Tips for engineers: The essence of PCB design grounding issues

Analog ground/digital ground and analog power supply/digital power supply are just relative concepts. The main reason for proposing these concepts is that the interference of digital circuits on analog circuits has reached an intolerable level. The current standard treatment methods are as follows:

　　1. The ground wire is divided into two after rectification and filtering. One of them is used as analog ground, and all the circuit grounds of the analog part are connected to this analog ground; the other is used as digital ground, and all the circuit grounds of the digital part are connected to this digital ground.

　　2. The DC power supply voltage regulator chip comes out and is also divided into two after filtering. One of them is used as an analog power supply after LC/RC filtering. All analog circuit power supplies are connected to this analog power supply; the other is a digital power supply. All digital circuit power supplies are connected to this digital power supply.

　　Note : The analog ground/digital ground and analog power/digital power should not be connected in any way except at the beginning of the power supply.

　　AVCC: analog power supply; AGND: analog ground

　　DVCC: power supply for digital part; DGND: digital ground

　　This distinction is to isolate the digital part from the analog part and reduce the interference brought by the digital part to the analog circuit part. However, it is impossible to completely isolate the two parts. There is a connection between the digital part and the analog part. Therefore, when supplying power, at least the ground should be together. Therefore, AGND and DGND should be connected with a 0 ohm resistor, magnetic bead or inductor. Such a one-point connection can reduce interference. Similarly, if the power supply of the two parts is the same, this connection method should also be used.

　　In the design of electronic systems, in order to avoid detours and save time, the requirements for anti-interference should be fully considered and met, and anti-interference remedial measures should be avoided after the design is completed. There are three basic elements that form interference :

　　(1) Interference source refers to the components, equipment or signals that generate interference. It can be described in mathematical terms as follows: du/dt. The place where di/dt is large is the interference source. For example, lightning, relays, thyristors, motors, high-frequency clocks, etc. may all become interference sources.

　　(2) Propagation path refers to the path or medium through which interference propagates from the interference source to the sensitive device. Typical interference propagation paths are conduction through wires and radiation through space.

　　(3) Sensitive devices refer to objects that are easily interfered with, such as A/D, D/A converters, single-chip microcomputers, digital ICs, weak signal amplifiers, etc. The basic principles of anti-interference design are: suppressing interference sources, cutting off interference propagation paths, and improving the anti-interference performance of sensitive devices.

　　(Similar to the prevention of infectious diseases)

　　1. Suppress interference sources

　　Suppressing interference sources means reducing du/dt and di/dt of interference sources as much as possible. This is the most important and most important principle in anti-interference design, which often achieves twice the result with half the effort. Reducing du/dt of interference sources is mainly achieved by connecting capacitors in parallel at both ends of the interference source. Reducing di/dt of interference sources 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) Add a freewheeling diode to the relay coil to eliminate the back electromotive force interference generated when the coil is disconnected. Adding only a freewheeling diode will delay the disconnection time of the relay. After adding a voltage regulator diode, the relay can operate more times per unit time.

　　(2) Connect a spark suppression circuit (usually an RC series circuit, with a resistor ranging from a few K to tens of K and a capacitor of 0.01uF) in parallel across the relay contacts to reduce the impact of sparks.

　　(3) Add a filter circuit to the motor, and make sure the leads of the capacitor and inductor are as short as possible.

　　(4) Each IC on the circuit board should be connected in parallel with a 0.01μF~0.1μF high-frequency capacitor to reduce the impact of the IC on the power supply. Pay attention to the wiring of the high-frequency capacitor. The connection should be close to the power supply end and as thick and short as possible. Otherwise, it will increase the equivalent series resistance of the capacitor, which will affect the filtering effect.

　　(5) Avoid 90-degree bends when wiring to reduce high-frequency noise emissions.

　　(6) Connect an RC suppression circuit in parallel at both ends of the thyristor to reduce the noise generated by the thyristor (this noise may cause the thyristor to break down if it is severe).

　　According to the propagation path of interference, it can be divided into two categories: conducted interference and radiated interference.

　　The so-called conducted interference refers to the interference transmitted to the sensitive device through the wire. The frequency band of high-frequency interference noise is different from that of useful signals. The propagation of high-frequency interference noise can be cut off by adding filters to the wires, and sometimes it can be solved by adding isolation optocouplers. Power supply noise is the most harmful, so special attention should be paid to its treatment. The so-called radiated interference refers to the interference transmitted to the sensitive device through space radiation. The general solution is to increase the distance between the interference source and the sensitive device, isolate them with ground wires, and add shielding covers to the sensitive devices.

　　2 Common measures to cut off interference propagation paths are as follows:

　　(1) Fully consider the impact of power supply on the MCU. If the power supply is well made, the anti-interference of the entire circuit is mostly solved. Many MCUs are very sensitive to power supply noise. It is necessary to add a filter circuit or voltage regulator to the MCU power supply to reduce the interference of power supply noise on the single chip. For example, a π-shaped filter circuit can be formed by using magnetic beads and capacitors. Of course, if the conditions are not high, a 100Ω resistor can be used instead of a magnetic bead.

　　(2) If the I/O port of the microcontroller is used to control noisy devices such as motors, isolation should be added between the I/O port and the noise source (add a π-shaped filter circuit). If the I/O port of the microcontroller is used to control noisy devices such as motors, isolation should be added between the I/O port and the noise source (add a π-shaped filter circuit).

　　(3) Pay attention to the crystal oscillator wiring. The crystal oscillator and the microcontroller pins should be as close as possible, the clock area should be isolated with a ground wire, and the crystal oscillator shell should be grounded and fixed. This measure can solve many difficult problems.

　　(4) Reasonable partitioning of circuit boards, such as strong and weak signals, digital and analog signals. Keep interference sources (such as motors and relays) and sensitive components (such as microcontrollers) as far away as possible.

　　(5) Use ground wire to isolate the digital area from the analog area. The digital ground and analog ground should be separated and finally connected to the power ground at one point. The A/D and D/A chip wiring also follows this principle. The manufacturer has taken this requirement into consideration when allocating the A/D and D/A chip pinouts.

　　(6) The ground wires of the microcontroller and high-power devices should be grounded separately to reduce mutual interference. High-power devices should be placed at the edge of the circuit board as much as possible.

　　(7) Using anti-interference components such as magnetic beads, magnetic rings, power filters, and shielding covers in key places such as microcontroller I/O ports, power lines, and circuit board connecting lines can significantly improve the anti-interference performance of the circuit.

　　3. Improve the anti-interference performance of sensitive devices

　　Improving the anti-interference performance of sensitive devices means minimizing the pickup of interference noise from the sensitive devices' side, and recovering from abnormal conditions as quickly as possible.

　　Common measures to improve the anti-interference performance of sensitive devices are as follows:

　　(1) When wiring, try to minimize the area of ​​the loop to reduce induced noise.

　　(2) When wiring, the power line and ground line 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 ports of the microcontroller, do not leave them floating, but connect them to ground or power. The idle terminals of other ICs should be connected to ground or power without changing the system logic.

　　(4) Using power supply monitoring and watchdog circuits on microcontrollers, such as IMP809, IMP706, IMP813, X25043, X25045, etc., can greatly improve the anti-interference performance of the entire circuit.

　　(5) On the premise that the speed can meet the requirements, try to reduce the crystal oscillator of the microcontroller and use low-speed digital circuits.

　　(6) Try to solder IC devices directly onto the circuit board and use IC sockets less often.

　　In order to achieve good anti-interference, we often see ground splitting wiring on PCB boards. However, not all mixed digital and analog circuits must have ground plane splitting. This is because the purpose of this splitting is to reduce noise interference.

　　Theory : In digital circuits, the general frequency is higher than that in analog circuits, and their own signals will form a return current with the ground plane (because in signal transmission, there are various inductances and distributed capacitances between copper wires). If we mix the ground wires together, then this return current will crosstalk between digital and analog circuits. And we separate them so that they only form a return current within themselves. They are connected only by a zero-ohm resistor or a magnetic bead because they are originally the same physical ground. Now the wiring separates them, and they should be connected in the end.

　　How to analyze whether they belong to the digital part or the analog part? This question is often considered when we draw the PCB. My personal opinion is that the key to determine whether a component is analog or digital is to see whether the main chip related to it is digital or analog. For example: the power supply may supply power to the analog circuit, then it is analog. If it supplies power to the microcontroller or data chip, then it is digital. When they are the same power supply, a bridge method is needed to lead one power supply from the other part. The most typical one is D/A, which should be a chip that is half digital and half analog. I think if the digital input can be processed well, the rest can be drawn to the analog part.

　　Analog circuits involve weak signals, but digital circuits have higher threshold levels, so the power supply requirements are lower than those of analog circuits. In a system with both digital and analog circuits, the noise generated by the digital circuit will affect the analog circuit, making the small signal indicators of the analog circuit worse. The solution to this problem is to separate the analog ground and the digital ground.

　　For low-frequency analog circuits, in addition to thickening and shortening the ground wire, using single-point grounding for each part of the circuit is the best choice to suppress ground interference, which can mainly prevent mutual interference between components caused by the common impedance of the ground wire.

　　For high-frequency circuits and digital circuits, the inductance effect of the ground wire will have a greater impact. Single-point grounding will cause the actual ground wire to be lengthened and bring adverse effects. At this time, a combination of separate grounding and single-point grounding should be adopted.

　　In addition, for high-frequency circuits, we must also consider how to suppress high-frequency radiation noise. The method is: make the ground wire as thick as possible to reduce the noise impedance to the ground; fully ground, that is, except for the printed wire that transmits the signal, all other parts are used as ground wires. Do not have useless large areas of copper foil.

　　The ground wire should form a loop to prevent high-frequency radiation noise, but the area surrounded by the loop should not be too large to avoid induction current when the instrument is in a strong magnetic field. However, if it is only a low-frequency circuit, ground loops should be avoided. It is best to isolate the digital power supply and analog power supply, and lay the ground wires separately. If there is an A/D, only a single point is used for common ground.

　　There is not much impact at low frequencies, but it is recommended to ground the analog and digital grounds at one point. At high frequencies, the analog and digital grounds can be grounded at one point through a magnetic bead.

　　If the analog ground and digital ground are directly connected over a large area, it will cause mutual interference. It is not appropriate not to short-circuit them, for the following reasons. There are four ways to solve this problem: 1. Connect with magnetic beads; 2. Connect with capacitors; 3. Connect with inductors; 4. Connect with 0 ohm resistors.

　　The equivalent circuit of the magnetic bead is equivalent to a band-stop limiter, which can only significantly suppress the noise at a certain frequency. When using it, it is necessary to estimate the noise frequency in advance in order to select the appropriate model. For situations where the frequency is uncertain or unpredictable, magnetic beads are not suitable.

　　The capacitor blocks the direct current and the alternating current, causing a floating ground.

　　The inductor is large in size, has many stray parameters, and is unstable.

　　0 ohm resistor is equivalent to a very narrow current path, which can effectively limit the loop current and suppress the noise. The resistor has an attenuation effect in all frequency bands (0 ohm resistor also has impedance), which is stronger than the ferrite bead.