Is your PCB ground wire right? Why should we have a master land?

PCB layout It needs the support of rich experience and solid theoretical foundation , And step on a few more pits , Do more simulations to deepen the understanding of routing , To form a closed-loop routing design , Today I'd like to introduce a and GND Cases related to routing , In the field of mobile phones, it will affect the image quality of cameras 、 In the medical field, it will affect the signal-to-noise ratio of bioelectric signal acquisition , If you don't understand the principle behind it , Only schematic or... Will be copied PCB Words , The best performance of the circuit is often not achieved .

The ground wire is PCB Routing , There are usually three functions ：

1. backflow
2. Control impedance
3. shielding

The case introduced today is related to reflux , The voltage fluctuation on the ground wire will affect the noise sensitive analog circuit . The figure above is a schematic diagram of ground wire routing , Of digital and analog circuits GND Finally, they all have to be connected to the ground of the battery , That is, digital current Id And analog current Ia Eventually they all come together , So these two currents Id and Ia Just use the public ground part , As shown in brackets in the figure above , Generally speaking, digital current Id The fluctuation of is relatively large , And analog current Ia The fluctuation is slightly smaller . Digital electricity Id The fluctuation of voltage will cause voltage fluctuation in the part of common ground wire , This fluctuation will be sensed by the analog circuit , This leads to the decline of signal quality , For example, the resistance of the common ground part is 20 mΩ, And digital current Id Volatility is 1A, Then the voltage fluctuation caused by △V Namely 0.02*1=0.02V, This 20mV Sensed by the amplifier of the analog circuit, it will appear in the form of noise , This is the consequence of high ground impedance .

The mitigation methods are as follows ： Reduce the resistance of the ground wire , Shorten the common ground wire between analog circuit and digital circuit , The analog circuit and digital circuit are isolated by magnetic beads to further suppress interference , If the current fluctuation of digital circuit remains unchanged , Is still 1A, The resistance of the common ground is reduced to 2 mΩ, At this time, the voltage fluctuation caused by the common ground part of the digital circuit is only 0.002*1=0.002V, Than the one above 20mV A lot smaller , meanwhile , The presence of magnetic beads will further suppress this noise , Improve the signal-to-noise ratio or common mode rejection ratio of analog circuits . Intuitively speaking ： Whatever the ground of your digital circuit / How does the power jump , Can't affect the ground of my analog circuit / Power Supply .

Based on the above introduction , Therefore, the general circuit board will be covered with copper in a large area （ Large area floor plan , Reduce impedance , Increase reflux capacity ）, Reduce the resistance of the ground . The above describes the treatment of ground wire , It is also a similar method to deal with the common power supply of analog circuit and digital circuit （ But generally speaking , It is not recommended that analog circuits and digital circuits share the same power supply ）.

So , Some people do not recommend drilling a large number of holes with other electrical properties on the ground plane , Or routing , This is what is usually said Fragmented land , This will easily increase the resistance of the ground wire （ Or impedance ）, Even there are hidden resistance bottlenecks, which are ignored by engineers and cause serious problems . For example, the red copper sheet highlighted in the figure below , The copper skin in the two white boxes looks big , In fact, their connection is only a narrow one in the green part , Here is the impedance bottleneck .

The above mentioned is only DC voltage fluctuation , For high frequency digital circuits , Current fluctuations are more complex , It contains very rich voltage or current harmonics , At this time, the parameter of frequency should be considered , Around the frequency parameter , It is necessary to consider the parameters such as parasitic capacitance or parasitic inductance of the ground wire , This is at the high end CPU It is particularly important in ,PCB The routing requirements are more strict , This is the of the power supply PDN Design , In the rear DCR Simulation and PDN The simulation will be introduced in more detail .