Analysis of four grounding modes in PCB design

GND, which refers to the shorthand of the ground of the wire. Represents ground or 0 line.

GND (Ground) on the circuit diagram and on the board represents the ground or 0 line. GND is the meaning of the common end, it can also be said to be ground, but this place is not a true ground. It is a ground that is assumed for the application. For the power supply, it is the negative pole of a power supply. It is different from the earth. Sometimes it needs to be connected to the earth, sometimes not, depending on the situation.

The signal ground of the device may be a ground reference point for the signal at a point or piece of metal in the device, which provides a common reference potential for all signals in the device.

There are single point grounding, multi-point grounding, floating ground and mixed grounding.

Single-point grounding means that only one physical point in the entire circuit system is defined as a ground reference point, and other points that need to be grounded are directly connected to this point. In low frequency circuits, there is not much influence between the wiring and the components. Generally, circuits with a frequency less than 1MHz are grounded at one point.

Multi-point grounding means that each grounding point in an electronic device is directly connected to the ground plane closest to it (ie, the metal backplane of the device). In high-frequency circuits, the effects of parasitic capacitance and inductance are large. Generally, circuits with a frequency greater than 10 MHz are often grounded at multiple points.

Floating, that is, the ground of the circuit is connected to the earth without a conductor. Virtual ground: A point that is not grounded but equipotential to ground.

The advantage is that the circuit is unaffected by the electrical performance of the earth. The floating ground can make the isolation resistance between the power ground (strong ground) and the signal ground (weak ground) large, so it can prevent the electromagnetic interference generated by the common impedance circuit coupling.

The disadvantage is that the circuit is susceptible to parasitic capacitance, which causes the ground potential of the circuit to fluctuate and increases the inductive interference to the analog circuit.

“Ground” is a very important concept in electronic technology. Since the classification and function of “land” are various and easy to be confused, the concept of “ground” is summarized.

“Grounding” has the signal grounding inside the device and the grounding of the device. The two concepts are different and the purpose is different. The classic definition of "ground" is "equipotential point or plane as a circuit or system reference."

One: The signal "ground", also known as the reference "ground", is the reference point of zero potential and is also the common end of the circuit signal loop.

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

(2) Exchange place: the neutral line of AC power. Should be distinguished from the ground.

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

(4) Analog ground: Zero potential reference point for amplifier, sample-and-hold, A/D converter and comparator.

(5) Digital ground: Also called logical ground, it is the zero potential reference point of the digital circuit.

(6) “Hot ground”: The switching power supply does not need to use a power frequency transformer. The “ground” of the switching circuit is related to the mains power grid, so-called “hot ground”, which is charged.

(7) “Cold ground”: Since the high-frequency transformer of the switching power supply isolates the input and output terminals; and because the feedback circuit is commonly used with a photocoupler, it can transmit the feedback signal and isolate the “ground” of both sides; It is called "cold land" and it is not charged.

Signal ground

The signal ground of the device may be a ground reference point for the signal at a point or piece of metal in the device, which provides a common reference potential for all signals in the device.

There are single point grounding, multi-point grounding, floating ground and mixed grounding. (The main introduction to floating ground here) Single point grounding means that only one physical point in the whole circuit system is defined as the ground reference point, and other points that need to be grounded are directly connected to this point. In low frequency circuits, there is not much influence between the wiring and the components. Generally, circuits with a frequency less than 1MHz are grounded at one point. Multi-point grounding means that each grounding point in an electronic device is directly connected to the ground plane closest to it (ie, the metal backplane of the device). In high-frequency circuits, the effects of parasitic capacitance and inductance are large. Generally, circuits with a frequency greater than 10 MHz are often used.

Multi-point grounding. Floating, that is, the ground of the circuit is connected to the earth without a conductor. 『Virtual ground: A point that is not grounded but equipotential to earth. The advantage is that the circuit is not affected by the electrical performance of the earth. The floating ground can make the isolation resistance between the power ground (strong ground) and the signal ground (weak ground) large, so it can prevent the electromagnetic interference generated by the common impedance circuit coupling. The disadvantage is that the circuit is susceptible to parasitic capacitance, which causes the ground potential of the circuit to fluctuate and increases the inductive interference to the analog circuit. A compromise is to connect a large bleeder resistor between the floating ground and the common ground to release the accumulated charge. Pay attention to the impedance of the control release resistor. Too low a resistor will affect the eligibility of the device leakage current.

1: Application of floating technology

a AC power ground and DC power ground

Generally, the neutral line of the AC power supply is grounded. However, due to the presence of the grounding resistance and the current flowing therethrough, the zero line potential of the power supply is not zero potential of the earth. In addition, there are often many interferences on the neutral line of the AC power supply. If the AC power ground is not separated from the DC power supply, it will affect the normal operation of the DC power supply and subsequent DC circuits. Therefore, the floating technology that separates the AC power ground from the DC power ground can isolate interference from the AC power ground.

b Amplifier floating technology

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

c Floating technology considerations

1) Try to improve the insulation resistance of the floating system to the ground, which is beneficial to reduce the common mode interference current entering the floating system.

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

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

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

2: Hybrid grounding

Hybrid grounding makes the grounding system exhibit different characteristics at low frequencies 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 the DC ground and the RF ground are separated, the DC ground of each subsystem is connected to the RF ground through a capacitor of 10 to 100 nF. The two grounds should be connected at a low impedance, and the connection point should be selected at the highest flip speed ( Di/dt) The point at which the signal is present.

Two: equipment connected to the earth

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

1) Protective ground, protective grounding is a good electrical connection between the metal casing (or frame) that is not charged when the equipment is in normal operation and the grounding device. A wiring method set to protect personnel safety. One end of the protection "ground" line is connected to the electrical enclosure, and the other end is reliably connected to the earth.

2) Anti-static grounding, venting the accumulated charge on the chassis, avoiding charge accumulation and increasing the potential of the chassis, resulting in unstable circuit operation.

3) Shielding the ground to prevent the device from changing the potential of the earth to the earth under the action of the external electromagnetic environment, resulting in unstable operation of the device.

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