An introduction to the design of grounding problem in PCB

- Dec 18, 2017-

Analog / digital and analog power / digital power are only relative concepts. The main reason for these concepts is that the interference of digital circuits to analog circuits has been intolerable.

1., the ground wire is divided into 2 parts after rectification and filtering. One of them is the analog ground, and all the analog parts of the circuit are all received on the analog ground. The other one is digitally, all the digital parts of the circuit are all connected to the number field.

2. DC power supply voltage regulator chip, the filter is also divided into 2, one after the LC/RC filter as the analog power, all analog circuit parts all received the analog power above; the other one is the digital power supply, power supply circuit of all digital part all received the above digital power supply

In the design of electronic system, in order to avoid detours and save time, we should fully consider and meet the requirements of anti-interference, and avoid remedial measures after the design is completed. There are three basic elements for the formation of interference.

(1) the source of interference refers to the components, devices, or signals that produce interference, which are described in the mathematical language as follows: du/dt, the large place of di/dt is the source of interference. Such as: lightning, relay, silicon controlled, motor, high frequency clock, and so on may be the source of interference.

(2) the path of propagation refers to the channel or medium that interferes from the source of the interference to the sensitive device. The typical path of interference propagation is through the conduction of the wire and the radiation of the space.

(3) sensitive devices, which refer to objects that are easily interfered. Such as: A/D, D/A converter, MCU, digital IC, weak signal amplifier and so on. The basic principle of anti-jamming design is to suppress the interference source, cut off the interference propagation path, and improve the anti-interference performance of the sensitive devices.

Suppressing interference source

The source of interference suppression is to minimize the interference source du/dt, di/dt., which is the most important and most important principle in the design of anti-jamming. It will often get twice the result with half the effort. The du/dt to reduce the interference source is realized mainly by the shunt capacitance at both ends of the interference source. The di/dt, which reduces the interference source, is realized by the series inductor or resistance in the interference source circuit and the increase of the continuous current diode.

The commonly used measures to suppress interference sources are as follows:

(1) the relay coil increases the continuous current diode to eliminate the anti EMF interference when the coil is disconnected. The interrupting time of the relay is delayed only by adding a continuous current diode, and the relay can move more times in unit time after increasing the voltage regulator diode.

(2) at the two ends of the relay contact, the spark suppression circuit (usually RC series circuit, resistors usually choose K to dozens of K, capacitance 0.01uF), reduce the effect of EDM.

(3) add the filter circuit to the motor, and pay attention to the capacitance and inductance of the wire as short as possible.

(4) each IC on the circuit board should be connected to a 0.01 - F - 0.1 - F high frequency capacitor to reduce the impact of IC on the power supply. Pay attention to the wiring of high frequency capacitance. The connection should be close to the power supply and try to be as short as possible. Otherwise, it will increase the equivalent series resistance of the capacitor and affect the filtering effect.

(5) avoid 90 degree fold line and reduce high frequency noise emission when wiring.

(6) the silicon controlled silicon at both ends of the SCR and the RC suppression circuit to reduce the noise produced by the silicon controlled silicon (the SCR may be penetrated when the noise is serious).

The propagation path of interference can be divided into two types: conduction interference and radiation interference.

The so-called conduction interference refers to the interference of transmission through a wire to a sensitive device. The frequency band of high frequency interference noise and useful signal is different, and the propagation of high-frequency interference noise can be cut off by adding filters on the wire, and sometimes isolated optocoupler can be used to solve it. The damage of the power noise is the greatest, and special attention should be paid to the treatment. The so-called radiation interference refers to the interference of transmission through space radiation to sensitive devices. The general solution is to increase the distance between the interference source and the sensitive device, isolating them with ground wire and adding a mask on the sensitive device.

The common measures to cut off the interference propagation path are as follows:

(1) give full consideration to the influence of power supply on single chip microcomputer. The power of the power is well done, and the anti-interference of the whole circuit is about half. Many singlechips are sensitive to the noise of the power supply. It is necessary to add a filter circuit or a voltage regulator to the power supply to reduce the interference of the power noise to the single chip. For example, magnetic beads and capacitors can be used to form a pion filter circuit. Of course, 100 Omega resistors can be used instead of magnetic beads when the conditions are not high.

(2) if the I/O port of the single chip microcomputer is used to control the noise components of the motor, it should be isolated between the I/O port and the noise source (adding the pion filter circuit). Control motor and other noise devices should be isolated between the I/O port and the noise source (adding a pion filter circuit).

(3) pay attention to crystal oscillator wiring. The crystal oscillator is as close as the MCU pin, isolating the clock area with the ground wire, and the crystal oscillator shell is grounded and fixed. This measure can solve many difficult problems.

(4) reasonable zoning of circuit boards, such as strong and weak signals, digital and analog signals. As far as possible, the interference sources (such as motor, relay) and sensitive elements (such as single chip) are far away.

(5) the digital area is isolated from the analog area by the ground line, and the digital area is separated from the analog ground, and at the end it is connected to the power source. A/D, D/A chip wiring is also based on this principle, manufacturers distribution A/D, D/A chip pin arrangement has taken into account this requirement.

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