Simulated and digitally detailed

Transferred from: http://blog.csdn.net/kevinhg/article/details/12080751

The two are essentially the same, that is, both the digital and the simulated land. To understand why to separate, listen to a story first, our company's business building, 2 floor is engaged in simulation, 3 floor is engaged in digital, the whole building has only one elevator, usually less time is good to do, on the 2 upstairs 3 floor without influence, but every day to work, people are very much, make the number of 3 floor, is always influenced by the people who are impersonating the 2 floor, 2 floor simulation of people to go downstairs, always have to wait for the elevator on the 3 floor again down, mutual influence is troublesome, business building to solve this problem, put forward 2 programs: 1th (laughter dead) elevator expansion, can be installed more people, elevator big is good, but the company will recruit, more people, Change the elevator again, recruit again ... Forever die Cycle, there is a way to very good, we simply do not lift, straight down, regardless of the 2 floor of the 3 floor, definitely solve the problem, but certainly will be wrong (1th was shot off). The 2nd way to install 2 elevators, a dedicated 2 floor, another dedicated to the 3 floor, wonderful! Too witty, so the 2-storey staff will not affect each other. Got it?

The digital, simulated ground will affect each other not because of a called number, one is called simulation, but they used the same elevator: ground, and this elevator is used by the well is we on the PCB cloth ground. Analog circuit of the current go this line, the digital circuit of the current also go this line, it is understandable that the line cloth is used to conduction current, can be the problem in this line has a resistor! And the most fundamental problem is that the current of this line goes to 2 different circuits. Suppose: There are 2 currents, several streams, and the mold flow starts at the same time from the ground. There are 2 parts: digital and analogue. If the 2 loops are not separated, the number flow mode flow to the digital part of the ground end of the time, the loss of the voltage of v= (stream + mode) x line resistance, the equivalent of the digital device ground end relative to the ground raised V, the digital device is not satisfied, I admit that will raise a little voltage, the number of the part I recognized But why does the mold flow add to my head? Similarly, analog devices will complain!

Two solutions: the 1th: your cloth PCB line without impedance, nature will not cause interference, like 2, 3 floor directly down jump, that is the most wide, that is, can be installed an unlimited elevator, nature who do not affect who, but who all know, this is mission impossible! 2nd: 2 loops separated, the number of streams, mold flow separate, not only the number of ground, mold to separate.

Similarly, sometimes although in the analog circuit, but also to divide large, small current circuit, is to avoid mutual interference. The so-called interference is: 2 different circuits of the current in the PCB traces caused by the voltage, the 2-part voltage superimposed on each other produced.

The following is described in detail, in short, the digital circuit is a common reference terminal, that is, the digital voltage signal of the reference terminal, the analog circuit is the common reference terminal, analog signal voltage reference terminal (0 potential point).
first, the reasons for the digital and simulated ground
Because the digital signal is generally rectangular wave, with a large number of harmonics. If the number in the circuit board is not separated from the analog ground 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 high frequency or strong electrical signal, it will also affect the normal operation of the digital circuit. The analog circuit involves the weak signal, but the digital circuit threshold level is higher, the demand for the power supply is lower than the analog circuit. Both the digital circuit and the analog circuit, the noise generated by the digital circuit will affect the analog circuit, so that the small signal of the analog circuit becomes worse, and the way to overcome it is to separate the analog and 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 that the access point is separated digitally from the analog ground in order to minimize the co-ground resistance of the digital ground and the analog ground.
Second, the basic principles of digital and simulated ground treatment are as follows
If the simulated ground and the large area of the digital directly connected, will cause mutual interference. It's not a short connection. For low-frequency analog circuit, in addition to the bold and shorten the ground, the circuit each part of the use of a little ground is the best choice to suppress ground interference, mainly to prevent the common impedance caused by the ground between the components of mutual interference.
And for high-frequency circuits and digital circuits, due to the inductance effect of the ground will be greater, a little grounding will lead to the actual ground length and bring adverse effects, then should take a separate grounding and a bit of grounding combined way. In addition, the high-frequency circuit should also consider how to suppress high-frequency radiation noise, the method is: As far as possible to add coarse ground, to reduce noise to ground impedance, full ground, that is, in addition to the transmission of the signal printed line, the rest of the whole as a ground. Do not have the useless large area copper foil.
Ground wire should constitute a loop to prevent high-frequency radiation noise, but the surrounding area of the loop should not be too large, so as to avoid the instrument in a strong magnetic field when the induced current. However, if it is a low frequency circuit, the ground loop should be avoided. The digital power supply and analog power are best isolated, the ground is divided, if there is a/d, then only a single point of co-site. The low frequency does not have much effect, but it is recommended to simulate and digital a little ground. At high frequencies, the analog and digital ground can be shared by a magnetic bead.
three or four ways to solve
The serial connection between the analog ground and the digital ground can be used in four ways: 1, connect with the magnetic beads, 2, the electric capacitance connection (using the principle of the capacitor to pass through), 3, the electricity sense connection (generally with a few uh to dozens of uh); 4, connect with 0 ohm resistor. The following highlights the bead and 0 ohm resistors:

In general, with 0 ohms is the best choice, 1, can guarantee the same DC potential, 2, a single point of grounding, limit the noise, 3, the noise of all frequencies have attenuation effect (0 ohms also has impedance, and the current path is narrow, can limit the noise current through); 4, capacitance (using the capacitance of the direct intersection principle).
The magnetic beads are sintered on a ferrite material with good impedance characteristics in high frequency bands, designed to suppress high-frequency noise and spike interference on signal lines, power lines, and the ability to absorb electrostatic pulses. The bead has a high resistivity and permeability, equivalent to the resistance and inductance in series, but the resistance and inductance values vary with the frequency. It has better high frequency filtering characteristics than ordinary inductors, and exhibits resistance at high frequencies, so it can maintain high impedance in a fairly wide frequency range, thus improving the effect of FM filtering. Magnetic beads on the high-frequency signal has a large barrier effect, the general specification has 100 Euro/100mmhz, it in the low frequency resistance than inductance is much smaller. Ferrite beads (ferrite bead) is an anti-jamming component that is developed rapidly in application, which is inexpensive, easy to use, and has significant effect on filtering high frequency noise.

Ferrite beads can be used not only to filter out high-frequency noise in power circuits (available for DC and AC output), but also to be widely used in other circuits where the volume can be made very small. Especially in the digital circuit, because the pulse signal contains high frequency harmonics, is also the main source of high-frequency circuit radiation, so 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 impedance to the low-frequency current, while the current at higher frequencies can have a large attenuation effect.

four. The difference between inductance and bead
There is more than one turn of the coil habit called inductance coil, less than one turn (lead through the magnetic ring) coil is used to call the bead. Inductance is a energy storage element, and the magnetic bead is a power conversion (consumption) device, the inductor is used for the supply filter circuit, the magnetic beads are used for the signal circuit, for EMC countermeasures; The magnetic beads are mainly used to suppress electromagnetic radiation interference, while the inductor is used in this respect to suppress conductive interference. Both can be used to deal with EMC, EMI problems, inductors are generally used for circuit matching and signal quality control, where the analog and digital combination of magnetic beads.
As a power filter, you can make sense of electricity. Magnetic bead circuit symbol is inductance but the model can be seen in the use of magnetic beads in the circuit function, the magnetic beads and inductors are the same principle, but the frequency characteristics are different; Magnetic beads are composed of oxygen magnets, inductance by the magnetic core and coil composition, magnetic beads to convert the AC signal to thermal energy, inductance to the Exchange storage, slowly released.
The inductor is a energy storage element, and the magnetic bead is a power conversion (consumption) device, the inductance is used for the supply filter circuit, the bead is used for the signal circuit, the magnetic beads are mainly used to suppress electromagnetic radiation interference, and the inductor is used in this respect to suppress the conductive interference. Both can be used to handle EMC and EMI issues. Magnetic beads are used to absorb UHF signals, such as some RF circuits, PLLs, oscillation circuits, including UHF memory circuits (DDR Sdram,rambus, etc.) need to be in the power input part of the magnetic beads, and inductance is an energy storage element, used in the LC oscillation circuit, low-frequency filter circuit, etc. The application frequency range is rarely more than 50MHZ.
Summary of several methods
Capacitance-to-pass, resulting in floating. Capacitance does not pass the DC, will lead to pressure difference and static accumulation, touch the machine shell will be hemp hand. If the capacitor and the magnetic beads in parallel, is the superfluous, because the bead pass straight, the capacitor will fail. In series, the words are nondescript.
The inductance is large, the spurious parameters are many, the characteristics are not stable, the discrete distribution parameters are not well controlled and the volume is large. Inductors are also notch, LC resonant (distributed capacitance), with special effects on noise.
The equivalent circuit of the magnetic beads is equivalent to the band trap, only to a certain frequency point of the noise inhibition, if not predictable noise, how to choose the model, moreover, the noise frequency is not necessarily fixed, so the magnetic bead is not a good choice.
The 0 Ohm resistor corresponds to a very narrow current path, which effectively limits the loop current and suppresses the noise. Resistance is attenuated in all frequency bands (0 ohms also has impedance), which is stronger than magnetic beads.
In short, the key is to simulate ground and digital ground to a point. It is suggested that a 0 ohm resistor is used to connect the different kinds of wires, the magnetic beads are used when the power supply is introduced into high frequency devices, and the small capacitance is used for high frequency signal coupling.

Simulated and digitally detailed