PCB News - Analysis of four grounding modes in PCB design

GND is short for the ground end of a power line. Represents the ground or zero line.

GND (Ground) on the circuit diagram and printed circuit board represents the Ground or 0 line. GND is the common end of the GND. It's a hypothetical ground for application, and for power, it's a negative pole of power. It is different from the earth. Sometimes you need to connect it to the earth, sometimes you don't, depending on the situation. The signal grounding of the equipment may be a point in the equipment or a piece of metal as the grounding reference point of the signal, which provides a common reference potential for all signals in the equipment. There are single point grounding, multi-point grounding, floating grounding and hybrid grounding. Single point grounding refers to that only one physical point in the whole circuit system is defined as the grounding reference point, and all other points to be grounded are directly connected to this point. In low frequency circuits, there is not much effect between the wiring and the components. Generally, circuits with frequencies less than 1MHz are grounded at one point.

Multi-point grounding means that each grounding point of an electronic device is directly connected to the nearest grounding plane (that is, the metal base plate of the device). In high frequency circuits, parasitic capacitance and inductance are more important. Multipoint grounding is usually used for circuits with a frequency greater than 10MHz.

Analysis of four grounding modes in PCB design

Floating ground, that is, the ground of the circuit has no conductor connection to the ground. Virtual ground: a point that is not grounded but is equipotential with ground.

Its advantage is that the circuit is not affected by geoelectric performance. Floating ground makes the isolation resistance between power ground (strong electric ground) and signal ground (weak electric ground) very large, so it prevents electromagnetic interference caused by circuit coupling of common ground impedance.

Its disadvantage is that the circuit is affected by the parasitic capacitance, which makes the circuit's ground potential change and increases the inductive interference to the analog circuit.

Earth is a very important concept in electronic technology. As there are many kinds of classification and functions of "ground", it is easy to confuse, so the concept of "ground" is summarized.

"Grounding" refers to the signal grounding inside the device and the device grounding. The two concepts are different and the purpose is different. The classical definition of "ground" is "the equipotential point or plane that serves as the reference of a circuit or system".

A: signal "ground", also known as reference "ground", is the reference point of zero potential, but also constitute the common end of the circuit signal loop.

(1) DC ground: dc circuit "ground", zero potential reference point.

(2) Ac: neutral line of ac. Should be distinguished from ground.

(3) Power ground: zero-potential reference point of high-current network devices and power amplifier devices.

(4) Analog: zero-potential reference points of amplifiers, samplers, A/D converters and comparators.

(5) Digitally: also called logically, is the zero-potential reference point of the digital circuit.

(6) "hot ground" : switching power supply without power frequency transformer, the "ground" of the switching circuit and the municipal power grid, the so-called "hot ground", it is charged.

(7) "cold ground" : because the high-frequency transformer of the switching power supply will isolate the input and output end; And because of its feedback circuit commonly used optocoupler, can not only transmit feedback signal, but also the two sides of the "ground" isolation; So the ground at the output is called the "cold ground", and it has no charge.

The signal ground

The signal grounding of the equipment may be a point in the equipment or a piece of metal as the grounding reference point of the signal, which provides a common reference potential for all signals in the equipment.

There are single point grounding, multi-point grounding, floating grounding and hybrid grounding. (Floating ground is mainly introduced here) single point grounding refers to the whole circuit system only one physical point is defined as the grounding reference point, all other points to be grounded are directly connected to this point. In low frequency circuits, there is not much effect between the wiring and the components. Generally, circuits with frequencies less than 1MHz are grounded at one point. Multi-point grounding means that each grounding point of an electronic device is directly connected to the nearest grounding plane (that is, the metal base plate of the device). In high frequency circuits, parasitic capacitance and inductance are more important. Usually used in circuits with frequencies greater than 10MHz

Multipoint grounding. Floating ground, that is, the ground of the circuit has no conductor connection to the ground. "Virtual ground: a point that is not grounded but is equipotential with ground." Its advantage is that the circuit is not affected by geoelectric performance. Floating ground makes the isolation resistance between power ground (strong electric gro

und) and signal ground (weak electric ground) very large, so it prevents electromagnetic interference caused by circuit coupling of common ground impedance. Its disadvantage is that the circuit is affected by the parasitic capacitance, which makes the circuit's ground potential change and increases the inductive interference to the analog circuit. A compromise is to connect a large discharge resistor between the floating ground and the common ground to release the accumulated charge. Pay attention to the resistance of the control release resistor, too low resistance will affect the qualification of the equipment leakage current.

1: Application of floating technology

A Ac power source is separated from DC power source

Generally, the neutral line of ac power supply is grounded. However, the zero-line potential of the power supply is not the zero potential of the earth due to the existence of grounding resistance and current flowing through it. In addition, there are often a lot of interference on the neutral line of AC power supply. If the ac source and DC source are not separated, the normal operation of DC power supply and subsequent DC circuit will be affected. Therefore, the interference from the ground line of ac power can be isolated by the floating technology that separates the source of AC power from the source of DC power.

Floating technology of B amplifier

For amplifiers, especially for small input signals and high gain amplifiers, any small interference signal at the input may lead to abnormal operation. Therefore, the floating technology of the amplifier can block the interference signal and improve the electromagnetic compatibility of the amplifier.

C Matters needing attention in floating technique

1) Try to improve the insulation resistance of the floating system to the ground, so as to reduce the common mode interference current into the floating system.

2) Pay attention to the parasitic capacitance of the floating system to the ground. High-frequency interference signals may still be coupled to the floating system through the parasitic capacitance.

3) The floating technology must be combined with electromagnetic compatibility technologies such as shielding and isolation to achieve better expected results.

4) When using the floating technology, attention should be paid to the harm of static electricity and voltage counterattack to equipment and human body.

2: mixed grounding

Mixed grounding enables the grounding system to exhibit different characteristics at low and high frequencies, which is necessary in wideband sensitive circuits. The capacitor has a high impedance to low frequency and DC, thus avoiding the formation of ground loops between the two modules. When separating THE DC ground from the RF ground, connect the DC ground of each subsystem to the RF ground through a capacitor of 10 ~ 100nF. The two ground should be connected at a point of low impedance, and the connection point should be chosen at the point where the flip rate (DI/DT) signal exists.

Two: equipment ground

In engineering practice, in addition to carefully considering the signal grounding inside the equipment, usually the signal ground of the equipment, the chassis and the earth are connected together, with the earth as the grounding reference point of the equipment. The purpose of equipment grounding is

1) Protection ground, protection grounding is to make a good electrical connection between the metal shell (or frame) and the grounding device which is not charged during normal operation of the equipment. A type of wiring designed to protect human safety. One end of the protection "ground" line is connected with the electrical shell, and the other end is reliably connected with the earth.

2) Anti-static grounding, discharge the accumulated charge on the chassis, avoid the charge accumulation to increase the potential of the chassis, resulting in unstable circuit work.

3) Shield the ground to avoid the potential change of the equipment to the earth under the action of the external electromagnetic environment, resulting in unstable equipment work.

In addition, there are lightning protection grounding and audio in the audio special ground and so on.