25 principles of PCB interference suppression design
Principle 1: On the "clean ground", no other devices can be placed except filtering and protection devices.
Reason: The purpose of the "clean ground" design is to ensure that the interface radiation is minimal, and the "clean ground" is easily coupled by external interference, so there should be no other unrelated circuits and devices on the "clean ground".
Principle 2: Crystals, crystal oscillators, relays, switching power supplies and other strong radiation devices should be at least 1000mil away from the single board interface connector.
Reason: These devices will directly radiate external interference, or couple out the current on the outgoing cable to radiate outward.
Principle 3: Sensitive circuits or devices (such as reset circuits, watchdog circuits, etc.) are at least 1000 mil away from each edge of the board, especially the side edges of the board interface.
Reason: Similar to the board interface, it is the place that is most likely to be coupled by external interference (such as static electricity), and sensitive circuits such as reset circuit and watchdog circuit are likely to cause system misoperation.
Principle 4: Filter capacitors for IC filtering should be placed as close as possible to the power supply pins of the chip.
Reason: The closer the capacitor is to the pin, the smaller the high-frequency loop area, and therefore the smaller the radiation.
Principle 5: The series matching resistor at the beginning should be placed close to its signal output.
Reason: The purpose of the series matching resistor at the beginning is to add the output impedance of the chip output and the series resistance to the characteristic impedance of the trace. The matching resistor is placed at the end, which cannot meet the above equation.
Principle 6: PCB traces must not have right-angle or sharp-angle traces.
Reason: Right-angle wiring leads to discontinuous impedance, which causes signal transmission, which causes ringing or overshoot, and forms strong EMI radiation.
Principle 7: Try to avoid setting adjacent wiring layers as much as possible. When it is unavoidable, try to make the traces in the two wiring layers perpendicular or parallel to each other, and the trace length is less than 1000mil.
Cause: Reduce crosstalk between parallel traces.
Principle 8: If the board has an internal signal routing layer, key signal lines such as clocks are routed on the inner layer (first consider the preferred wiring layer).
Reason: Layout of key signals on the internal routing layer can provide shielding.
Principle 9: A ground wire is recommended on both sides of the clock line. Ground wire should be grounded vias every 3000 mils.
Reason: Ensure that the potentials of the points on the ground line are equal.
Principle 10: Clock, bus, RF line and other key signal traces and other parallel traces on the same layer should meet the 3W principle.
Reason: Avoid crosstalk between signals.
Principle 11: The pads of surface-mount fuses, magnetic beads, inductors, and tantalum capacitors for power supplies with a current of ≥1A should be connected to the plane layer through no less than two vias.
Reason: Reduce the equivalent impedance of the via.
Principle 12: Differential signal lines should be on the same layer, of equal length, and run in parallel, with the same impedance, and there should be no other traces between the differential lines.
Reason: Ensure that the common-mode impedance of the differential pair is equal and improve their anti-interference ability.
Principle 13: Key signal traces must not be routed across partitions (including reference plane gaps caused by vias and pads).
Reason: A trace across a partition will increase the signal loop area.
Principle 14: When it is unavoidable that the signal line is split across its reflow plane, it is recommended to use a bridge capacitor approach near the signal across the split. The capacitance value is 1nF.
Reason: When the signal is split across, it often causes its loop area to increase. The bridge ground method is used to set the signal loop for it.
Principle 15: Do not route other unrelated signals under the filter (filter circuit) on the board.
Reason: Distributed capacitance will weaken the filtering effect of the filter.
Principle 16: The input and output signal lines of the filter (filter circuit) cannot be parallel to each other and cross-routed.
Reason: Avoid direct noise coupling of the traces before and after filtering.
Principle 17: The key signal line is ≥3H from the edge of the reference plane (H is the height of the line from the reference plane).
Reason: Suppression of edge radiation effects.
Principle 18: For grounded components with metal enclosures, ground copper should be laid on the top layer of the projection area.
Reason: The distributed capacitance between the metal case and the ground copper can be used to suppress external radiation and improve immunity.
Principle 19: In single-layer or double-layer boards, you should pay attention to "minimize the loop area" when wiring.
Reason: The smaller the loop area, the smaller the external radiation of the loop, and the stronger the anti-interference ability.
Principle 20: When changing layers of signal lines (especially key signal lines), design ground vias near the layer changing vias.
Reason: Can reduce the area of the signal loop.
Principle 21: Clock lines, bus lines, radio frequency lines, and other strong radiation signal lines should be kept away from interface signal lines.
Reason: Avoid interference from strong radiation signal lines to outgoing signal lines and radiate outward.
Principle 22: Sensitive signal lines such as reset signal lines, chip selection signal lines, system control signal lines, etc. should be kept away from the interface signal lines.
Reason: Outgoing signal lines of the interface often bring in external interference, which will cause system malfunction when coupled to sensitive signal lines.
Principle 23: In single and double panels, the traces of the filter capacitor should be filtered by the filter capacitor before reaching the device pins.
Reason: The power supply voltage is filtered before supplying power to the IC, and the noise fed back from the IC to the power supply is also filtered by the capacitor.
Principle 24: In single- or double-panel, if the power line is very long, decoupling capacitors should be added to the ground every 3000 mils.
Reason: Filter out high frequency noise on the power line.
Principle 25: The ground and power lines of the filter capacitor should be as thick and short as possible.
Reason: The equivalent series inductance will reduce the resonant frequency of the capacitor and weaken its high-frequency filtering effect.