What is digital ground and analog ground, what is the principle of processing?

What is the digital ground and the analog ground, what is the principle of treatment, in fact, they are essentially cultivated, that is, the digital ground and the simulated ground are both ground. But there are some differences, then how do we distinguish them and whether they have any influence on each other. Digital ground and analog ground will affect each other not because one is called a number, the other is called simulation, but they use the same elevator: the ground, and the hoistway used by this elevator is the ground wire we put on the PCB. The current of the analog loop goes this line, and the current of the digital loop also goes this line. It is understandable that the line is used to conduct the current. The problem is that there is resistance on this line! And the most fundamental problem is that the current going through this line goes to 2 different loops. Assume that there are 2 currents, several streams, and the mold flow starts from the ground at the same time. There are 2 devices: digital and analog. If the two loops are not separated and the stream current flows to the ground of the digital part, the loss voltage is V=(number stream + mode stream) X trace resistance, which is equivalent to the ground of the digital device relative to the ground. The terminal is raised by V, and the digital device is not satisfied. I admit that it will raise a little voltage. I recognize the part of the stream, but why should the mold flow be added to my head? Similarly, analog devices will complain as well!

The first one: the PCB line of your cloth has no impedance, and naturally it will not cause interference. It is like jumping directly down the 2nd and 3rd floors. That is the widest time of the hoistway, that is, you can install an infinite elevator. Naturally, everyone Does not affect who, but everyone knows, This is mission impossible!

The second one: two loops are separated, the number flow, the mold flow are separated, and the ground is separated from the ground.

In the same way, sometimes in the analog loop, but also to divide the large, small current loop, is to avoid mutual interference. The so-called interference is: the voltage caused by the current in two different circuits on the PCB trace, which is generated by superimposing the two voltages on each other.

Specifically, the digital ground is the common reference end of the digital circuit part, that is, the reference end of the digital voltage signal; the analog ground is the common reference end of the analog circuit part, and the voltage reference end of the analog signal (zero potential point) .

Since digital signals are generally rectangular waves with a large number of harmonics. If the digital ground and analog ground in the board are not separated from the access point, the harmonics in the digital signal can easily interfere with the waveform of the analog signal. When the analog signal is a high frequency or strong electrical signal, it will also affect the normal operation of the digital circuit. 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.

The root cause of the problem is that there is no guarantee that the resistance of the copper foil on the board is zero, and the digital ground and analog ground are separated at the access point in order to minimize the common ground resistance of the digital ground and the analog ground.

If the analog ground and the digital ground are directly connected to each other, it will cause mutual interference. Not short and not appropriate. 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, only a single point is common 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.

The serial connection between the analog ground and the digital ground can be carried out in four ways: 1. Connecting with magnetic beads; 2. Connecting with capacitors (using the principle of direct separation of capacitors); 3. Connecting with inductors (usually using a few uH to several Ten uH); 4, connected with a 0 ohm resistor. The following highlights the magnetic beads and 0 ohm resistors:

Under normal circumstances, the use of 0 ohm resistor is the best choice, 1, can ensure that the DC potential is equal; 2, single point grounding, limiting noise; 3, the attenuation of all frequencies of noise (0 ohm also has impedance, and current path Narrow, can limit the passage of noise current); 4, the capacitor (using the principle of the capacitor through the straight through).

The magnetic beads are made of a ferrite material sintered surface with good impedance characteristics in a high frequency band. They are designed to suppress high-frequency noise and spike interference on signal lines and power lines, and have the ability to absorb electrostatic pulses. Magnetic beads have high resistivity and permeability, equivalent to series connection of resistance and inductance, but both resistance and inductance values ​​vary with frequency. It has better high-frequency filtering characteristics than ordinary inductors, and exhibits resistivity at high frequencies, so it can maintain a high impedance over a relatively wide frequency range, thereby improving the frequency modulation filtering effect. Magnetic beads have a great hindrance to high-frequency signals. The general specification is 100 ohms/100mMHZ, which is much smaller than the inductor at low frequencies. Ferrite Bead is a kind of anti-jamming component that has been developed rapidly. It is cheap, easy to use, and has a significant effect on filtering high frequency noise.

Ferrite beads can be used not only for filtering high-frequency noise in power circuits (for DC and AC outputs), but also for other circuits, and their size can be made small. Especially in digital circuits, since the pulse signal contains high-order harmonics with high frequency, it is also the main source of high-frequency radiation of the circuit, so it can play the role of magnetic beads in this case. As long as the wire passes through it in the circuit. When the current in the wire passes, the ferrite has little resistance to the low-frequency current, and the higher-frequency current has a large attenuation.

More than one coil is accustomed to be an inductive coil, and a coil of less than one turn (a straight through magnetic ring) is used to be called a magnetic bead. Inductors are energy storage components, while magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power supply filter circuits. Magnetic beads are mostly used in signal loops for EMC countermeasures; magnetic beads are mainly used to suppress electromagnetic radiation interference, and inductors are used for inductors. This aspect focuses on suppressing conducted interference. Both can be used to deal with EMC and EMI problems; inductors are generally used for circuit matching and signal quality control, using magnetic beads where analog ground and digital ground are combined.

As a power supply filter, an inductor can be used. The circuit symbol of the magnetic bead is the inductance. However, it can be seen that the magnetic bead is used in the circuit function. The magnetic bead and the inductor are the same principle, but the frequency characteristics are different. The magnetic bead is composed of an oxygen magnet, and the inductance is composed of a core and a coil. Composition, the magnetic beads convert the AC signal into heat energy, and the inductor stores the AC and slowly releases it.

Inductors are energy storage components, while magnetic beads are energy conversion (consumption) devices; inductors are mostly used in power supply filter circuits, magnetic beads are mostly used in signal loops for EMC countermeasures; magnetic beads are mainly used to suppress electromagnetic radiation interference, and inductors are used for inductors. This aspect focuses on suppressing conducted interference. Both can be used to handle EMC and EMI issues. Magnetic beads are used to absorb ultra-high frequency signals. Like some RF circuits, PLLs, oscillator circuits, and ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.), it is necessary to add magnetic beads to the input part of the power supply. The energy components are used in LC oscillating circuits, low-frequency filter circuits, etc., and their application frequency ranges rarely exceed 50 MHz.

The capacitor is connected straight to the ground, causing floating. If the capacitor does not pass through the DC, it will cause pressure difference and static electricity to build up. If the capacitor and the magnetic bead are connected in parallel, it is a superfluous addition. Because the magnetic beads are straight, the capacitance will fail. If you are connected in series, it will look nondescript.

The inductor has a large volume, many stray parameters, unstable characteristics, poor control of discrete distribution parameters, and large volume. The inductor is also a notch, LC resonance (distributed capacitance), and has special effects on noise.

The equivalent circuit of the magnetic bead is equivalent to the band-stop filter, which only suppresses the noise of a certain frequency. If the noise cannot be predicted, how to choose the model, and the noise frequency is not necessarily fixed, so the magnetic beads are not good. s Choice.

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.

In short, the key is to ground the analog ground and the digital ground. It is recommended that different types of grounds be connected by a 0 ohm resistor; a magnetic bead is used when the power source is introduced into the high frequency device; a small capacitor is used for the coupling of the high frequency signal line; and the inductor is used at a high power low frequency.

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