Decrease interference between digital circuit and analog circuit

Analog ground/digital ground and analog power/digital power are merely relative concepts. The main reason for these concepts is that the interference of digital circuits to analog circuits has reached an intolerable point. The current standard treatment is as follows.

1. ground from the rectifier filter is divided into two, one as an analog ground, all analog part of the circuit ground connect to this analog ground; the other for the digital ground, all digital part of the circuit ground connect to this digital ground.

2. From DC power regulator chip, after filtering the same divided into two, one after LC/RC filtering as an analog power supply, all analog part of the circuit power connect to the analog power supply; the other for the digital power supply, all digital part of the circuit power supply connect to the digital power supply.

Note: analog ground / digital ground, analog power supply / digital power supply except a little connection at the beginning of the power supply, there can not be any connection.

AVCC: analog power supply;

AGND: analog ground.

DVCC: digital power supply;

DGND: digital ground.

This distinction is to isolate the digital part and the analog part to reduce the interference brought by the digital part to the analog circuit part. But the two parts can not be completely separated, the digital part and the analog part is still connected, so in the power supply at least ground should be together, so AGND and DGND, we will use 0 ohm resistor or magnetic beads or inductors to connect up, such a little connection will be able to reduce interference. Similarly, if the two parts of the power supply is the same should also use such a connection.

How to analyze whether they belong to the digital part or the analog part?

This issue is often the problem we have to consider when PCB layout. My personal opinion is to determine whether a component belongs to the analog, or digital is to look at the main chip associated with it, is digital or analog. For example: the power supply it may give power to the analog circuit, it is the analog part, if it is to the microcontroller or data class chip power, then it is digital. When they are the same power supply will need to use a bridge method to lead a power supply from another part. The most typical form is the D/A, which should be a half digital, half analog chip. I think if you can handle the digital input, the rest can be drawn to the analog part.

Analog circuits involve weak signals, but digital circuits have higher threshold levels, and the power requirements are a little lower than analog circuits. In systems with both digital and analog circuits, the noise generated by the digital circuit can affect the analog circuit and make the small signal indicator of the analog circuit worse. The way to overcome this is to separate the analog and digital ground.

For low-frequency analog circuits, in addition to thickening and shortening the ground, each part of the circuit using a point of ground is the best choice to suppress ground interference, mainly to prevent mutual interference between components due to the common impedance of the ground.

For high-frequency circuits and digital circuits, due to the inductive effect of the ground will have a greater impact, a point of grounding will lead to the actual ground line lengthening and bring adverse effects, then should take a combination of separate grounding and a point of grounding.

In addition, for high-frequency circuit should also consider how to suppress high-frequency radiation noise, the method is: try to thicken the ground to reduce the noise to ground impedance; full ground, that is, except the transmission signal line, the other parts of the whole as ground. Do not have a large area of useless copper foil.

Ground should constitute a loop to prevent the generation of high-frequency radiation noise, but the area surrounded by the loop should not be too large, so as not to produce induced currents when the instrument is in a strong magnetic field. However, if only low-frequency circuits, ground loops should be avoided. Digital power supply and analog power supply is best isolated, the ground is arranged separately, if there is A/D, only here a single point of common ground.

There is little effect in low frequency, but it is recommended that analog and digital one point ground. At high frequencies, the analog and digital ground can be grounded together at one point by magnetic beads.

If the analog and digital ground is directly connected to a large area, will lead to mutual interference. Not short connection is inappropriate, the reason as above. there are four ways to solve this problem: 

• connected with magnetic beads; 

• connected with capacitors; 

• connected with inductors; 

• connected with 0 ohm resistors.
  

The equivalent circuit of magnetic beads is equivalent to band resistance limiter, which only has a significant suppression effect on the noise at a certain frequency point, so it is necessary to estimate the frequency of the noise point in advance when using in order to choose the appropriate model. For the case where the frequency is uncertain or unpredictable, the magnetic beads do not fit.

Capacitance isolation DC and cross AC, resulting in a floating ground.

Inductors are large, with many stray parameters and unstable.

0 ohm resistor is equivalent to a very narrow current path, which can effectively limit the loop current and enable noise suppression. Resistors have attenuation in all frequency bands (0 ohm resistors also have impedance), which is stronger than magnetic beads.