Electronic Design - Digital-analog mixed PCB layout design

Digital-analog mixed PCB layout design
1. Understanding the basic concepts of digital-analog hybrid design
Many products include mixed digital-analog PCB designs, and different signals have different anti-interference capabilities. In the interconnection design process, the crosstalk between different signals must be reasonably controlled in order to ensure the index requirements of the final product.
It is very important to understand the following basic concepts. Mastering the basic concepts of digital-analog hybrid design will help understand the strict layout and wiring design rules that will be formulated later, so that the terminal product will not be easily discounted when designing the digital-analog hybrid. Implement the important constraint rules. And it helps to flexibly and effectively deal with crosstalk problems that may be encountered in digital-analog hybrid design.

1. The important difference between analog signal and digital signal in anti-interference ability
The digital signal level has a strong anti-interference ability, while the analog signal has a poor anti-interference ability.
For example, a 3V level digital signal can tolerate even a 0.3V crosstalk signal and will not affect the logic state. However, in the field of analog signals, some signals are extremely weak. For example, the receiving sensitivity of a GSM mobile phone can achieve an index of -110dBm, which is only equivalent to an effective value of a sine wave of 0.7uV. Even if the in-band interference noise of the order of uV is received at the front end of the LNA, it is enough to greatly degrade the receiving sensitivity of the base station. This slight interference may come from small noises on the digital control signal line or power ground line.
From the point of view of the system, digital signals are generally only transmitted on the board or in the frame. For example, memory bus signals, power control signals, etc., as long as it is ensured that the interference received from the sending end to the receiving end is not enough to affect the judgment of the logic state. The analog signal needs to undergo a series of processes such as modulation, frequency conversion, amplification, transmission, spatial propagation, reception, and demodulation before it can be recovered. During this process, the noise continuously drops to the signal. From a system perspective, it is necessary to ensure that the final signal-to-noise ratio meets the requirements in order to demodulate correctly. The biggest interference comes from the attenuation and noise of space propagation. In order to achieve better communication performance, the crosstalk introduced by the interconnection on the board must be reduced as much as possible.
Therefore, it can be considered that the requirements for crosstalk of analog signals are several tens of times higher than that of digital signals, and may even reach tens of thousands of times.

2. High-precision ADC and DAC circuits
In an ideal situation, the relationship between the signal-to-noise ratio of linear ADC and DAC circuits and the number of conversion bits is:
SNR=10Log(F2/N2)=10Log[A2/2/(A2/3*2n)]=6.02n+1.76 dB
For 14-bit linear ADCs and DACs, if the least significant bit (LSB) is valid, the theoretical signal-to-noise ratio can be calculated to be 86dBc. Compared with the digital circuit's crosstalk requirement of about 20dBc, the high-precision 14-bit linearity The noise requirements of ADCs and DACs are at least 1000 times higher than that of digital signals. Of course, if only 11 bits are required for the least significant number of bits, the crosstalk requirement can be appropriately lowered, but it is still much higher than the requirement for digital signals.
The two situations above on the note indicate that the analog circuit in the digital-analog hybrid single board is very susceptible to interference, which will affect the signal-to-noise ratio and other indicators. Therefore, in the process of digital-analog hybrid single-board PCB design, very high requirements must be put forward for layout and routing.

3. Digital signal is a strong source of interference to analog signal
The level of the digital signal is very high compared to the analog signal, and the digital signal contains rich harmonic frequencies, so the digital signal itself is a strong interference source for the analog signal. In particular, high-current clock signals and switching power supplies are strong interference sources that need to be paid attention to in digital-analog hybrid designs.
4. The fundamental purpose of digital-analog hybrid interconnection design
We can understand the digital-to-analog design problem in this way. For digital circuits, we follow the design rules of digital circuits. In the area of digital circuits, large interference can be allowed, as long as it does not affect the implementation of system functions and external emc indicators.
The "larger" we are talking about here is relative to analog circuits. For digital circuits, it is not necessary and impossible for us to control the existence of crosstalk like analog circuits. For analog circuits, we must follow the design rules of analog circuits, and the interference allowed in the analog circuit area is much smaller than in the digital circuit area.
The purpose of digital-analog hybrid interconnection design is to ensure that the interference of digital signals only exists in the digital signal area through reasonable layout, wiring, shielding, filtering, and power supply division.

What we need to focus on includes interference sources, sensitive circuits, and interference paths. The following will describe the layout and routing principles adopted from these three aspects. Successful digital-analog hybrid single-board design must be realized with careful attention to every step and every detail in the entire process. This means that thorough and careful planning must be carried out at the beginning of the design, and every design step must be carefully planned. The progress of the work is fully and continuously evaluated. The layout and routing must be carefully checked and verified to ensure 100% compliance with the layout and routing rules. Otherwise, improper routing of a signal line will completely destroy an otherwise very good circuit board.
The rules are dead. Only through a deep understanding of the rules can we ensure that we can use the rules correctly and complete an excellent design.
Second, the circuit type distinction
Before explaining the layout rules of digital-analog hybrid design, we now distinguish the interference sources, sensitive circuits and interference paths on the terminal board. Understanding these interference sources and sensitive circuits can help us correctly formulate the layout and wiring plan. Understanding is essential.
1. Analog circuit
For end products, analog circuits include all radio frequency circuits, radio frequency power supplies, radio frequency control circuits, digital-to-analog conversion circuits, and audio circuits. All of the above analog circuits are sensitive circuits. Among them, sensitive circuits that need special attention include frequency termination circuits (including local oscillator signals, frequency synthesis circuit power and control signals), receiving front-end circuits, and audio circuits.
2. Sources of interference
Interference sources include all digital circuits, high-power radio frequency circuits (power amplifiers, antennas and other high-power radio frequency circuits). Among them, the interference sources that need special attention include clock circuits, switching power supplies, high-current power lines, power amplifier circuits, and antenna circuits. The interference of radio frequency signals such as power amplifiers and antennas is analyzed in the radio frequency design part of this specification.
3. Interference path
The interference paths that need to be paid attention to for digital-analog hybrid design include: space radiation, power ground (plane or wiring), digital-to-analog conversion circuits, and various control signals of analog circuits.
(1) Space radiation: The circuits that are close to each other will generate crosstalk through radiation, which is the same as the concept of digital signal crosstalk, but it should be noted that the crosstalk that analog signals can tolerate is much smaller than that of digital signals, so there is It is necessary to control crosstalk at the layout stage. The way to reduce the space radiation is generally to extend the distance of the layout and use the shielding box.
(2) Power ground: The power ground is a common loop between digital and analog circuits, so interference signals may be conducted to sensitive circuits through the power ground conductor. The way to control the crosstalk of the power supply ground is to use filter components and power supply ground division reasonably.
(3) Digital-to-analog conversion circuit: It is an interface between analog and digital signals. If the layout or wiring is handled improperly, such as the unclear layout of digital and analog circuits and the interlaced wiring, it may cause crosstalk.
(4) Analog control signal: The ideal analog device should be that the control signal and the analog circuit are isolated inside the device, and the control signal only needs to ensure the correct logic level. However, the device often cannot do this, and the interference number on the control signal may be directly coupled to the analog circuit. The solution is to minimize the interference of the analog circuit control signal and use filter components reasonably.