Basic principles of digital and simulated ground handling

In the design of electronic system, in order to avoid detours and save time, we should fully consider and meet the requirements of anti-jamming
After the design is completed, the remedial measures for anti-jamming are carried out. There are three basic factors that form interference:
(1) The interference source, which is the component, device or signal that produces the disturbance, is described in mathematical language as follows: DU/DT, di/dt large land
Side is the source of interference. Such as: Lightning, relays, SCR, motor, high-frequency clock, etc. may become a source of interference.
(2) Transmission path, refers to the interference from the source of interference transmitted to the sensitive device path or medium. The typical interference propagation path is through
Conduction of conductors and radiation of space.
(3) Sensitive devices refer to objects that are susceptible to interference. such as: A/D, D/a converter, single chip microcomputer, digital IC, weak signal amplification
and other device.

The basic principle of anti-jamming design is to suppress the interference source, cut off the interference propagation path and improve the anti-jamming performance of the sensitive device.
1. Suppressing interference sources
To suppress the interference source is to reduce the du/dt,di/dt of the interference source as much as possible. This is the highest priority and most important in anti-jamming design
The principle, often can play the multiplier effect. The main du/dt to reduce the interference source is through the shunt capacitance at both ends of the interference source
To achieve. The DI/DT to reduce the interference source is realized by interfering the series inductance or resistance of the source loop and adding the freewheeling diode.
Common measures to suppress interference sources are as follows:
(1) The relay coil increases the freewheeling diode, eliminating the anti-EMF interference that occurs when the coil is disconnected. Only the freewheeling diode will
So that the time lag of the relay, increase the voltage regulator diode relay in the unit time can move more times.
(2) at both ends of the relay contact with the spark suppression circuit (generally RC series circuit, the resistance is generally selected from a few k to dozens of K, electricity
Jongxian 0.01uF) To reduce the impact of EDM.
(3) Add a filter circuit to the motor, pay attention to capacitance, inductance lead should be as short as possible.
(4) Each IC on the circuit board should be connected with a 0.01μf~0.1μf high-frequency capacitor to reduce the IC's influence on the power supply. Attention
High-frequency capacitance wiring, should be close to the power supply side and as short as possible, otherwise, equal to increase the capacitance of the equivalent series power
affect the filter effect.
(5) Avoid 90-degree line when wiring, reduce high-frequency noise emission.
(6) SCR at both ends of the RC suppression circuit, reduce the noise generated by the SCR (this noise may be serious when the SCR
Breakdown).

2, according to the interference of the transmission path can be divided into conduction interference and radiation interference two categories.
The so-called conduction interference refers to the interference transmitted through the wire to the sensitive device. The high frequency interference noise and the useful signal frequency band are different, can
The propagation of high frequency interference noise is cut off by adding a filter on the wire, and sometimes it can be solved by isolating the optocoupler. Electric
Source noise is the most harmful, pay special attention to treatment. The so-called radiation interference is transmitted through the space radiation to the sensitive device dry
Disturbance rejection. The general solution is to increase the distance between the source of interference and the sensitive device, to isolate them with ground wire and to cover the sensitive devices.
Cover.
The usual measures to cut off interference propagation paths are as follows:
(1) Fully consider the effect of power supply on single-chip microcomputer. The power supply is done well, and the anti-interference of the whole circuit solves the more than half. Many single
Chip microcomputer is sensitive to power supply noise, to the SCM power supply filter circuit or voltage regulator to reduce the power supply noise to the microcontroller
of interference. For example, the use of magnetic beads and capacitors to form the π-shaped filter circuit, of course, when the requirements are not high can also be used 100ω
Resistors instead of magnetic beads.
(2) If the microcontroller's I/O port is used to control the motor and other noise devices, the I/O port and the noise source should be separated (increase π
Filter circuit). Control of the motor and other noise devices, between the I/O port and the noise source should be isolated (increased π-shaped filter
Circuit).
(3) Pay attention to the crystal oscillator wiring. Crystal oscillator and single-chip computer pin as close as possible, using ground to isolate the clock area, crystal shell grounding
and fixed. This measure resolves many problematic issues.
(4) Reasonable zoning of the circuit board, such as strong, weak signal, digital, analog signal. As far as possible the source of interference (such as motor, relay
and sensitive components (such as single-chip microcomputer) away from the device.
(5) using ground to isolate the digital area from the simulation area, the digital and analog ground to separate, and finally in a point connected to the power supply ground. A/D,
D/A chip wiring is also based on this principle, manufacturers assigned A/D, d/a chip pin arrangement has been considered this requirement.
(6) Single-chip microcomputer and high-power device ground to separate ground to reduce mutual interference. High-power devices in the circuit as much as possible
The edge of the board.
(7) In the microcontroller I/O port, power cord, circuit board cable and other key areas of the use of anti-interference elements such as magnetic beads, magnetic ring, electricity
Source filter, shielding cover, can significantly improve the anti-jamming performance of the circuit.

3, improve the anti-jamming performance of sensitive devices
Improving the anti-jamming performance of sensitive devices is to consider minimizing noise pickup from sensitive devices and never
The method of recovering the normal state as soon as possible.
The common measures to improve the anti-jamming performance of sensitive devices are as follows:
(1) Minimize the area of the loop loop when wiring, to reduce the induced noise.
(2) When wiring, the power cord and ground should be as thick as possible. In addition to reducing the pressure drop, it is more important to reduce the coupling noise.
(3) for the single-chip computer idle I/O port, do not hover, to ground or to connect power. Idle end of other ICS without changing the system
Grounding or connecting the power supply in the case of logic.
(4) Use Power monitor and watchdog circuit for SCM, such as: imp809,imp706,imp813,x25043,x25045
And so on, can greatly improve the anti-jamming performance of the whole circuit.
(5) The speed can meet the requirements of the premise, as far as possible to reduce the single-chip crystal oscillator and selection of low-speed digital circuit.
(6) IC devices as far as possible directly welded on the circuit board, less use of IC block.
*************************************************************************************************************** *********

In order to achieve good anti-jamming, so we often see the PCB board on the ground division of the wiring method. But not all of the digital circuit and analog circuit mix must be ground plane division. Because this segmentation is to reduce the noise interference.

Theory: In a digital circuit the general frequency is higher than the frequency in the analog circuit, and their own signal will be formed with the ground plane to form a reflux (because in the signal transmission, there are a variety of inductance and distribution capacitance between copper wire and copper wire), if we mix the ground together, Then this reflux will be crosstalk between the digital and analog circuits. And we separate is to let them only in their own internal formation of a reflux. They are connected only with a 0 ohm resistor or bead because they are the same physical ground, and now the wiring separates them, and finally they should be connected together.

How do you analyze whether they belong to the digital part or the simulation part? This is often the problem when we draw the PCB in the specific filter. My personal opinion is to determine whether a component is a simulation, or the key to the number is to see whether the main chip associated with it is digital or analog. For example: the power supply it may give the analog circuit power supply, it is the analog part, if it is to the microcontroller or data class chip power supply, it is digital. When they are the same power supply it is necessary to use a bridge to bring a power supply from the other part. The most classic is a D/A, it should be a half is a number, half is analog chip. I think if we can handle the digital input, the rest can be drawn to the analog part.

Linking between analog and digital ground

(1) What is the general value of the serial inductance between analog and digital ground?

(2) with 0 ohms is the best choice (1) to ensure that the DC potential is equal, (2) Single-point grounding (limit 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).
Magnetic beads equivalent to the band trap, only 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.
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 characteristic is not stable, the discrete distribution parameter is not control, the volume is large. Inductors are also notch, LC resonant (distributed capacitance), with special effects on noise.
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.

The basic principles of digital and simulated ground processing are as follows:

1), if the low-frequency simulation, bold and shorten the ground, single-point grounding, can effectively prevent the common ground impedance caused by the mutual interference between components. and the high-frequency and digital, the inductance effect of the ground is more serious, single-point grounding will lead to the actual ground length, it should be multi-point grounding and single-point grounding combined.

2), high frequency should also consider how to suppress high-frequency radiation noise. The method is as follows: The ground should be as thick as possible to reduce the noise to ground impedance; Large area (full) ground, that is, in addition to the transmission signal and power of the printed line, the rest of the whole copper as ground, but do not leave dead useless large area copper foil.

3), the ground should constitute a loop to prevent the generation of high-frequency radiation noise, but the loop area is not too large, so as not to produce a larger induction current. Note that the ground loop should be avoided if it is low frequency.

4), digital power and analog power is best isolated, ground separate layout, if there is a/D conversion, only as close as possible to the single point of the device grounding.

1), if the low-frequency simulation, bold and shorten the ground, single-point grounding, can effectively prevent the common ground impedance caused by the mutual interference between components. and the high-frequency and digital, the inductance effect of the ground is more serious, single-point grounding will lead to the actual ground length, it should be multi-point grounding and single-point grounding combined.

2), high frequency should also consider how to suppress high-frequency radiation noise. The method is as follows: The ground should be as thick as possible to reduce the noise to ground impedance; Large area (full) ground, that is, in addition to the transmission signal and power of the printed line, the rest of the whole copper as ground, but do not leave dead useless large area copper foil.

3), the ground should constitute a loop to prevent the generation of high-frequency radiation noise, but the loop area is not too large, so as not to produce a larger induction current. Note that the ground loop should be avoided if it is low frequency.

4), digital power and analog power is best isolated, ground separate layout, if there is a/D conversion, only as close as possible to the single point of the device grounding.

Basic principles of digital and simulated ground handling