Simulate the difference of the ground and signal ground

 Many people can not tell the difference between analog ground and signal ground, sometimes not distinguish between digital and analog, thus making the circuit quality, affecting the performance of the circuit:
 Analog circuits involving weak signal, but digital circuit threshold level higher power requirements than the analog circuit. Both digital circuits and analog circuits, digital circuit noise will affect the analog circuit, analog circuit small-signal indicators worse way to overcome is to separate analog and digital ground.
 For low-frequency analog circuits, in addition to bold and shorten the ground, various parts of the circuit point grounding is the best choice for inhibition ground interference, mainly to prevent mutual interference between the parts due to common impedance ground.
 This point ground inductance effect will be greater, point grounding will lead to the actual ground longer for high-frequency circuits and digital circuits, and an adverse impact, then take a combination of separate grounding point grounding .
 In addition, the high-frequency circuits should also consider how to suppress high-frequency radiated noise, the method is: try to bold the ground to reduce noise to ground impedance; full ground addition to the trace of the transmission signal, the other parts as the ground . Do not whether to use the large area of ​​copper foil.
 Ground shall constitute a loop, to prevent the generation of high-frequency radiated noise, but the loop area is surrounded by not too large, so that the instrument is in a strong magnetic field, the induced current. However, if only low-frequency circuit, you should avoid ground loops. Digital and analog power supply is the best isolated ground separate arrangement, if the A / D, only here a single point of common ground.
 The low frequency is not much impact, but suggested that the analog and digital point grounding. At high frequencies through the bead analog and digital ground a little common ground.
  If a large area of ​​analog and digital ground is directly connected, will lead to mutual interference. Not less then another wrong reasons as there are four ways to solve this problem: 1 beads connected; two capacitors connected; 3, inductively connected; 0-ohm resistors connected.
  The bead equivalent circuit is equivalent to band-stop limit filter, only the noise of a frequency significantly inhibited the use of advance estimate of the noise frequency in order to choose the appropriate model. Frequency uncertain or unforeseen circumstances, the beads substandard.
  Capacitor separated from direct pay, resulting in floating.
  Inductor bulky, stray parameters, unstable.
  0 Ohm resistor is equivalent to a very narrow current paths, which can effectively limit the loop current, so that the noise to be suppressed. Resistance in all band attenuation (0 ohm impedance), this is stronger than the bead.
  Circuit PCB design, you must understand that the electromagnetic compatibility (EMC), two basic principles: The first principle is to minimize the current loop area; The second principle is that the system only with a reference plane. Conversely, if the system there are two reference surface, it may be the formation of a dipole antenna (Note: the small dipole antenna radiation size and line length, flowing through the current size is proportional to the frequency); If the signal can not be as much as possible small loop return, it is possible to form a large loop antenna (Note: the size of the loop area of ​​small loop antenna radiation flowing through the loop current is proportional to the square of the size and frequency). In the design as possible to avoid these two cases.
 
Complex mixed-signal PCB design is a complex process, the design process to note the following:

1.The PCB partition as a separate analog part and digital part.
2.Two components layout.
3.A / D converter inter-partition placement.
4.Do not split. In the analog part and digital part of the board following the laying of the uniform.
5. in all layers of the circuit board, the digital signal can only digital part of the circuit board wiring.
6.In all layers of the circuit board, the analog signal only in the circuit board of the analog portion of the wiring.
7 analog and digital power split.
8.Wiring can not split across the gap between the power plane.
9.Must be split across the gap between the power supply signal line is to be located close to a large area of ​​the wiring layer.
10 analysis of the actual flow through the return current path and way.