PCB Tech - PCB common impedance and suppression

PCB common impedance and suppression

Common resistance interference is caused by a large number of ground wires on the PCB. When two or more circuits share a ground wire, different circuit currents will produce a certain voltage drop on the shared ground wire. This voltage drop will affect the circuit performance when amplified; when the current frequency is very high, it will produce a lot of The circuit is disturbed by the large inductance.

In order to suppress common impedance interference, the following measures can be adopted:

1. Separate the ground wires of D/A digital/analog and circuit

The ground wires of the two circuits are independent of each other, and then are connected to the ground wires of the power supply terminal to prevent them from interfering with each other.

2. Grounding at multiple points nearby

There are a large number of common ground wires distributed on the edge of the PCB board, and present a semi-closed loop to prevent magnetic field interference, and all levels of circuits are grounded nearby to prevent the ground wire from being too long. It is suitable for high-frequency circuits with signal operating frequency greater than 10MHz.

3. The busbar is grounded

The bus bar is made of copper foil plated with silver, and the ground wires of all integrated circuits on the PCB are connected to the bus bar. The bus bar has the low impedance characteristics of a strip symmetrical transmission line. In high-speed circuits, it can increase the signal transmission speed and reduce interference.

4. One point grounding

The several grounding points of the unit circuits of the same level should be concentrated as much as possible to avoid the AC signals of other loops from channeling to this level, or the AC signals of this level from channeling to other loops. It is suitable for low-frequency circuits with signal working frequency less than 1MHZ. If the working frequency is 1-10MHz and one-point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength. In short, one-point grounding can eliminate the common impedance interference of the ground wire. The basic principle.

5. Large area grounding

In the high-frequency circuit board, all unused areas on the PCB are evenly distributed to the ground wire to reduce the inductive reactance in the ground wire, thereby weakening the high-frequency signal generated on the ground wire and shielding the electric field interference.

6. Bold the ground wire

If the ground wire is very thin, the ground potential will change with the current change, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Its width should be at least 3mm.

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