off the circuit board is much less likely to induce voltages in the loop. Differences in routing strategies in the analog and digital domains.

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. This common sense reduces the dI/dt (current versus time) effect in digital circuits, which can change the potential of ground and introduce noise into analog circuits. Wiring techniques for digital and analog circuits are basically the same, with one exception. Another point to note with analog circuits is to keep digital signal lines and loops in the ground plane as far away as possible from analog circuits. This can be accomplished by connecting the analog ground plane alone to the system ground connection, or by placing the analog circuitry on the far end of the board, at the end of the line. This is done to keep the signal path free from external interference. This is not necessary for digital circuits, which can tolerate a lot of noise on the ground plane without problems.

1.2 Location of components
As mentioned above, in every PCB design, the noisy and "quiet" (non-noisy) parts of the circuit are separated. In general, digital circuits are "rich" in noise and insensitive to noise (because digital circuits have a larger voltage noise margin); in contrast, analog circuits have a much smaller voltage noise margin. Of the two, analog circuits are sensitive to switching noise.

In the wiring of mixed-signal systems, these two circuits are separated. Two basic parasitic components that can cause problems are easily formed in PCB board design: parasitic capacitance and parasitic inductance. When designing a board, placing two traces close to each other creates parasitic capacitance. This can be done: on two different layers, place one trace above the other; or on the same layer, place one trace next to the other. In both trace configurations, a change in voltage over time (dV/dt) on one trace may generate current on the other trace. If the other trace is high impedance, the current created by the electric field will be converted to voltage. Fast voltage transients often occur on the digital side of analog signal designs. This error can seriously affect the accuracy of analog circuits if traces with fast voltage transients are placed close to high-impedance analog traces. In this environment, analog circuits have two disadvantages: their noise margin is much lower than that of digital circuits; high-impedance traces are more common. This phenomenon can be reduced using one of the two techniques described below. A common technique is to vary the size between traces according to the equation for capacitance. The effective dimension to change is the distance between the two traces. Note that the variable d is in the denominator of the capacitance equation, and as d increases, the capacitive reactance decreases. Another variable that can be changed is the length of the two traces. In this case, the length L is reduced and the capacitive reactance between the two traces is also reduced.

Another technique is to route a ground trace between these two traces. The ground wire is low impedance, and adding another trace like this will attenuate the interfering electric field. The principle of parasitic inductance in the circuit board is similar to that of parasitic capacitance. It is also to lay two traces. On two different layers, place one trace on top of the other trace; or on the same layer, place one trace next to the other, as shown in Figure 6. In these two trace configurations, the change of current on one trace with time (dI/dt) will generate voltage on the same trace due to the inductive reactance of this trace; and due to the existence of mutual inductance, it will A proportional current is drawn on the other trace. If the voltage variation on a trace is large enough, the interference can reduce the voltage tolerance of the digital circuit and cause errors. This phenomenon is not unique to digital circuits, but it is common in digital circuits where large transient switching currents exist. To eliminate potential noise from EMI sources, separate "quiet" analog lines from noisy I/O ports. To try to achieve a low impedance power and ground network, the inductive reactance of the digital circuit wires should be minimized, and the capacitive coupling of the analog circuit should be minimized.

2. Summary
Cabling strategies are often presented as a rule of thumb because it is difficult to test the ultimate success of a product in a laboratory environment. Therefore, despite the similarities in the routing strategies of digital and analog circuits, it is important to recognize and take seriously the differences in routing strategies. Once the digital and analog ranges are determined, careful routing is critical to a successful PCB board.