The digital signal only has 0 and 1, so the digital ground is the 0 signal, but this signal is usually not 0, and will be somewhat higher than 0. But if it does not reach 1, it is still 0. That is to say, its high and low levels are all within a range. Because of this, the digital signal has strong anti-interference ability.
As we all know, the rising edge of the digital signal is relatively steep and contains rich high harmonics. These high harmonics may not have much influence in the digital circuit, because usually the high frequency interference voltage is very low, and the anti-interference ability of the digital signal is strong.
The analog ground is a real zero potential. The problem lies here. If the two grounds are directly connected together, the digital ground will pull up the analog ground and pass the high harmonics in the digital circuit to the analog circuit through the analog ground. It will really affect your analog measurement.
The fundamental reason for the interference is that the resistance of the copper foil on the circuit board is not 0. Only by separating the analog ground from the digital ground and minimizing the common ground resistance between the two grounds can the degree of interference of the analog ground be minimized.
2. Processing method between digital ground and analog ground
Usually there are two processing methods between digital ground and analog ground:
1. Connect with magnetic beads
① Magnetic beads are ferrite materials with good impedance characteristics in high frequency bands. They are specially used to suppress high frequency noise and spike interference on signal lines and power lines, and they also have the ability to absorb electrostatic pulses.
② Magnetic beads have high resistivity and permeability, which is equivalent to a series connection of resistance and inductance, but the resistance value and inductance value change with frequency.
③It has better high-frequency filtering characteristics than ordinary inductors, and exhibits resistance at high frequencies, so it can maintain a higher impedance in a relatively wide frequency range, thereby improving the frequency modulation filtering effect.
2. Connect with 0 ohm resistor
①The DC potential can be ensured to be equal.
②Single point grounding to limit noise.
③It has an attenuation effect on noise of all frequencies (0 ohms also have impedance, and the current path is narrow, which can limit the passage of noise current).