In PCB design, why is the difference between analog circuits and digital circuits so big?

The number of digital designers and digital circuit board design experts in the engineering field continues to increase, reflecting industry trends. 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, a simple circuit routing design is no longer optimal due to their different routing strategies.

The number of digital designers and digital circuit board design experts in the engineering field continues to increase, reflecting industry trends. 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, a simple circuit routing design is no longer optimal due to their different routing strategies.

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 routing.

Similarities Between Analog and Digital Routing Strategies

Bypass or decoupling capacitors

When wiring, both analog and digital devices need these types of capacitors, and they all need to connect a capacitor close to their power pins, and the value of this capacitor is usually 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 location of these capacitors is shown in Figure 1. The capacitor value range is between 1/10 and 10 times the recommended value. But the pins must be short and as close as possible to the device (for 0.1uF capacitors) or the power supply (for 10uF capacitors).

In PCB design, why is the difference between analog circuits and digital circuits so big?

figure 1

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.

In analog and digital PCB design, bypass or decoupling capacitors (0.1uF) should be placed as close to the device as possible. The power supply decoupling capacitor (10uF) should be placed at the power line entry of the circuit board. In all cases, the leads for these capacitors should be short.

In PCB design, why is the difference between analog circuits and digital circuits so big?

figure 2

On the circuit board in Figure 2, different routes are used to route the power lines and ground lines. Due to this inappropriate coordination, the Electronic components and lines of the circuit board are more likely to be subject to electromagnetic interference.

In PCB design, why is the difference between analog circuits and digital circuits so big?

image 3

In the single panel of Figure 3, the power and ground lines to the components on the board are close to each other. The matching ratio of power line and ground line in this circuit board is appropriate as shown in Figure 2. Electronic components and circuits in circuit boards are 679/12.8 times less likely to be affected by electromagnetic interference (EMI) 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, 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, and the circuit board traces have parasitic inductance. The following formula can be used to calculate the voltage change: V = LdI/dt. Where: V = change in voltage, L = inductive reactance of circuit board traces, dI = change in current flowing through the trace, and dt = time for current 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 and ground wires should be routed together

The location of the power and ground wires is well matched to 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.

An example of PCB design in which power and ground wires are not properly matched is shown in Figure 2. On this circuit board, the designed loop area is 697cm². Using the method shown in Figure 3, the likelihood of radiated noise on or off the circuit board to induce voltages in the loop is greatly reduced.

Differences between analog and digital cabling strategies

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. 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 connections, or by placing the analog circuitry at the farthest end of the board, at the end of the line. This is done to keep external interference to the signal path to a minimum.

This is not necessary for digital circuits, which can tolerate a lot of noise on the ground plane without problems.

In PCB design, why is the difference between analog circuits and digital circuits so big?

Figure 4

Figure 4 (left) isolates the digital switching action from the analog circuit, separating the digital and analog parts of the circuit. (Right) To separate high and low frequencies as much as possible, keep high frequency components close to the circuit board connectors.

In PCB design, why is the difference between analog circuits and digital circuits so big?

Figure 5

Figure 5. Two closely spaced traces are placed on the PCB, which can easily form parasitic capacitances. Because of this capacitance, a rapid voltage change on one trace can generate a current signal on the other trace.

In PCB design, why is the difference between analog circuits and digital circuits so big?

Image 6

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 detrimental to the operation of circuits including digital switching circuits.

component location

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 the most sensitive to switching noise. In the wiring of mixed-signal systems, these two circuits are separated, as shown in Figure 4.

Parasitic elements created by PCB design

There are two basic parasitic elements that can easily create problems in PCB design: parasitic capacitance and parasitic inductance.

When designing a board, placing two traces close to each other creates parasitic capacitance. This can be done by placing one trace on top of the other on two different layers, or by placing one trace next to the other on the same layer, as shown in Figure 5.

In both trace configurations, the 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 most 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; and high-impedance traces are more common.

This phenomenon can be reduced using one of the two techniques described below. The most common technique is to vary the size between traces according to the equation for capacitance. The most effective dimension to change is the distance between the two traces. It should be noted 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 such an additional trace will attenuate the interfering electric field, as shown in Figure 5.

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 variation of current on one trace with time (dI/dt), due to the inductive reactance of this trace, will generate voltage on the same 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 the first trace is large enough, the interference can cause errors by reducing the voltage tolerance of the digital circuit. 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, it is best to 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.

Epilogue

Once the digital and analog ranges are determined, careful routing is critical to a successful PCB. 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.

The number of digital designers and digital circuit board design experts in the engineering field continues to increase, reflecting industry trends. 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, a simple circuit routing design is no longer optimal due to their different routing strategies.

The number of digital designers and digital circuit board design experts in the engineering field continues to increase, reflecting industry trends. 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, a simple circuit routing design is no longer optimal due to their different routing strategies.

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 routing.

Similarities Between Analog and Digital Routing Strategies

Bypass or decoupling capacitors

When wiring, both analog and digital devices need these types of capacitors, and they all need to connect a capacitor close to their power pins, and the value of this capacitor is usually 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 location of these capacitors is shown in Figure 1. The capacitor value range is between 1/10 and 10 times the recommended value. But the pins must be short and as close as possible to the device (for 0.1uF capacitors) or the power supply (for 10uF capacitors).

In PCB design, why is the difference between analog circuits and digital circuits so big?

figure 1

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.

In analog and digital PCB design, bypass or decoupling capacitors (0.1uF) should be placed as close to the device as possible. The power supply decoupling capacitor (10uF) should be placed at the power line entry of the circuit board. In all cases, the leads for these capacitors should be short.

In PCB design, why is the difference between analog circuits and digital circuits so big?

figure 2

On the circuit board in Figure 2, different routes are used to route the power lines and ground lines. Due to this inappropriate coordination, the Electronic components and lines of the circuit board are more likely to be subject to electromagnetic interference.

In PCB design, why is the difference between analog circuits and digital circuits so big?

image 3

In the single panel of Figure 3, the power and ground lines to the components on the board are close to each other. The matching ratio of power line and ground line in this circuit board is appropriate as shown in Figure 2. Electronic components and circuits in circuit boards are 679/12.8 times less likely to be affected by electromagnetic interference (EMI) 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, 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, and the circuit board traces have parasitic inductance. The following formula can be used to calculate the voltage change: V = LdI/dt. Where: V = change in voltage, L = inductive reactance of circuit board traces, dI = change in current flowing through the trace, and dt = time for current 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 and ground wires should be routed together

The location of the power and ground wires is well matched to 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.

An example of PCB design in which power and ground wires are not properly matched is shown in Figure 2. On this circuit board, the designed loop area is 697cm². Using the method shown in Figure 3, the likelihood of radiated noise on or off the circuit board to induce voltages in the loop is greatly reduced.

Differences between analog and digital cabling strategies

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. 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 connections, or by placing the analog circuitry at the farthest end of the board, at the end of the line. This is done to keep external interference to the signal path to a minimum.

This is not necessary for digital circuits, which can tolerate a lot of noise on the ground plane without problems.

In PCB design, why is the difference between analog circuits and digital circuits so big?

Figure 4

Figure 4 (left) isolates the digital switching action from the analog circuit, separating the digital and analog parts of the circuit. (Right) To separate high and low frequencies as much as possible, keep high frequency components close to the circuit board connectors.

In PCB design, why is the difference between analog circuits and digital circuits so big?

Figure 5

Figure 5. Two closely spaced traces are placed on the PCB, which can easily form parasitic capacitances. Because of this capacitance, a rapid voltage change on one trace can generate a current signal on the other trace.

In PCB design, why is the difference between analog circuits and digital circuits so big?

Image 6

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 detrimental to the operation of circuits including digital switching circuits.

component location

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 the most sensitive to switching noise. In the wiring of mixed-signal systems, these two circuits are separated, as shown in Figure 4.

Parasitic elements created by PCB design

There are two basic parasitic elements that can easily create problems in PCB design: parasitic capacitance and parasitic inductance.

When designing a board, placing two traces close to each other creates parasitic capacitance. This can be done by placing one trace on top of the other on two different layers, or by placing one trace next to the other on the same layer, as shown in Figure 5.

In both trace configurations, the 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 most 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; and high-impedance traces are more common.

This phenomenon can be reduced using one of the two techniques described below. The most common technique is to vary the size between traces according to the equation for capacitance. The most effective dimension to change is the distance between the two traces. It should be noted 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 such an additional trace will attenuate the interfering electric field, as shown in Figure 5.

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 variation of current on one trace with time (dI/dt), due to the inductive reactance of this trace, will generate voltage on the same 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 the first trace is large enough, the interference can cause errors by reducing the voltage tolerance of the digital circuit. 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, it is best to 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.

Epilogue

Once the digital and analog ranges are determined, careful routing is critical to a successful PCB. 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.

The Links:   CM75TU-24H G084SN05-V8

Author: Yoyokuo