PCB Tech - On the overall consideration of the PCB grounding method

1. Overall consideration of PCB grounding method

1.1 Advantages of commonly used star point grounding (one point grounding) method: no series mutual interference will occur

If you can't follow 100%, you need to carefully consider how to choose the star point? There are 2 templates:

The first board-the large capacitor of the power supply filter is the star point

Common star point grounding

The second board-the case is a star point

Power input ground wire

1.2 Tuner (RF) ground and small signal ground

The tuner RF front end and its shielding case must be connected to the chassis as the ground wire, and the low signal ground can be branched from the tuner ground wire to the tuner (RF) ground and small signal ground

1.3 Grounding of MCU and KB

MCU and KB can be grounded together, and the ground point is connected to the main ground or chassis through a narrow lead

1.4 Servo PCB grounding method

Four types of grounding classification, motor driver/audio/digital/RF circuit grounding method. Each piece of separate copper foil is the ground, connected through a narrow lead. The motor ground is tightened by screws.

1.5 Signal transmission method

Parallel transmission of signal lines and signal ground lines at the same time can reduce noise

2. Audio considerations

The signal current generates a magnetic field, and the power line has many noise signals and noise electromagnetic fields generated by large noisy currents. Know the direction of the signal current and its magnitude and intensity, and reduce the area of the signal current circuit to reduce inductive coupling. Corresponding power line ground Lines should be distributed in parallel (parallel or parallel) to minimize the loop area and reduce the loop impedance. Small signal line traces should not be close to digital circuits or noise signals. Signal lines that can be shielded on adjacent layers of the PCB should be mutually. Vertical (into 90º), which minimizes crosstalk.

3. Noise considerations

The power supply should be decoupled at the entry point of the PCB.

The power supply should be located at the power entry point of the PCB and close to the high-current circuit (power amplifier IC) as soon as possible. Minimize the area between the wires and thus minimize the inductance). When attaching the cable to the PCB, if possible, provide multiple ground loops to minimize the loop area. VCC (clean power) lines and signal lines must not be parallel to unfiltered (dirty) lines that carry batteries, ignition, high current, or fast switching signals.

Usually the signal line and the related ground loop should be placed as close as possible to minimize the current loop area

a) Low frequency signal current passes through the minimum resistance line b) High frequency signal current passes through the minimum inductance line

Small signals or peripheral circuits should be as close as possible to the I/O connector, and away from high-speed digital circuits, high-current circuits, or unfiltered power circuits.

4. EMC considerations

Each digital IC power supply pin adds a high-frequency, low-inductance ceramic capacitor for decoupling. Capacitors of 0.1 µF are used on ICs up to 15 MHz, and capacitors of 0.01 µF are used on ICs greater than 15 MHz. The RF decoupling component of the battery or ignition device should be placed at the power inlet of the PCB (near the I/O connector). The oscillator and MCU should be far away from the I/O connector or tuner, and as close as possible to their chips, preferably on the same side of the PCB to keep the loop area to a minimum. RF decoupling capacitors should be added to the RF circuit. The shielding of low-frequency signals (below 10MHz) should only be terminated and grounded on the source to prevent unwanted ground loops.

5. The 3-W rule of PCB layout

In PCB routing, we should follow the 3-W rule of routing. Crosstalk will occur between the traces on the PCB. This crosstalk will not only occur between the clock signal and its surrounding signals, but also on other key signals, such as data, address, control, input and output signal lines, etc., There may be crosstalk and coupling effects. In order to solve the crosstalk of these signals, we can take a measure from the PCB trace, that is, we should follow the 3-W rule of the trace when we trace. Using the 3-W rule can reduce the coupling between signal traces.

The 3-W rule is to satisfy the separation distance of all signals (key signals such as clock, audio, video, reset, data, address, etc.): the distance between the edge of the trace should be greater than or equal to 2 times the width of the trace, that is, the center of the trace The distance between them is 3 times the width of the trace. For example, if the clock line width is 8mils, the distance between the edge of the clock trace and the edges of other traces should be 16mils.

Note: For traces close to the edge of the board, the distance from the edge of the board to the edge of the trace should be greater than 3W.

The 3-W rule can be used in various wiring situations, not just for clock signals or high-frequency periodic signals. If there is no ground reference plane in the I/O area, then the differential trace pair does not have a mirror plane, and the 3-W rule can be used for routing at this time.

Generally, the distance between the two signal traces of a differential pair trace should be W, and the distance between the differential trace and other traces should meet the 3-W rule, that is, the minimum distance between the trace and other traces should be It is 3W, as shown in Figure 3. For differential pair traces, noise and other signals from the power plane are coupled to the differential pair traces. If the distance between the signal lines of the differential pair is too large (more than 3W), and the distance to other signal lines is too small (Less than 3W), then the data transmission may be disrupted.

6: PCB corner routing

Sudden changes in the impedance of the signal line will cause discontinuity and therefore reflection, so avoid this discontinuity in the PCB trace. Especially when designing a high-speed signal PCB, especially when the signal rise time is ns (microsecond) level, special attention should be paid to the corner processing of the trace.

When the trace has a right-angled corner, the width and cross-sectional area of the trace at the corner increase, so additional parasitic capacitance will be generated, so the impedance will be reduced, and thus the discontinuity of the trace impedance will be generated. In the case of such a right-angle corner, two 45° or rounded corners can be used at the corner to achieve a right-angle corner. In this way, the line width and cross-sectional area of the trace can be kept the same, thereby avoiding the problem of discontinuity in impedance. As shown in Figure 4, it is the right-angle corner processing method. From the comparison in the figure, it can be seen that the round corner method is the best. Usually 45° can be applied to signals of 10GMz, and rounded corners can be applied to signals above 10GMz.