First, the basic conceptual understanding of digital-analog hybrid design
Many products include digital-analog mixed PCB designs with different signals with different immunity to interference.
In the interconnection design process, the crosstalk between different signals must be properly controlled to ensure the 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 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. Perform 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.
- The important difference between the analog signal and digital signal in an anti-interference ability
Digital signal levels have the strong anti-interference ability, while analog signals have poor anti-interference ability.
For example, a digital signal at a 3V level can be tolerated even if it receives a 0.3V crosstalk signal 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 equivalent to a sine wave RMS of 0.7uV. Even if the in-band interference noise of the order of up 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 affect 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, noise is constantly superimposed on 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:
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
the digital circuit of about 20dBc. The DAC requires at least 1000 times more noise than a digital signal. Of course,
if the least significant digit requires only 11 bits, the crosstalk requirement can be lowered appropriately,
but still much higher than the digital signal requirement.
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.
- 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.
- 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 region through reasonable layout, wiring, shielding, filtering, power ground division and other design methods.
The areas we need to focus on include interference sources, sensitive circuits, and interference paths.
The layout and routing principles adopted will be described in the following three aspects.
The successful digital-analog mixed-board design must pay careful attention to every step and every detail in the whole process,
which means that thorough and careful planning must be carried out at the beginning of the design,
and each design step must be carried out. The progress of the work is carried out in a comprehensive and continuous assessment.
Care must be taken to check and verify the layout and wiring, and 100% compliance with the layout rules is guaranteed.
Otherwise, a signal line improperly routed will completely destroy a very good board.
The rules are dead, and a deep understanding of the principles through rules can ensure that we can correctly apply the rules and complete the excellent design.
Second, the circuit type distinction
Before explaining the layout rules of digital-analog hybrid design, we will make a distinction between the interference source,
the sensor circuit and the interference path on the terminal board. Understanding these interference sources and
sensitive circuits can help us to correctly formulate the layout and wiring scheme, and also the interference path.
Understanding is crucial.
Analog circuit
For the terminal products, the analog circuit includes all RF circuits, RF power supplies, RF control circuits,
digital-to-analog conversion circuits, and audio circuits. All of the above analog circuits are sensitive circuits,
and the sensitive circuits that need special attention include frequency termination circuits (including local oscillator signals,
power supply and control signals of frequency synthesizer circuits), receiving front-end circuits, and audio circuits.
- Source of interference
Sources of interference include all digital circuits, high-power RF circuits (amplifiers, antennas, and other high-power RF circuits). 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 RF signals such as power amplifiers and antennas is analyzed in the RF design part of this specification.
- Interference pathway
The interference paths that need to be paid attention to for digital-analog hybrid design include space radiation,
power ground (plane or trace), the digital-to-analog conversion circuit, and various control signals of analog circuits.
(1) Space radiation: Crosstalk is generated by radiation between circuits close to each other,
which is the same as the concept of digital signal crosstalk, but it should be noted that
analog signals can tolerate much less crosstalk than digital signals, so It is necessary to control crosstalk during the layout phase.
The way to reduce spatial radiation is generally to distance the layout and use a shielded box.
(2) Power ground: The power ground is a common circuit between digital and analog circuits,
so the interference signal may be conducted to the sensing circuit through the ground conductor of the power supply.
The way to control the crosstalk of the power supply is to use the filter components and the power ground.
(3) Digital-to-analog conversion circuit: It is an interface analog to digital signal. If the layout or wiring is improperly processed,
such as the digital and analog circuit layout is not clear, the traces are interlaced, which 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 can ensure the correct logic level.
However, devices often fail to do this, and the interference number on the control signal may be directly
coupled into the analog circuit. The solution is to minimize the interference of the analog circuit control signal
and use the filter components reasonably.
Third, the layout rules of digital-analog hybrid design
Rule 1: The analog device is laid out in the analog area.
Rule 2: The digital device is laid out in the digital area.
Rule 3: The digital-analog hybrid chip is processed as an analog device and placed in the analog area,
but the digital interface needs to be close to the corresponding digital device layout.
Rule 4: The following circuits are protected as much as possible using a shielded box
1. Receive the front-end circuit, including the filter between the antenna and the receiving chip, LNA, impedance matching circuit, and so on.
- Frequency source circuit: VCO, phase-locked loop chip, the loop filter, crystal oscillator and other circuits.
- Power amplifier circuit: When the layout is as different as possible, the circuit has an independent power supply path.
Rule 5: Place the filter capacitor before the power supply enters the analog zone.
Rule 6: Digital power and analog power are supplied from different directions.
Rule 7: The power supply path in the same direction is powered by a path from a large signal to a small signal.
As shown in the figure: The power supply path from large to small can reduce the interference of the large signal loop to the small signal circuit.
Rule 8: The power supply wiring should be as short as possible to reduce the line voltage drop.
Earlier mobile phone battery connectors were typically designed in the middle of the phone board with RF circuitry on top.
The advantage of this layout is that the RF and digital part of the power supply path is independent, and the power supply path of the attack and discharge is short.
Rule 9: When the power module is laid out and routed, the heat-dissipation copper area is reserved according to the power consumption.
Rule 10: Layout is the space reserved for ground vias for important pins.
The ground pin of the RF device needs to be grounded nearby and needs to be connected to the reference layer of the RF signal.
For example, if the second layer is hollowed out, the ground pin must be connected to the third layer.
Rule 11: The filter capacitor is close to the pin of the power module, and the high-frequency filter capacitor is close to the pin.
Alice Lu,
Email: sales16@andwinpcb.com
Skype: andwinpcb16@outlook.com