Explain in detail difference between analog and digital circuits in PCB design.

The number of digital designers and digital board designers in engineering field is growing, reflecting industry trends. Although emphasis on digital design has led to significant changes in electronics, there is and always will be a part of circuit design that interfaces with analog or real environments. There are some similarities between layout strategies in analog and digital domains, but simple layout schemes are no longer optimal when it comes to achieving better results due to their different layout strategies.

This article discusses main similarities and differences between analog and digital layouts in terms of bypass capacitors, power supply, grounding design, voltage errors, and electromagnetic interference (EMI) caused by PCB layout.

01 Similarities between analog and digital wiring

Bypass or decoupling capacitor

When connecting to both analog and digital devices, these types of capacitors are required, and they all need to connect a capacitor near power pins. The capacitance of this capacitor is typically 0.1uF. A different type of capacitor is required on power supply side of system, typically around 10uF.

The location of these capacitors is shown in fig. 1. The value of capacitor ranges from 1/10 to 10 times recommended value. However, contact should be short and as close as possible to device (for a 0.1uF capacitor) or power supply (for a 10uF capacitor).

Picture 1

The placement of bypass or decoupling capacitors on board, as well as their placement on board, is common sense for both digital and analog circuits. Interestingly, reasons for this are different.

In analog wiring design, bypass capacitors are typically used to bypass high frequency signals in power supply. If no bypass capacitors are added, these high frequency signals can get to sensitive analog ICs via power pins. Typically, frequency of these high frequency signals exceeds ability of analog devices to suppress high frequency signals. If shunt capacitors are not used in analog circuits, noise can occur in signal path, and in severe cases, even vibration can occur.

In analog and digital circuit board designs, bypass or decoupling capacitors (0.1 uF) should be placed as close to device as possible. A power supply decoupling capacitor (10 µF) must be placed at input of board's power line. In all cases, leads of these capacitors must be short.

Picture 2

On printed circuit board shown in fig. 2, different routes are used for power and ground wires. Due to improper matching, electronic components and PCB circuits are more likely to be subject to electromagnetic interference.

Picture 3

On one panel, shown in fig. 3, power and ground lines of devices on board are close to each other. The matching ratio of power line and ground line on this PCB is correct, as shown in Figure 2. The electronic components and lines on PCB are 679/12.8 times less susceptible to electromagnetic interference (EMI), or about 54 times.

Digital devices such as controllers and processors also require decoupling capacitors, but for different reasons. One of functions of these capacitors is to act as a "miniature" reservoir of charge.

In digital circuits, it usually takes a lot of current to switch gate states. Since switching transients are generated on chip and flow through board when switching, it is useful to have an additional "standby" charge. If there is not enough charge when performing switching action, it will cause a large change in power supply voltage. Too much voltage fluctuation can cause digital signal level to go into an undefined state and likely cause state machine in digital device to malfunction.

Switching current flowing through a PCB track causes a voltage change. The PCB trace has parasitic inductance. The following formula can be used to calculate voltage change: V = LdI/dt. Where: V = voltage change, L = board trace inductance, dI = change in current flowing through trace, dt = current change time.

It is therefore recommended to use bypass (or decoupling) capacitors on power supply or power pins of active devices for several reasons.

The power line and ground line must be laid together

The location of power and ground wires is well coordinated to reduce possibility of electromagnetic interference. If power and ground lines are not properly connected, system loops can occur and noise is likely to occur.

An example of a PCB design in which power line and ground line are not properly matched is shown in fig. 2. The calculated circuit area on this PCB is 697 cm². When using method shown in Fig. 3, chance that noise radiated into or out of board induces voltages in loop is greatly reduced.

02 Differences between analog and digital domain connection strategies

Grounding problem

The basics of PCB layout apply to both analog and digital circuits. The basic rule of thumb is to use a continuous ground plane. This common sense reduces effect of dI/dt (current change with time) in digital circuits, which can change ground potential and allow noise to creep into analog circuits.

Digital and AN layout methodsThe analog circuits are basically same, with one exception. For analog circuits, there is one more thing to pay attention to: loops in digital signal lines and ground plane should be as far away from analog circuits as possible. This can be achieved by connecting analog ground pad separately to system ground, or by placing analog circuitry on farthest side of board, at end of track. This is done in order to minimize external interference in signal path.

This is not necessary for digital circuits, which can handle a lot of ground plane noise without issue.

Picture 4

Figure 4 (left) isolates digital switching action from analog circuit by separating digital and analog parts of circuit. (Right) To maximize separation of high and low frequencies, high-frequency components are placed close to connectors on board.

Picture 5

As shown in fig. 5, it is easy to create parasitic capacitance by laying two closely spaced traces on a PCB. Because of this capacitance, a rapid voltage change on one track can generate a current signal on another track.

Picture 6

Figure 6. If you do not pay attention to placement of tracks, tracks on PCB can generate linear inductance and mutual inductance. This parasitic inductance is very detrimental to operation of circuits, including digital switching circuits.

Element Position

As mentioned above, in every PCB design, noisy part of circuit is separated from "quiet" part (non-noisy part). In general, digital circuits are "rich" in noise and insensitive to noise (because digital circuits have more voltage noise margin); conversely, analog circuits are much less immune to voltage noise.

Of two, analog circuits are most sensitive to switching noise. In a mixed-signal system diagram, these two circuits are separated, as shown in Figure 4.

Stray components created during PCB design

It's easy to form two main parasitic components that can cause PCB design problems: parasitic capacitance and parasitic inductance.

When designing a board, placing two traces next to each other creates parasitic capacitance. You can do this: on two different layers, place one trace above other, or on same layer, place one trace next to other, as shown in Figure 5.

In both trace configurations, change in voltage over time (dV/dt) on one trace can induce current on other trace. If other trace has a high impedance, current generated by electric field will be converted to voltage.

Fast voltage transients most commonly occur on digital side of analog signal systems. This error can seriously affect accuracy of an analog circuit if traces that experience fast voltage transients are close to high impedance analog traces. In this environment, analog circuits have two drawbacks: their noise margin is much lower than that of digital circuits, and high impedance traces are more common.

This phenomenon can be reduced using one of two methods described below. The most common method is to resize between tracks according to capacitance equation. The most efficient size to change is distance between two traces. It should be noted that variable d is in denominator of capacitance equation, with an increase in d, capacitive reactance decreases. Another variable that can be changed is length of two traces. In this case, length L decreases, as does capacitance between two tracks.

Another way is to run a ground wire between these two tracks. Ground traces are low impedance, and adding such an extra trace will attenuate interfering electric field, as shown in Figure 5.

The principle of parasitic inductance in a printed circuit board is similar to principle of parasitic capacitance.You also need to lay two traces: on two different layers, place one trace above other, or on one layer, place one trace next to other, as shown in Figure 6.

In both track configurations, change in current in one track over time (dI/dt) due to inductive reactance of that track will generate a voltage in same track, and due to presence of mutual inductance, a proportional current is induced in other track. If voltage change on first trace is large enough, disturbance can reduce voltage tolerance of digital circuit and lead to errors. This phenomenon does not only occur in digital circuits, it is more common in digital circuits due to high instantaneous switching currents in digital circuits.

To eliminate potential noise from EMI sources, it is best to separate "quiet" analog lines and noisy I/O ports. To try to create a low impedance power and ground network, it is necessary to minimize inductive reactance of digital circuit wires and minimize capacitive coupling of analog circuit.

03 Conclusion

Once digital and analog ranges are determined, careful layout is critical to a successful PCB. Routing strategies are often presented as a rule of thumb because it is difficult to test ultimate success of a product in a laboratory setting. Thus, while routing strategies for digital and analogue circuits are similar, differences in routing strategies need to be considered and taken seriously.