Experience in PCB layout design - similarities and differences between analog and digital layout

Experience in PCB layout design - similarities and differences between analog and digital layout

Similarities between analog and digital routing strategies

1) Bypass or decoupling capacitors

Both analog and digital devices require these types of capacitors when routing. They need to be connected to a capacitor near their power supply pins. This capacitor is typically 0.1mF. Another type of capacitor is required on the system power supply side. Typically, this capacitor is approximately 10mF.

 

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

 

Adding bypass or decoupling capacitors to the board and the location of these capacitors on the board are common sense for both digital and analog designs. But the interesting thing is that the reasons are different. In analog wiring designs, bypass capacitors are typically used to bypass high-frequency signals on the power supply. If no bypass capacitors are added, these high-frequency signals may enter the sensitive analog chip through the power supply pins.

 

In general, the frequency of these high frequency signals exceeds the ability of analog devices to reject high frequency signals. If a bypass capacitor is not used in the analog circuit, noise may be introduced into the signal path, and even more severe conditions may cause vibration.

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In analog and digital PCB designs, bypass or decoupling capacitors (1mF) should be placed as close as possible to the device. The power supply decoupling capacitor (10mF) should be placed at the power line inlet of the board. In all cases, the pins of these capacitors should be short.

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On this board, different routes are used to route the power and ground lines. Because of this improper fit, the electronic components and lines of the board are more likely to be electromagnetically disturbed.

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In this single panel, the power and ground wires to the devices on the board are close to each other. The matching of the power and ground wires in this board is more appropriate than in Figure 2. The possibility of electromagnetic interference (EMI) in electronic components and circuits 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 required, but for different reasons. One function of these capacitors is to act as a "mini" charge bank. In digital circuits, switching the gate state typically requires a large current. Because switching transient currents are generated on the chip and flow through the board during switching, it is advantageous to have additional "standby" charge. If there is not enough charge when performing the switching action, the power supply voltage will change greatly. A voltage change that is too large can cause the digital signal level to go into an indeterminate state and is likely to cause the state machine in the digital device to malfunction. The switching current flowing through the board trace will cause a voltage change, and the board trace has parasitic inductance. The voltage change can be calculated by the following formula: V = LdI/dt

 

Where V = voltage change; L = board trace inductance; dI = current flow through the trace; dt = current change time.

 

Therefore, it is preferable to apply a bypass (or decoupling) capacitor at the power supply or at the power supply pin of the active device for a variety of reasons.

 

2) The power cord and ground wire should be put together

The position of the power cord and the ground wire are well matched to reduce the possibility of electromagnetic interference. If the power and ground wires are not properly matched, a system loop is designed and noise is likely to occur. An example of a PCB design with improper power and ground wiring is shown in Figure 2 above.

 

On this board, the designed loop area is 697cm2. With the method shown in Figure 3 above, the possibility of radiated noise on the board or outside the board inducing a voltage in the loop can be greatly reduced.

 

Differences between analog and digital domain routing strategies

Ground plane is a problem

The basics of board layout apply to both analog and digital circuits. A basic rule of thumb is the use of uninterrupted ground planes. This common sense reduces the dI/dt (current vs. time) effect in digital circuits, which changes the ground potential and causes noise to enter the analog circuit. . The wiring techniques for digital and analog circuits are basically the same, except one thing. Another point to note about analog circuits is to keep the digital signal lines and the loops in the ground plane as far away as possible from the analog circuits. This can be done by connecting the analog ground plane to the system ground connection alone or by placing the analog circuitry at the far end of the board, the end of the line. This is done to keep the external interference to the signal path to a minimum. This is not required for digital circuits, which can tolerate a large amount of noise on the ground plane without problems.

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(Left) Separate the digital switching action from the analog circuitry to separate the digital and analog sections of the circuit. (Right) To separate the high and low frequencies as much as possible, the high frequency components should be close to the connectors on the board.

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It is easy to form parasitic capacitance by laying two adjacent traces on the PCB. Due to the presence of such a capacitor, a rapid voltage change on one trace can produce a current signal on the other trace.

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If you do not pay attention to the placement of the traces, the traces in the PCB may cause line inductance and mutual inductance. This parasitic inductance is very detrimental to the operation of circuits containing digital switching circuits.

" >If you do not pay attention to the placement of the traces, the traces in the PCB may cause line inductance and mutual inductance. This parasitic inductance is very detrimental to the operation of circuits containing digital switching circuits.