Summary of grounding problems of PCB design proofing in Xi ‘an

In Xi ‘an PCB design proofing, the concepts of analog ground/digital ground and analog power supply/digital power supply are often mentioned by engineers. In fact, these concepts are only relative, and the premise of their existence is that the interference of digital circuits to analog circuits has reached a certain level. According to Xiaobian, there are two standard solutions at present: one is that the ground wire is divided into two after rectification and filtering, and one of them is used as analog ground, and all circuits of all analog parts are connected to this analog ground. The other one is a digital ground, and the circuit grounds of all digital parts are all connected to this digital ground. The second method is that the DC power supply voltage stabilizing chip comes out, and after filtering, it is also divided into two pieces, one of which is used as an analog power supply after LC/RC filtering, and all circuit power supplies of all analog parts are connected to this analog power supply; And the other one is a digital power supply, and all circuit power supplies of all digital parts are connected to this digital power supply.
Based on the above problems, it should be noted that the analog ground/digital ground and the analog power supply/digital power supply can’t be connected except at the beginning of the power supply.
AVCC: analog part of the power supply; AGND: simulated ground
DVCC: digital part of the power supply; DGND: Digital ground
The purpose of this distinction is to separate the digital part from the analog part and reduce the interference of the digital part to the analog circuit part. However, these two parts can’t be completely isolated. The digital part and the analog part are connected, so they should at least be together when supplying power. Therefore, the AGND and DGND should be connected with a 0 ohm resistor or magnetic bead or inductor. Such a little connection can reduce the interference. Similarly, if the power supply of the two parts is the same, this connection should be adopted.
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 taking remedial measures against anti-interference after the design is completed. There are three basic elements that form interference:
(1) Interference source, that is, the components, equipment or signals that produce interference, is described in mathematical language as follows: du/dt, where di/dt is large is the interference source. For example, lightning, relay, thyristor, motor, high-frequency clock, etc. can all become interference sources.
(2) Propagation path is the channel or medium through which interference spreads from interference source to sensitive devices. Typical interference propagation paths are conduction through wires and radiation in space.
(3) Sensitive devices, objects that are easily disturbed. For example: A/D, D/A converter, single chip microcomputer, digital IC, weak signal amplifier, etc. The basic principle of anti-interference design is to suppress interference sources, cut off interference propagation paths and improve the anti-interference performance of sensitive devices. (similar to the prevention of infectious diseases)
Then, how to suppress the interference source in the design of power supply circuit becomes the primary problem:
To suppress the interference source is to reduce the du/dt and di/dt of the interference source as much as possible. This is the best first consideration and the most important principle in anti-jamming design, and it often gets twice the result with half the effort. Reducing the du/dt of the interference source is mainly achieved by connecting capacitors in parallel at both ends of the interference source, while reducing the di/dt of the interference source is achieved by connecting inductors or resistors in series with the interference source loop and adding freewheeling diodes.
Common measures to suppress interference sources are as follows:
(1) The free-wheeling diode is added to the relay coil to eliminate the back EMF interference generated when the coil is disconnected. Only adding freewheeling diode will delay the turn-off time of relay, and after adding voltage stabilizing diode, relay can act more times per unit time.
(2) Connect spark suppression circuits (generally RC series circuits, with resistance of several to several tens of K and capacitance of 0.01uF) at both ends of relay contacts in parallel to reduce the influence of electric spark;
(3) Add filter circuit to the motor, pay attention to the capacitor and inductor leads to be as short as possible;
(4) Each IC on the circuit board should be connected in parallel with a high-frequency capacitor of 0.01μ f ~ 0.1μ f to reduce the influence of IC on the power supply. Pay attention to the wiring of high-frequency capacitors. The wiring should be close to the power supply end 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 line during wiring to reduce high-frequency noise emission;
(6) Connect the RC suppression circuit at both ends of the thyristor in parallel to reduce the noise generated by the thyristor (this noise may break down the thyristor when it is serious).
According to the propagation path of interference, it can be divided into conduction interference and radiation interference. Conducted interference refers to the interference that propagates to sensitive devices through wires. The frequency band of high-frequency interference noise is different from that of useful signal, so the transmission of high-frequency interference noise can be cut off by adding a filter on the wire, and sometimes it can be solved by adding an isolation optocoupler. Noise is the most harmful, so special attention should be paid to it. Radiation interference refers to the interference that propagates to sensitive devices through space radiation. The general solution is to increase the distance between interference sources and sensitive devices, isolate them with ground wires and cover the sensitive devices.
Common measures to cut off the propagation path of interference are as follows:
(1) Fully consider the influence of power supply on single chip microcomputer. If the power supply is done well, the anti-interference of the whole circuit will be solved by more than half. 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 power supply noise on the single chip. For example, a π-shaped filter circuit can be composed of magnetic beads and capacitors. Of course, a 100 Ω resistor can be used instead of magnetic beads when conditions are not high.
(2) Pay attention to crystal oscillator wiring. The crystal oscillator is as close as possible to the pin of the single chip microcomputer, and the clock area is isolated by the ground wire, and the crystal oscillator shell is grounded and fixed. This measure can solve many difficult problems;
(3) Reasonable division of circuit board, such as strong and weak signals, digital and analog signals. Try to keep interference sources (such as motors and relays) away from sensitive components (such as single chip microcomputer) as much as possible;
(4) Separate the digital area from the analog area with a land line, separate the digital ground from the analog ground, and finally connect to the power supply ground at one point. The wiring of A/D and D/A chips is also based on this principle, and this requirement has been taken into account when the manufacturer allocates the pin arrangement of A/D and D/A chips;
(5) The grounding wires of single-chip microcomputer and high-power devices should be grounded separately to reduce mutual interference. High-power devices should be placed on the edge of the circuit board as much as possible;
(6) If I/O port of single chip microcomputer is used to control noise devices such as motor, isolation should be added between I/O port and noise source (π-shaped filter circuit should be added). Noise devices such as motor control should be isolated between I/O port and noise source (adding π-shaped filter circuit);
(7) Anti-interference components such as magnetic beads, magnetic rings, power filters and shielding covers are used in key places such as I/O port of single chip microcomputer, power cord and circuit board connecting line, which can significantly improve the anti-interference performance of the circuit.
Improving the anti-interference performance of sensitive devices can be understood as the way to minimize the picking up of interference noise and recover from abnormal state as soon as possible. Common measures include:
(1)IC devices should be directly welded on the circuit board as far as possible, and IC seats should be used less.
(2) Minimize the area of the loop during wiring to reduce the induced noise;
(3) When wiring, the power cord and ground wire should be as thick as possible. Besides reducing the voltage drop, it is more important to reduce the coupling noise.
(4) For the idle I/O port of the single chip microcomputer, don’t hang up, but connect to the ground or power supply. The idle terminals of other IC are grounded or connected to the power supply without changing the system logic;
(5) Using power supply monitoring and watchdog circuits, such as IMP809, IMP706, IMP813, X25043, X25045 and so on, can greatly improve the anti-interference performance of the whole circuit;
(6) On the premise that the speed can meet the requirements, try to reduce the crystal oscillator of single chip microcomputer and choose low-speed digital circuit;
In order to achieve good anti-interference, it is common to see the wiring mode of ground division on PCB, but not all digital circuits and analog circuits must be ground division. Because this segmentation is to reduce the interference of noise.
The reason for the analysis is that the general frequency in digital circuits is higher than that in analog circuits, and their own signals will form a backflow with the ground plane (because there are various inductances and distributed capacitances between copper wires in signal transmission). If the ground wires are mixed together, the backflow will cross talk with each other in digital and analog circuits, and the need to separate them is to make them form a backflow only within themselves. They are only connected by a zero ohm resistor or magnetic bead because they are the same physical ground. Now the wiring separates them, and finally they should be connected.
How to analyze whether they belong to digital part or analog part? This problem is often tested when we draw PCB. If you want to judge whether a component is analog or digital, the key is whether the main chip related to it is digital or analog. For example, the power supply may supply power to the analog circuit, so it is the analog part, and if it supplies power to a single-chip microcomputer or a data chip, it is digital. When they are the same power supply, it is necessary to use a bridge method to lead one power supply from another part. The most canonical one is D/A, which should be a chip that is half digital and half analog.
Analog circuits involve weak signals, but the threshold level of digital circuits is higher, so the requirement of power supply is lower than that of analog circuits. In the system with both digital circuits and analog circuits, the noise generated by digital circuits will affect analog circuits, making the small signal index of analog circuits worse. The way to overcome this problem is to separate analog ground from digital ground.
For low-frequency analog circuits, in addition to thickening and shortening the ground wire, it is the best choice to use a point grounding in all parts of the circuit to suppress the ground wire interference, which can mainly prevent the mutual interference between components caused by the common impedance of the ground wire.
For high-frequency circuits and digital circuits, because the inductance effect of the ground wire will be greater at this time, one-point grounding will lead to the lengthening of the actual ground wire and bring adverse effects. At this time, a combination of separate grounding and one-point grounding should be adopted; In addition, for high-frequency circuits, how to suppress high-frequency radiation noise should be considered. The method is to thicken the ground wire as much as possible to reduce the noise impedance to the ground. Grounded, that is, except for the printed circuit that transmits signals, all other parts are used as ground wires. Don’t have useless large-area copper foil.
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 the 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 analog power supply should be isolated, and the ground wires should be arranged separately. If there is A/D, only a single point should be shared here.
Low frequency does not have much influence, but it is recommended that analog and digital be grounded at one point. At high frequency, analog and digital ground can be shared by magnetic beads.
If the analog ground and digital ground are directly connected in a large area, it will lead to mutual interference. There are four ways to solve this problem: magnetic bead connection, capacitor connection, inductor connection and 0 ohm resistance connection.
The equivalent circuit of magnetic beads is equivalent to band-stop wave limiter, which can only significantly suppress the noise at a certain frequency point. When using, it is necessary to estimate the noise frequency in advance so as to select the appropriate model. When the frequency is uncertain or unpredictable, the magnetic beads do not match.
However, the capacitor isolation leads to AC, which will cause floating ground. Large inductance and many stray parameters will also lead to instability.
Finally, the 0 ohm resistor, which is equivalent to a narrow current path, can effectively limit the loop current and suppress the noise. Resistors have attenuation in all frequency bands (0 ohm resistors also have impedance), which is stronger than magnetic beads.