Common impedance and rejection

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 generate a certain voltage drop on the common ground wire. This voltage drop will affect the circuit performance after being amplified; when the current frequency is high, it will produce a very large voltage drop. The large inductive reactance causes the circuit to be disturbed.

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

(1) One point ground

Make the several ground points of the same level unit circuit as concentrated as possible to avoid the AC signals of other circuits from entering this stage, or the AC signals of this stage from entering other circuits. It is suitable for low-frequency circuits with a signal operating frequency less than 1MHZ. If the operating frequency is between 1 and 10MHz and a point ground is used, the length of the ground wire should not exceed 1/20 of the wavelength. In short, a point ground is to eliminate the common impedance interference of the ground The basic principle.

(2) Multi-point grounding nearby

A large number of common ground wires are distributed on the edge of the PCB and present a semi-closed loop (anti-magnetic field interference). The circuits at all levels should be 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) Bus grounding

The busbar is made of silver-plated copper foil, and the ground wires of all integrated circuits on the PCB are connected to the busbar. The bus bar has the low impedance characteristic of the bar-shaped 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 set as ground wires to reduce the inductive reactance in the ground wire, thereby weakening the high-frequency signals generated on the ground wire and shielding the electric field interference.

(5) Bold ground wire

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

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

The ground wires of the two circuits are independent, and then connected to the power supply ground wires to suppress their mutual interference.