Shenzhen kingford is a professional PCB design company with many years of work experience PCB design team, can provide multi-layer, high density, high speed PCB routing design and PCB design proofing services. Next, I will share with you how to distinguish between analog and digital PCB design.
Why should PCB design distinguish between analog and digital?
Simply put, the digital ground is the digital circuit part of the common reference end, that is, the digital voltage signal reference end; The analog ground is the common reference end of the analog circuit and the voltage reference end of the analog signal (zero potential point).
Since the digital signal is generally rectangular wave, it has a large number of harmonics. If the digital and analog parts of the board are not separated from the access point, harmonics in the digital signal can easily interfere with the waveform of the analog signal. When the analog signal is high frequency or strong electric signal, it will also affect the normal operation of the digital circuit. Analog circuits involve weak signals, but the digital circuit threshold level is higher, the requirements of the power supply is lower than analog circuits. In the system with both digital circuit and analog circuit, the noise generated by digital circuit will affect analog circuit board and make the small signal index of analog circuit deteriorate. The way to overcome this is to separate analog and digital ground.
The root cause of the problem is that the resistance of the copper foil on the board cannot be guaranteed to be zero, and the separation of the digital ground and the analog ground at the access point is to minimize the common ground resistance of the digital ground and the analog ground.
What are the ways to distinguish analog ground from digital ground in PCB design?
There are several general ways to deal with analog ground and digital ground:
Ø directly separate, in the heart of the schematic diagram connected to DGND digital area, ground connection for the AGND simulation area, and then in the PCB ground plane segmentation for digital and analog ground, and right spacing widening;
Magnetic beads are used to connect digital ground and analog ground.
The digital ground and analog ground are connected with capacitors, using the principle of capacitor separation through communication;
The digital ground and analog ground are connected by inductance, and the sensing value varies from uH to dozens of uH.
Digital ground and analog ground are connected with zero ohm resistance.
In summary, the capacitor is separated through the intersection, resulting in floating ground. Capacitor without DC, will lead to differential pressure and static accumulation, touch the case will be numb. If the capacitor and the bead are connected in parallel, it is gilding the lily, because the bead is straight, the capacitor will fail. In tandem, it's not very homely.
The inductor has large volume, many stray parameters, unstable characteristics, discrete distribution parameters are not easy to control, and large volume. Inductance is also notch, LC resonance (distributed capacitance), special effect on noise.
The equivalent circuit of magnetic bead is equivalent to a trap, only for a certain frequency of noise suppression effect, if you can not predict the noise, how to choose the model, moreover, noise frequency is not necessarily fixed, so magnetic bead is not a good choice.
0 ohm resistance is equivalent to a very narrow current path, can effectively limit the loop current, so that the noise is suppressed. The resistance attenuates in all frequency bands (the 0 ohm resistor also has an impedance), which is stronger than the magnetic bead.
Anyway, the key is a little bit of analog and digital grounding. It is suggested that different kinds of ground are connected with 0 ohm resistance; Magnetic beads are used when the power supply is introduced into the high-frequency device; Small capacitance for high frequency signal line coupling; Inductors are used for high power and low frequency.
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