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

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

1. Reasons for being divided into digital ground and analog ground:

Because the digital signal is generally a rectangular wave, with a lot of harmonics. If the digital ground and analog ground in the circuit 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 electric signal, it will also affect the normal operation of the digital circuit. The analog circuit involves weak signals, but the threshold level of the digital circuit is higher, and the requirements on the power supply are lower than that of the analog circuit. In a system with both digital circuits and analog circuits, the noise generated by the digital circuits will affect the analog circuits and make the small signal indicators of the analog circuits worse. The way to overcome them is to separate the analog and digital grounds.

The root cause of the problem is that the resistance of the copper foil on the circuit board cannot be guaranteed to be zero. The separation of the digital ground and the analog ground at the access point is to minimize the common ground resistance of the digital ground and the analog ground.

2. The basic principles of digital and analogue processing are as follows:

If the analog ground and digital ground are directly connected in a large area, it will cause mutual interference. Not short-circuited and inappropriate.

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

For high-frequency circuits and digital circuits, since the inductance effect of the ground wire will be greater at this time, one-point grounding will cause the actual ground wire to be lengthened 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 radiated noise should also be considered. The method is: thicken the ground wire as much as possible to reduce the impedance of the noise to the ground; full ground, that is, except for the printed line of the transmission signal, all other parts are used as ground wires. . Do not have useless large area copper foil.

The ground wire should form a loop to prevent the generation of high-frequency radiated noise, but the area enclosed by the loop must not be too large to prevent the induced current from being generated when the instrument is in a strong magnetic field. But if it is only a low-frequency circuit, you should avoid ground loops. The digital power supply and the analog power supply are best isolated, and the ground wires are arranged separately. If there is A/D, there is only a single point of common ground here.

There is not much effect at low frequencies, but it is recommended that analog and digital 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) Connect with magnetic beads; 2) Connect with capacitor; 3) Connect with inductance; 4) Connect with 0 ohm resistor.

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

2. Use 0 ohm resistor is the best choice

a. Ensure that the DC potential is equal

b. Single point grounding (limit noise)

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

3. Magnetic beads

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

The main parameters:

Nominal value: Because the unit of the magnetic bead is nominal according to the impedance it generates at a certain frequency, the unit of the impedance is also ohms. Generally, 100MHz is used as the standard, such as 2012B601, which means that the impedance of the magnetic bead at 100MHz is 600 ohms.

Rated current: Rated current refers to the current that can ensure the normal operation of the circuit.

The magnetic beads have a very high resistivity and permeability, which is equivalent to a series connection of resistance and inductance, but the resistance value and inductance value change with frequency. It has better high-frequency filtering characteristics than ordinary inductors, and exhibits resistance at high frequencies, so it can maintain a higher impedance in a relatively wide frequency range, thereby improving the effect of FM filtering. The magnetic bead has a great hindrance to high-frequency signals. The general specification is 100 ohms/100mMHZ. Its resistance is much smaller than the inductance at low frequencies. Ferrite Bead (Ferrite Bead) is a kind of anti-interference component that has been rapidly developed. It is cheap, easy to use, and has a significant effect of filtering high-frequency noise.

Some magnetic beads have multiple holes, and passing wires can increase the impedance of the component (the square of the number of times the magnetic beads are passed), but the increased noise suppression capability at high frequencies cannot be as much as expected. A few magnetic beads will be better. Ferrite is a magnetic material, which causes magnetic saturation due to excessive current flow and a sharp drop in permeability. High-current filtering should use magnetic beads specially designed in structure, and also pay attention to its heat dissipation measures.

Ferrite beads can not only be used to filter high-frequency noise in power circuits (can be used for DC and AC output), but also widely used in other circuits, and their volume can be made very small. Especially in digital circuits, because the pulse signal contains high-frequency harmonics, which is also the main source of high-frequency radiation in the circuit, it can play the role of magnetic beads in such occasions.

As long as the wire passes through it in the circuit. When the current in the wire passes through, the ferrite has little resistance to low-frequency currents, and it has a greater attenuation effect on higher-frequency currents. The high-frequency current is dissipated in the form of heat. The equivalent circuit is an inductor and a resistor connected in series. The values of the two components are proportional to the length of the bead. There are many types of magnetic beads, manufacturers should provide technical specifications, especially the curve of the relationship between the impedance and frequency of the magnetic beads.

4. Capacitance

Use the principle of capacitors to cut through.

4. The difference between inductance and magnetic beads:

A coil with more than one turn is used to be called an inductive coil, and a coil with less than one turn (a wire passing through a magnetic ring) is used to be called a magnetic bead; the inductance is an energy storage element, and the magnetic bead is an energy conversion (consumption) device; the inductance is mostly used The power supply filter circuit, magnetic beads are mostly used for signal circuits and EMC countermeasures; magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductance is used in this regard to focus on suppressing conductive interference. Both can be used to deal with EMC and EMI problems Inductance is generally 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 inductor, but it can be seen from the model 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 the core and the coil Composition, the magnetic beads convert the AC signal into heat energy, and the inductor stores the AC and slowly releases it.

The inductance is an energy storage element, and the magnetic bead is an energy conversion (consumption) device; the inductance is mostly used in the power supply filter circuit, the magnetic bead is mostly used in the signal circuit, and is used for EMC countermeasures; the magnetic bead is mainly used to suppress electromagnetic radiation interference, and the inductance is used This aspect focuses on suppressing conductive interference. Both can be used to deal with EMC and EMI problems.

Magnetic beads are used to absorb UHF signals. Like some RF circuits, PLLs, oscillating circuits, including UHF memory circuits (DDR SDRAM, RAMBUS, etc.), magnetic beads need to be added to the power input part, and the inductance is a kind of storage Energy components, used in LC oscillation circuits, low-frequency filter circuits, etc., its application frequency range rarely exceeds 50MHZ.

V. Summary of several methods

Capacitors are connected directly, causing floating ground. If the capacitor is not connected to DC, it will cause pressure difference and static electricity accumulation, and it will be troublesome to touch the case. If the capacitor and the magnetic bead are connected in parallel, it is even more addictive, because the magnetic bead is straight, the capacitor will fail. If it is connected in series, it will be nondescript.

The inductor has a large volume, many stray parameters, unstable characteristics, discrete distribution parameters are difficult to control, and a large volume. The inductance is also notch, LC resonance (distributed capacitance), which has a special effect on noise.

The equivalent circuit of the magnetic bead is equivalent to a band trap, which only suppresses the noise at 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 bead is not a good s Choice.

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

In short, the key is that the analog ground and digital ground should be grounded at one point. It is recommended to connect 0 ohm resistors between different types of grounds; use magnetic beads when the power supply is introduced into high-frequency devices; use small capacitors for coupling high-frequency signal lines; and use inductances at high power and low frequencies.