Analyzing Digital Analog Mixed PCB Design Together

Many PCB products contain mixed digital and analog, and different signals have different anti-interference capabilities. In the process of interconnection design, the crosstalk between different signals must be reasonably controlled 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 is helpful to understand the layout and PCB wiring design rules that are strictly formulated later, so that the important constraint rules will not be easily implemented at a discount during the design of digital analog hybrid terminal products. It is also helpful to deal with the crosstalk problem that may be encountered in digital analog hybrid design flexibly and effectively.

PCB design

1. Important differences between analog and digital signals in anti-interference capability

Digital signal level has strong anti-interference ability, while analog signal has poor anti-interference ability.

For example, the digital signal of 3V level can be tolerated even if the crosstalk signal of 0.3V is received, 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 GSM mobile phones can reach - 110dBm, which is only equivalent to the effective value of 0.7uV sine wave. Even if the LNA front-end receives in band interference noise of the order of uV, it is enough to significantly degrade the reception sensitivity of the base station. This slight interference may come from the small noise on the digital control signal line or power supply ground wire.

From a system point of view, digital signals are generally transmitted only on the board or in the frame. For example, memory bus signal, power control signal, 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 logic state judgment. The analog signal can only be recovered after a series of processes such as modulation, frequency conversion, amplification, transmission, space propagation, reception and demodulation. In this process, the noise continuously depreciates to the signal. From the perspective of the system, the final signal-to-noise ratio must meet the requirements to correctly demodulate. The maximum interference comes from the attenuation and noise of space propagation. In order to achieve better communication performance, the crosstalk introduced by the interconnection in the board must be reduced as much as possible.

Therefore, it can be considered that the requirements for crosstalk of analog signals are dozens of times higher than that of digital signals, and even tens of thousands of times higher.

2. High precision ADC and DAC circuits

Ideally, the relationship between SNR and conversion bits of linear ADC and DAC circuits is:

SNR=10Log(F2/N2)=10Log[A2/2/(A2/3 × 2n)]=6.02n+1.76 dB

For 14 bit linear ADC and DAC, if the lowest bit data (LSB) is valid, the theoretical signal-to-noise ratio can be calculated to be 86dBc. Compared with the crosstalk requirement of about 20dBc for digital circuits, the noise requirement for high-precision 14 bit linear ADC and DAC is at least 1000 times higher than that for digital signals. Of course, if the LSB only needs 11 bits, the crosstalk requirement can be appropriately lowered, but it is still much higher than that of digital signals.

The above two situations indicate that analog circuits in a mixed digital analog board are vulnerable to interference, which will affect signal-to-noise ratio and other indicators. Therefore, in the process of digital analog mixed single board PCB design, high requirements should be put forward for layout and routing.

3. Digital signal is a strong interference source to analog signal

The level of the digital signal is very high compared with the analog signal, and the digital signal contains rich harmonic frequencies. Therefore, the digital signal itself is a strong interference source for the analog signal. Especially the clock signal and switching power supply with large current are the strong interference sources that need to be paid attention to in the digital analog hybrid design.

4. Fundamental purpose of digital analog hybrid interconnection design

We can understand the problem of digital analog design in this way. For digital circuits, we follow the design rules of digital circuits. In the digital circuit area, large interference can be allowed, as long as it does not affect the system function realization and external EMC indicators.

The "larger" we are talking about here is relative to the analog circuit. For digital circuits, it is unnecessary and impossible to control the existence of crosstalk as for analog circuits. For analog circuits, we must follow the design rules of analog circuits. The interference allowed in the analog circuit area is far less than that in the digital circuit area.

The purpose of digital analog hybrid interconnection design is to ensure that digital signal interference only exists in the digital signal area through reasonable PCB layout, wiring, shielding, filtering, power and ground separation and other design methods.

The content we need to focus on includes interference sources, sensitive circuits and interference channels. The following will describe the layout and routing principles adopted from these three aspects. A successful digital analog mixed board PCB design must pay close attention to every step and every detail in the whole process before it can be realized. This means that thorough and careful planning must be carried out at the beginning of the design stage, and the work progress of each design step must be comprehensively and continuously evaluated. The layout and wiring must be carefully checked and checked to ensure 100% compliance with PCB layout and wiring rules. Otherwise, a very good circuit board will be completely destroyed if a signal line is improperly routed.

Rules are dead. Only by deeply understanding the rules can we ensure that we can correctly use the rules and complete excellent PCB design.