Single-point ground
Is the single-point ground system, only one physical reference point is defined as a ground point , each of the other points to be grounded is connected to this point .
Single-point ground for low -frequency circuits (1MHZ less ) . If the system frequency is very high, so that the operating wavelength and system ground lead length can be compared , the single-point grounding have a problem. The length of the ground wire is close to 1 / 4 wavelength , it is like a short-circuited transmission line terminals , ground current, the voltage standing wave distribution form , ground into a radiating antenna , and can not act as a " ground" effect .
To reduce ground impedance to avoid radiation, the length of the ground should be less than 1 /20 of the wavelength . In dealing with the power circuit , generally can be considered a single point grounding. For PCB extensive use of digital circuits , because of its rich in high harmonics, generally do not recommend the use of single-point grounding.
Multi-point grounding
Multi-point grounding is the grounding point of each device is directly connected to the ground plane from its recent , so the length of the shortest ground lead .
Multi-point grounding circuit structure is simple , significantly reducing ground line may appear high frequency standing wave phenomena applicable to the frequency higher (> 10MHZ) occasions. But multi-point grounding may result in the formation of many ground loop inside the device , thereby reducing equipment resilience to external electromagnetic fields . In the case of multi-point grounding , pay attention to the ground loop problems, especially networking between different modules , devices . Electromagnetic interference caused by ground loops :
Should be an ideal ground zero potential , physical entity zero impedance . However, the actual ground resistance component have the inherent reactance component , when the current through the ground , it is necessary to generate a voltage drop. Ground with other connection ( signal, power lines, etc. ) constitute a loop , when the time -varying electromagnetic field coupled to the circuit , the electromotive force induced in the ground loop, loop coupled to the load by the way , constitute a potential threat to EMI .
Floating
Refers to a floating ground system grounding device electrically insulated from the earth .
Since some floating own weaknesses , not suitable for large-scale systems in general , and its grounding rarely used
Grounding on the general principle of selecting :
For a given device or system , the highest frequency of interest ( corresponding wavelength ) into the upper , when the length of the transmission line L> in, high-frequency circuit is considered , on the contrary , low frequency circuit is considered . According to a rule of thumb , for less than 1MHZ circuits using single point grounding is good; For higher 10MHZ, the multi-point ground better. For frequencies in between , as long as the longest transmission line length L is less than / 20 in, you can single- point ground to avoid common impedance coupling.
For grounded generally selected the following principles:
( 1 ) low-frequency circuit (<1MHZ), recommend using single point grounding ;
( 2 ) high-frequency circuits (> 10MHZ), recommended the use of multi-point grounding ;
( 3 ) high-frequency mixing circuit mixing the ground .
