In circuit design, what are differences between six types of grounds?
Types and functions of ground wire
1. Analog ground wire AGND
AGND analog ground wire is mainly used in analog circuit part, such as analog sensor ADC data acquisition circuit, working gain circuit, etc.
In these analog circuits, because signal is an analog signal, it is a small signal that is easily affected by high current of other circuits. If no distinction is made, high current will cause a large voltage drop in analog circuit, which will distort analog signal and can seriously disrupt operation of analog circuit.
2. Digital ground wire DGND
DGND digital ground wire, obviously compared with AGND analog ground wire, is mainly used for digital circuit parts, such as key detection circuit, USB communication circuit, single-chip microcomputer circuit and so on.
The reason for installing a DGND digital ground wire is because digital circuits have one thing in common, they are all discrete switching signal, and only "0" and "1" are different.
During transition from digital voltage "0" to digital voltage "1" or from digital voltage "1" to digital voltage "0", voltage changes according to Maxwell electromagnetic field. Theoretically, a magnetic field will be created around changing current, which will form electromagnetic radiation for other circuits.
Under no circumstances should a separate DGND digital ground wire be used to effectively isolate other circuits to reduce effects of electromagnetic radiation from a circuit.
3. Power ground wire PGND
Whether analog ground wire AGND or digital ground wire DGND, they are all low power circuits. Large power circuits such as motor drive circuits, solenoid valve drive circuits, etc. also have a separate reference ground called PGND power ground.
Power circuits, as name implies, are circuits with relatively large currents. Apparently, large currents can easily cause ground shifts between different functional circuits.
(Earth Shift Phenomenon)
Because there is a ground bias in circuit, original 5V voltage may not be 5V, but become 4V. Since 5V voltage refers to GND 0V ground wire, if ground bias increases voltage of GND ground wire from 0V to 1 V, then previous voltage 5V (5V-0V=5V) becomes current 4V (5V-1V=4V). too much.
4. Ground GND
Analog ground wire AGND, digital ground wire DGND and power ground wire PGND are classified as DC ground wire GND. These different types of ground wires are eventually combined into a 0V reference ground wire. This ground wire is called GND power ground wire.
The power supply is source of power for all circuits. The voltage and current required to operate all circuits are supplied by power supply. Therefore, ground wire GND of power supply is 0 V reference point for all circuits.
This is why other types of ground, whether analog ground AGND, digital ground DGND, or power ground PGND, must be connected to power ground GND at end.
5. AC ground wire CGND
The CGND AC ground wire is typically present in circuit elements containing AC power, such as AC to DC power circuits.
The AC-DC power circuit is divided into two parts. The front stage in circuit is AC part and back stage in circuit is DC part, resulting in two ground wires: one is AC ground wire and other is DC ground wire.
The AC ground wire is used as 0V reference point of AC circuit, and DC ground wire is used as 0V reference point of DC circuit. Typically, to combine GND ground wire in circuits, engineers connect AC ground wire to DC ground wire through a decoupling capacitor or inductor.
The safety voltage of human body is below 36V. If a voltage greater than 36V is applied to human body, it will cause damage to human body. This is common sense for engineers when developing and designing circuit solutions. .
In order to increase circuit's safety factor, engineers commonly use EGND ground wire in high voltage and high current projects such as household appliances such as electric fans, refrigerators and televisions.
(Socket with EGND protection function)
Why 3 pins for a household appliance socket? For 220V AC, only need a live wire and a neutral wire, two are ok. So why nest 3 terminals?
Three socket terminals, two of which are used for live wire and 220V neutral wire, and other terminal is EGND ground wire for protection.
It is important to note that EGND ground wire is connected to our earth only for high voltage protection, it does not participate in circuit function of project and has nothing to do with function of circuit.
There is therefore an obvious difference in circuit assignment between EGND ground wire and other types of GND ground wires.
GND line separation value
Engineers may ask, how can there be so many differences in GND ground wire, and how can a simple circuit problem be so complex? Why do you need to introduce so many separate GND functions?
For this kind of ground wire design problem, engineers usually refer to it as simply GND, which is not distinguished during circuit design process, making it difficult to effectively identify GND ground wires with different circuit functions in PCB layout. Just connect all GND wires together directly.
While this is easy to do, it will cause a number of problems:
Problem 1: Signal Crosstalk
If GND ground wires with different functions are directly connected to each other, a high power circuit passing through GND ground wire will affect 0 V GND reference point of low power circuit, creating crosstalk between signals of different circuits.
Question 2. Signal accuracy
Analog circuit, main indicator of its evaluation is accuracy of signal. Without precision, analog circuit loses its original functional meaning.
Because ground wire CGND of AC power supply is a sine wave, it periodically oscillates up and down, and its voltage also fluctuates up and down, instead of always maintaining a constant 0 V, as on DC ground wire GND.
Connect GND wires of different circuits together. Periodically changing AC ground wire CGND will cause analog ground wire AGND to change, which will affect analog signal voltage accuracy value.
Question 3. Electromagnetic compatibility experiment
The weaker signal, weaker EMC of external electromagnetic radiation; stronger signal, stronger EMC of external electromagnetic radiation.
If GND wires of different circuits are connected together, GND ground wire of strong signal circuit will directly interact with GND wire of weak signal circuit. The consequence of this isthe fact that initially weak EMC electromagnetic radiation has also become a signal source with strong external electromagnetic radiation, which complicates processing of EMC experiment scheme.
Question 4. Chain reliability
The smaller signal coupling between systems of a circuit, greater ability of circuit to operate independently; greater signal coupling, weaker circuit's ability to operate independently.
Just imagine, if there is no intersection between two circuits A and circuits B, function of circuits A obviously cannot affect normal operation of circuits B, and function of circuits B cannot affect normal operation either. Normal operation of contour system A.
It's like a couple of strange men and women. Before they become lovers, girl's emotional changes won't affect boy's mood because they don't have an intersection.
If in a system of grounding circuits circuits with different functions are connected together, this is tantamount to an increase in interference link between circuits, that is, a decrease in reliability of circuit.
- What is drowning in gold? Why Shen Jin?
- This is a metaphor for EMI/EMS/EMC that can be understood at a glance.
- How many types of pads have you seen in PCB design?
- Summary of Common PCB Repair Techniques
- What is three anti-paint? How to use it correctly?
- Knowing these rules, you will not get confused looking at circuit diagram.
- How to make anti-interference PCB design?
- Can diodes do this?