How to distinguish the types of circuits, and what are the layout rules for mixed digital and analog design?
Circuit type distinction
Before explaining the digital-analog mixed design layout rules, we are now making a distinction between 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 scheme. Understanding is crucial.
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. The above analog circuits used are all sensitive circuits. Among them, the sensitive circuits that need special attention include frequency end circuits (including local oscillator signals, power supply for frequency synthesis circuits and control signals), receiving front-end circuits, and audio circuits.
Interference sources include all digital circuits, high-power RF circuits (amplifiers, antennas, and other high-power RF 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 to RF signals such as power amplifiers and antennas is analyzed in the RF design section of this specification.
For the digital-analog mixed design, the interference paths that need attention include: space radiation, power ground (plane or trace), digital-to-analog conversion circuits, and various control signals of analog circuits.
(1) Space radiation: Crosstalk between adjacent circuits 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 can be tolerated by analog signals is much smaller than digital signals, so there are It is necessary to control crosstalk during the layout stage. The way to reduce space radiation is generally to extend the distance of the layout and use a shield box.
(2) Analog control signal: The ideal analog device should be that the control signal is isolated from the analog circuit inside the device, and the control signal only needs to ensure the correct logic level. However, devices often fail to do this, and interference numbers on control signals may be directly coupled to analog circuits. The solution is to minimize the interference to the analog circuit control signal and use the filter reasonably.
(3) Power ground: The power ground is a common circuit between digital and analog circuits, so the interference signal may be conducted to the sensitive circuit through the power ground conductor. The way to control the ground crosstalk of the power supply is to use the filter device and the power supply ground division reasonably.
(4) Digital-to-analog conversion circuit: It is an interface for analog digital signals. If the layout or wiring is not handled properly, for example, the layout of digital and analog circuits is not clear, and the wiring is staggered, it may cause crosstalk.
Layout rules of digital-analog mixed design
Rule 1: Analog devices are placed in the analog area.
Rule 2: Digital devices are placed in the digital area.
Rule 3: Digital power and analog power are supplied from different directions.
Rule 4: The digital-to-analog mixed chip is treated as an analog device and is located in the analog area, but the digital interface needs to be located close to the corresponding digital device layout.
Rule 5: Use the shielded box for protection of the following circuits as much as possible
1. Receiving front-end circuits, including filters, LNA, impedance matching circuits, etc. from antenna to receiving chip.
2. Frequency source circuit: VCO, phase-locked loop chip, loop filter, crystal and other circuits.
3. Power amplifier circuit. Separate power supply paths for different circuits as much as possible during layout
Rule 6: Place a filter capacitor before the power supply enters the analog area
Rule 7: The power supply path in the same direction uses the path from large signal to small signal for power supply.
As shown in the figure: The power supply path from large to small can reduce the interference of large-signal circuits on small-signal circuits.
Rule 8: The power supply traces of the power amplifier should be as short as possible to reduce the line voltage drop.
Earlier mobile phone battery connectors are generally designed in the middle of the mobile phone board, with the RF circuit on the top and the digital circuit on the bottom, as shown in the figure:
The advantage of this layout is that the power supply path of the radio frequency and the digital part is independent, and the power supply path of the attack amplifier is short.
Rule 9: During the layout and wiring of the power module, a heat dissipation copper area is reserved according to the power consumption.
Rule 10: The layout is to leave space for ground vias for important pins.
The ground pin of the RF device needs to be grounded nearby and connected to the RF signal reference layer. For example, if the second layer is hollowed out, the ground pin must be connected to the third layer nearby.
Rule 11: The filter capacitor is close to the pin of the power module, and the high-frequency filter capacitor is closer to the pin.
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