Analog ground/digital ground and analog power/digital power are just relative concepts. The main reason for proposing these concepts is that the interference of digital circuits to analog circuits has reached an intolerable level.
The current standard approach is as follows:
1. The ground wire is divided into two after rectifying and filtering, one of which is used as the analog ground, and all the circuit parts of the analog part are connected to the analog ground; the other is the digital ground, and all the circuit parts of the digital part are connected. Go to this digital ground.
2. The DC power supply voltage regulator chip is output, and after filtering, it is also divided into two. One of them is filtered by LC/RC and used as an analog power supply. All the circuit power supplies of the analog part are connected to the analog power supply; the other is a digital The power supply, all the digital part of the circuit power supply is connected to this digital power supply.
Note: Analog ground/digital ground and analog power/digital power supplies can be connected in any way except for a little connection at the beginning of the power supply.
AVCC: Analog part power supply; AGND: analog ground
DVCC: digital part power supply; DGND: digital ground
This distinction is made to isolate the digital portion from the analog portion, reducing the interference that the digital portion brings to the analog circuit portion. However, the two parts cannot be completely isolated. The digital part and the analog part are connected. Therefore, they should be at least connected together when power is supplied. Therefore, a 0 ohm resistor or magnetic bead should be used between AGND and DGND. The inductors are connected so that a single connection reduces interference. Similarly, if the two parts of the power supply are the same, this connection should be used.
In the design of electronic systems, in order to avoid detours and save time, the anti-interference requirements should be fully considered and avoided, and remedial measures against interference should be avoided after the design is completed.
There are three basic elements that form interference:
(1) Interference source refers to the component, device or signal that generates interference. It is described in mathematical language as follows: du/dt, where di/dt is large, it is the source of interference. Such as: lightning, relays, thyristors, motors, high-frequency clocks, etc. may become sources of interference.
(2) Propagation path refers to the path or medium that interferes with the propagation from the interference source to the sensitive device. Typical interference propagation paths are conduction through the wires and radiation from the space.
(3) Sensitive devices refer to objects that are easily disturbed. Such as: A / D, D / A converter, microcontroller, digital IC, weak signal amplifier. The basic principle of anti-interference design is to suppress the interference source, cut off the interference propagation path, and improve the anti-interference performance of sensitive devices.
1 suppression interference source
To suppress the interference source is to reduce the du/dt, di/dt of the interference source as much as possible. This is the most important and most important principle in anti-jamming design, and it often has a multiplier effect. Reducing the du/dt of the interference source is mainly achieved by connecting capacitors across the interference source. Reducing the di/dt of the interferer is achieved by connecting the inductor or resistor in series with the source loop and adding a freewheeling diode.
Common measures to suppress interference sources are as follows:
(1) The relay coil increases the freewheeling diode to eliminate the back EMF interference generated when the coil is disconnected. Adding a freewheeling diode will delay the turn-off time of the relay. After the Zener diode is added, the relay can move more times per unit time.
(2) Connect the spark suppression circuit at both ends of the relay contact (usually RC series circuit, the resistance is generally selected from a few K to tens of K, and the capacitor is selected as 0.01uF) to reduce the influence of spark.
(3) Add a filter circuit to the motor, pay attention to the capacitor and inductor leads as short as possible.
(4) Each IC on the board should be connected with a high frequency capacitor of 0.01μF ~ 0.1μF to reduce the impact of the IC on the power supply. Pay attention to the wiring of high-frequency capacitors. The wiring should be close to the power supply terminal and be as short and as short as possible. Otherwise, it will increase the equivalent series resistance of the capacitor, which will affect the filtering effect.
(5) Avoid 90-degree fold lines during wiring and reduce high-frequency noise emission.
(6) The RC suppression circuit is connected to both ends of the thyristor to reduce the noise generated by the thyristor (this thyristor may break down when the noise is severe).
According to the propagation path of interference, it can be divided into two types: conducted interference and radiated interference.
Conducted interference is the interference that propagates through a wire to a sensitive device. The high-frequency interference noise and the frequency band of the useful signal are different, and the propagation of the high-frequency interference noise can be cut off by adding a filter to the wire, and sometimes the isolation optocoupler can be added. Power supply noise is the most harmful, so pay special attention to handling. Radiation interference refers to interference that propagates through a spatial radiation to a sensitive device. The general solution is to increase the distance between the interferer and the sensitive device, isolate them with ground and add a shield to the sensitive device.
2 Common measures to cut off the interference propagation path are as follows:
(1) Fully consider the impact of the power supply on the microcontroller. The power supply is well done, and the anti-interference of the entire circuit is solved. Many single-chip microcomputers are very sensitive to power supply noise. It is necessary to add a filter circuit or a voltage regulator to the power supply of the single-chip microcomputer to reduce the interference of the power supply noise on the single chip. For example, a magnetic bead and a capacitor can be used to form a π-shaped filter circuit. Of course, a 100Ω resistor can be used instead of the magnetic bead when the condition is not high.
(2) If the I/O port of the MCU is used to control noise devices such as motors, isolation should be added between the I/O port and the noise source (increasing the π-shaped filter circuit). Control noise devices such as motors, and add isolation between the I/O port and the noise source (increasing the π-shaped filter circuit).
(3) Pay attention to the crystal wiring. The crystal oscillator and the MCU pins are placed as close as possible, and the clock region is isolated by the ground wire. The crystal oscillator case is grounded and fixed. This measure can solve many difficult problems.
(4) Reasonable division of the circuit board, such as strong and weak signals, digital and analog signals. Keep interference sources (such as motors, relays) away from sensitive components (such as microcontrollers) as much as possible.
(5) Separate the digital zone from the analog zone with a ground wire, separate the digital ground from the analog ground, and finally connect it to the power ground at one point. A/D and D/A chip wiring are also based on this principle. This requirement has been considered when the manufacturer assigns A/D and D/A chip pinouts.
(6) The ground wire of the MCU and the high-power device should be grounded separately to reduce mutual interference. Place high-power devices on the edge of the board as much as possible.
(7) Anti-interference components such as magnetic beads, magnetic rings, power supply filters, and shields can be used in key areas such as I/O ports, power lines, and circuit board connectors to significantly improve the anti-jamming performance of the circuit.
3 Improve the anti-jamming performance of sensitive devices
Improving the anti-interference performance of sensitive devices refers to the method of minimizing the pickup of interference noise and recovering from an abnormal state as soon as possible from the sensitive device side.
Common measures to improve the anti-jamming performance of sensitive devices are as follows:
(1) Minimize the loop loop area during wiring to reduce induced noise.
(2) When wiring, the power and ground wires should be as thick as possible. In addition to reducing the voltage drop, it is more important to reduce the coupling noise.
(3) For the idle I/O port of the MCU, do not hang it, ground it or connect it to the power supply. The idle ends of other ICs are grounded or connected to the power supply without changing the system logic.
(4) The power supply monitoring and watchdog circuit for the single-chip microcomputer, such as IMP809, IMP706, IMP813, X25043, X25045, etc., can greatly improve the anti-interference performance of the whole circuit.
(5) Under the premise that the speed can meet the requirements, try to reduce the crystal oscillator of the single-chip microcomputer and select the low-speed digital circuit.
(6) The IC device is soldered directly on the circuit board as much as possible, and the IC holder is used less.
In order to achieve good anti-interference, we often see the grounding of the PCB board. But not all digital circuits and analog circuits must be ground plane split. Because this is divided to reduce noise interference.
Theory: In digital circuits, the general frequency will be higher than the frequency in the analog circuit, and their own signals will form a reflow with the ground plane (because in the signal transmission, there are various kinds of copper and copper wires. Such inductance and distributed capacitance), if we mix the ground lines together, then this reflow will crosstalk in the digital and analog circuits. And we are separated so that they form a reflow only within themselves. They are only connected by a zero-ohm resistor or a magnetic bead because they are the same physical meaning. Now the wiring separates them and finally they should be connected.
How do you analyze whether they belong to the digital part or the analog part?
This problem is often the result of filtering when we draw PCBs. My personal opinion is to judge whether a component is analog or whether the key to the digital is whether the main chip associated with it is digital or analog. For example: the power supply may supply power to the analog circuit, then it is the analog part, if it is to supply power to the microcontroller or data chip, then it is digital. When they are the same power supply, a bridge is needed to bring one power supply from another. The most typical is D/A, it should be one half is a number, and half is a simulated chip. I think that if you can process the digital input, the rest can be drawn to the simulation part.
Analog circuits involve weak signals, but digital circuit threshold levels are higher, and power requirements are lower than analog circuits. In systems with both digital and analog circuits, the noise generated by the digital circuit can affect the analog circuit, making the small signal specifications of the analog circuit worse. The solution is to separate the analog ground and the digital ground.
For low-frequency analog circuits, in addition to thickening and shortening the ground line, the use of one-point grounding in each part of the circuit is the best choice to suppress ground-line interference, mainly to prevent mutual interference between components due to the common impedance of the ground line.
For high-frequency circuits and digital circuits, since the influence of the inductance of the ground wire will be greater at this time, the grounding at one point will cause the actual ground wire to lengthen and adversely affect. In this case, a combination of separate grounding and one-point grounding should be adopted.
In addition, for high-frequency circuits, it is also necessary to consider how to suppress high-frequency radiation noise by: thickening the ground line as much as possible to reduce the noise-to-ground impedance; full grounding, that is, except for the printed line of the transmitted signal, other parts are all used as ground lines. . Do not have a large area of ​​copper foil that is useless.
The ground wire should form a loop to prevent high-frequency radiation noise, but the area enclosed by the loop should not be too large to avoid induced current when the instrument is in a strong magnetic field. However, if it is only a low frequency circuit, the ground loop should be avoided. The digital power supply and the analog power supply are preferably isolated, and the ground lines are arranged separately. If there is an A/D, it is only a single point here.
There is not much impact in the low frequency, but it is recommended that the analog and digital grounds be grounded at one point. At high frequencies, the analog and digital grounds can be shared by magnetic beads.
If the analog ground and the digital ground are directly connected to each other, it will cause mutual interference. It is not short-circuited and not appropriate. There are four ways to solve this problem as above:
1. Connected with magnetic beads;
2. Connect with a capacitor;
3. Connect with an inductor;
4. Connect with a 0 ohm resistor.
The equivalent circuit of the magnetic bead is equivalent to the band-stopper, which only significantly suppresses the noise of a certain frequency point. When using it, it is necessary to estimate the noise frequency in advance so that the appropriate model can be selected. For frequencies that are uncertain or unpredictable, the beads do not match.
The capacitor is connected straight to the ground, causing floating.
The inductor is bulky, has many stray parameters, and is unstable.
The 0 ohm resistor is equivalent to a very narrow current path, which effectively limits the loop current and suppresses noise. The resistor has an attenuation in all frequency bands (0 ohm resistor also has impedance), which is stronger than the magnetic beads.
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