Common impedance and suppression of advanced PCB design

Common impedance and suppression of advanced PCB design

PCB online: Common resistance interference is caused by a large number of ground wires on the PCB. When two or more loops share a section of ground wire, different loop 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 high, it will produce a very high voltage drop. The circuit is disturbed by the large inductance.

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

(1) One point grounding

Make the several grounding points of the unit circuit of the same level as concentrated as possible to avoid the AC signals of other loops from fleeing into this class, or the AC signals of this class fleeing into 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.

(2) Grounding at multiple points nearby

There are a large number of common ground wires on the PCB that are distributed on the edge of the board and present a semi-closed loop (anti-magnetic field interference). 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 bus bar 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) Large area grounding

In the high-frequency circuit, all unused areas on the PCB are evenly distributed as 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.

(5) Thicken 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.

(6) The ground wire of D/A (digital/analog) circuit is separated

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 suppress their mutual interference.