Originally written by cuptree
Grounding is an important means of suppressing electromagnetic interference , and improving the electromagnetic compatibility of the electronic equipment. Proper grounding can suppress interference effects and suppression equipment to radiated interference and otherwise,wrong grounding would cause serious interference,or even making the electronic device in malfunction.
In electronic device, the 'GND' has two meaning: one is the 'earth ground' , the other is 'system benchmarks GND'. Grounding refers to the system of a selected point with a potential benchmarks between low resistance conductive path.
Connecting the 'earth ground' is making earth as the benchmark,and the earth as a zero potential.Using metal,electronics line selected points connected to the devices linking to the GND.
Connecting the the 'system benchmarks GND' means benchmark for signal circuit conductors.(Electronic devices usually using shield, or heavy metal base, copper wire, copper as the base conductor) and see this benchmarks as relative zero point, called system GND.
There are two purpose for connecting GND. First is for safety, called protected GND.Metal housing for electronic equipment must meet the land, so you can avoid the accident appeared on metal high voltage to ground and endanger the operator and equipment safety. Second, provide low impedance path for the electricity to return to their source.
In fact , the ground wire on the electrical or physical contact, do not necessarily have a clear divide. In the system, sometimes to assume a protectorate role also assume lightning; or undertake workplace and bear protection role. Different connections, electrical objects, focus of the approach will be different.
protective grounding is for protecting devices,equipment, circuit and person's safety from thundering,static electricity.Or protect people when devices are in malfunction.Thus equipment, devices, circuits, chassis and metal casing must be taken to protect Earth.
b, working Grounding
The working ground is the potential reference point or reference plane of single board, motherboard or system signals.It provides a low impedance signal return channel.Signal quality depends largely to whether the extent grounded quality is good or not.Due to the grounding influence of material properties and other technical factors,no matter how good the connection of grounding conductor or lap is, there is always a certain amount of impedance.Signal flow will produce a voltage drop across the ground, causing ripple problem, and affecting the signal quality; the weaker the signal is , the higher the frequency of signal, the more serious the affection will be. Although then, it is still very important to reduce the impedance of ground conductor when designing the circuit.
Filter is composed of inductors, capacitors, resistors, composition of ferrite beads and common mode coil frequency-selective network. Lowpass filter is widely used in electromagnetic compatibility technology. External radiation in order to reduce the power and signal cables, interface circuits and filtering the power supply circuit must be designed.
Filter performance depends on the filter circuit impedance on both sides, low-impedance circuit, simple inductor filter get 40dB of attenuation and high impedance circuit, almost no role in high impedance circuit, simple the filtering effect of capacitive filter circuit can be very good, almost in a low impedance circuit will not function. In the design of filter circuit, capacitors close to the high impedance circuit, inductance is close to low impedance circuit design.
Capacitor insertion loss increases as the frequency increases, until it reached after the self-resonant frequency, with wires and electrodes for capacitors and inductance in series with the capacitor on the circuit, and insertion loss began to decline.
Power EMC filter is a passive two-way network, it plugged in at one end, other end load. In the high frequency attenuation band of interest, power supply EMI filters can be considered"network impedance mismatch"
Network analysis results show that the filter impedance port impedance mismatches on both sides ,the greater attenuation of electromagnetic energy, the more effective it would be. Due to a power supply common mode impedance of the line tend to be low, so normally there is high impedance for the power side of the filter. In order to get a better result, low impedance power supply side, should be equipped with high input impedance of the filter; for high input impedance of the load side, you should be equipped with low output impedance of the filter.
Common filters for interference with tens of megabytes signal has better filtering, at high frequency, due to the inductance of the capacitor, its filtering performance will be degraded. For high frequency interference, and feed through system filter you want to use. Due to the structural characteristics of the filter, it has good filtering properties, the effective frequency range can be extended to GHz, so I use my wireless products. Filters used, the most important issue is a grounding problem. Only grounded filters can play a filtering role, otherwise it is of no value. Filters used to pay attention to the following issues: