Juding Circuit Technology PCB Digital-Analog Hybrid Circuit Board Design

2019-08-27 20:54:24 461

Many products include digital-analog mixed PCB designs, and different signals have different anti-interference capabilities. In the interconnection design process, the crosstalk between different signals must be properly controlled to ensure the target product requirements.

It is very important to understand the following basic concepts. Mastering the basic concepts of digital-analog hybrid design will help to understand the layout and wiring design rules that are very strict in the future, so that the design of the digital-analog hybrid of the terminal products will not be easily discounted. Execute the important constraint rules. It also helps to deal with crosstalk problems that may be encountered in digital-analog hybrid design in a flexible and efficient manner.

1. The important difference between analog signal and digital signal in anti-interference ability

Digital signal levels have strong anti-interference ability, while analog signals have poor anti-interference ability.

For example, a digital signal of 3V level can be tolerated even if it receives a crosstalk signal of 0.3V without affecting the logic state. However, in the field of analog signals, some signals are very weak. For example, the receiving sensitivity of GSM mobile phones can achieve an index of -110dBm, which is only equivalent to the sine wave RMS of 0.7uV. Even if the in-band interference noise of the order of uV is received at the LNA front end, it is enough to greatly degrade the base station receiving sensitivity. This slight interference can result from small noise on the digital control signal line or the power ground.

From a system point of view, digital signals are typically transmitted only on a board or in a frame. For example, the memory bus signal, the power control signal, etc., as long as the interference received from the transmitting end to the receiving end is insufficient to influence the determination 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 to be recovered.

During this process, the noise continues to drop to the signal. From a system perspective, it must be ensured that the final signal-to-noise ratio meets the requirements for proper demodulation. The largest interference comes from the attenuation and noise of spatial propagation. In order to achieve better communication performance, the crosstalk introduced by the interconnection in the board must be minimized.

Therefore, it can be considered that the analog signal requires tens of times higher than the digital signal, and may even reach tens of thousands of times.

2. High precision ADC, DAC circuit

Ideally, the relationship between the linear ADC, the DAC circuit signal-to-noise ratio, and the number of conversion bits is:

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

For a 14-bit linear ADC, DAC, if the least significant bit data (LSB) is valid, the theoretical signal-to-noise ratio can be calculated to be 86dBc, which is a high-precision 14-bit linear ADC compared to the crosstalk requirement of a digital circuit of about 20dBc. The DAC requires at least 1000 times more noise than the digital signal.

Of course, if the least significant digit requires only 11 bits, the crosstalk requirement can be properly lowered, but still much higher than the digital signal requirements.

In the above two cases, the analog circuit is extremely susceptible to interference in the digital-analog mixed board, which affects the signal-to-noise ratio and other indicators. Therefore, in the digital-analog mixed-board PCB design process, high requirements are placed on the layout and wiring.

3. Digital signals are strong sources of interference for analog signals

The level of the digital signal is very high relative to the analog signal, and the digital signal contains a rich harmonic frequency, so the digital signal itself is a strong source of interference for the analog signal. In particular, high-current clock signals, switching power supplies, etc. are more powerful sources of interference that need to be addressed in digital-analog hybrid designs.

4. The fundamental purpose of digital-analog hybrid interconnect design

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

The "larger" we are talking about here is relative to analog circuits. For digital circuits, we don't have to and cannot control the presence of crosstalk as we do analog circuits. For analog circuits, we must follow the design rules of the analog circuit, and the interference allowed in the analog circuit area is much smaller than the digital circuit area.

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

The areas that need to be focused on include interference sources, sensitive circuits, and interference paths. The layout and routing principles adopted will be described in the following three aspects.

Successful digital-analog mixed-board PCB design must pay careful attention to every step and every detail in the entire process, which means that thorough and careful planning must be carried out at the beginning of the design. The progress of the work of the steps is fully and continuously assessed.

Care must be taken to check and verify the layout and wiring, and 100% compliance with the place and route rules is guaranteed. Otherwise, a signal line improperly routed will completely destroy a very good board.

The rules are dead. Through deep understanding of the rules, we can ensure that we can correctly apply the rules and complete excellent PCB design.

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