Read the difference between analog ground and digital ground, magnetic beads and inductance

In simple terms, the digital ground is the common reference terminal of the digital circuit portion, that is, the reference terminal of the digital voltage signal; the analog ground is the common reference terminal of the analog circuit portion, and the voltage reference terminal (zero potential point) of the analog signal.

First, the reasons for dividing into digital ground and analog ground:

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.

Second, the basic principles of digital ground and analog ground processing are as follows:

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, 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.

Three, four solutions

Serial connection between analog ground and digital ground

1) connected with magnetic beads; 2) connected with capacitors; 3) connected with inductors; 4) connected with 0 ohm resistors.

1. Inductors generally use a few uH to tens of uH.

2. Use 0 ohm resistor is the best choice

a. can ensure that the DC potential is equal

b. Single point grounding (limited noise)

c. Attenuation of noise at all frequencies (0 ohm also has impedance, and the current path is narrow, which can limit the passage of noise current).

Magnetic beads

It is made of a ferrite material sintered surface with good impedance characteristics in the high frequency band. It is designed to suppress high-frequency noise and spike interference on signal lines and power lines, and has the ability to absorb electrostatic pulses.

The main parameters:

Nominal value: Since the unit of the magnetic bead is nominally based on the impedance it produces at a certain frequency, the unit of impedance is also ohms. Generally, the standard is 100MHz, for example, 2012B601, which means that the impedance of the magnetic bead is 600 ohms at 100MHz.

Rated current: The rated current is the current that can be guaranteed to work properly.

Magnetic beads have high resistivity and permeability, which is equivalent to the series connection of resistance and inductance, but the resistance value and inductance value change with frequency. He has better high-frequency filtering characteristics than ordinary inductors and resistive at high frequencies, so it can maintain high impedance over a wide frequency range, thus 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.

Some magnetic beads have a plurality of holes, and the passage of wires can increase the impedance of the component (the square of the number of passes through the beads), but the increased noise suppression capability at high frequencies is not as expected, but multiple series A few magnetic beads will work better. Ferrite is a magnetic material that is magnetically saturated due to excessive current passing, and the magnetic permeability drops sharply. High-current filtering should use magnetic beads specially designed for the structure, and pay attention to the heat dissipation measures.

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. The high-frequency current is radiated in the form of heat, and the equivalent circuit is an inductor and a resistor in series, and the values ​​of the two components are proportional to the length of the magnetic beads. There are many types of magnetic beads, and manufacturers should provide technical specifications, especially the relationship between impedance and frequency of magnetic beads.

capacitance

The principle of using a capacitor to block the intersection.

Fourth, the difference between inductance and magnetic beads:

More than one coil is used to be called an induction coil, and a coil of less than one turn (wire through-through magnetic ring) is used to be called a magnetic bead; an inductor is an energy storage component, and a magnetic bead is an energy conversion (consumption) device; Power filter loop, magnetic beads are mostly used in signal loops for EMC countermeasures; magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductors are used in this respect to focus on suppressing conducted interference. Both can be used to handle EMC and EMI problems; inductors are typically used for circuit matching and signal quality control. Magnetic beads are used 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. Energy components, used in LC oscillator circuits, low-frequency filter circuits, etc., the application frequency range rarely exceeds 50MHZ.

V. Summary of several methods

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.

Windshield Cell Phone Mount

Windshield Cell Phone Mount,Car Windscreen Mobile Phone Holder,Car Windshield Cell Phone Holder,Car Windscreen Cell Phone Holder

Ningbo Luke Automotive Supplies Ltd. , https://www.nbluke.com