Electronic Design - PCB design digital ground and analog ground connected?

First of all, let us understand why the digital-analog ground has attracted the attention of PCB design engineers. Most of them are derived from the names of the power pins and ground pins of components (such as ADC). In fact, the pin names of the analog ground and digital ground indicate the function of the internal component itself, but it does not necessarily mean that the external should act according to the internal function. There are two parts of the digital-analog circuit inside the chip. In order to prevent the digital signal from coupling to the analog circuit, the digital ground and the analog ground are separated. I personally think it is a partition, but the internal digital-analog ground is connected. I use a multimeter to measure Some chips, such as ADV7180, KS8995, TLK2541, etc., are connected internally.

Let's talk about the advantages and disadvantages of the above two digital-to-analog ground connections:

Connect with inductance or magnetic beads

As the name implies, the digital-to-analog is connected to a certain point of the PCB board with an inductor or a magnetic bead. This seems to solve the digital signal interference with the analog circuit, but it has created a new problem-EMI. When the high-frequency electrostatic discharge interference current flows through the inductor or the magnetic bead, it will produce a voltage drop across the inductor or the magnetic bead. If it is a high-voltage test, the voltage at both ends is very high and cannot be released to the end quickly, which will damage the chip. Or the device restarts.

I personally prefer direct connection or bridging. This also solves the problem of loops and avoids the generation of radiation. The digital-to-analog ground is equipotential, and there is no voltage drop or is very small, so that the equipment will not be affected during high-voltage testing., Damage the chip. Just pay attention to reasonable partitions or trenches and bridges. The premise of this method is to do digital and analog partitions. However, the power supply must be separated. A more secure method is to use a separate power supply, but the cost is higher. Generally, a ferrite ring is used to isolate the analog power supply from the digital power supply.

The best junction point between the digital ground and the analog ground is generally at the entrance of the power supply, so that the coupling between the analog circuit and the digital circuit is minimal, and the interference of the digital circuit to the analog circuit is minimal. If the dividing surface is done well, the drive current and loop current of the two do not intersect each other, and there will be no large current loop.

This is the basic principle, but it is difficult to handle in practical applications. The difficulty lies in the cross-connection of the analog and digital parts. The effect of a large current loop is not as good as an undivided one. Therefore, in the actual PCB manufacturing process, be careful to choose the form and location of the segmentation, which can better isolate the two without hindering the flow of drive current and loop current.

The advantage of using inductors or magnetic beads for connection is that it can isolate the high-frequency noise of the digital part and keep the analog circuit ground clean.