Electronic Design - Design "digital ground and analog ground" on the PCB

The following introduces the methods and techniques of designing "digital ground and analog ground" on the PCB board:

Method 1: Divide the ground plane according to the function of the circuit

Segmentation refers to the use of physical segmentation to reduce the coupling between different types of lines, especially through the coupling of power lines and ground lines. The example of dividing the ground wire according to the circuit function is shown in the figure. The ground planes of four different types of circuits are divided using the division technology, and the four ground planes are separated by non-metallic grooves on the ground plane. The power input of each circuit adopts an LC filter to reduce the coupling between the power surfaces of different circuits.

For L and C of the LC filter of each circuit, in order to provide different filtering characteristics for each circuit, it is better to use different values. High-speed digital circuits have high instantaneous power, so high-speed digital circuits are placed at the power entrance. The interface circuit considers factors such as electrostatic discharge (ESD) and transient suppression devices or circuits, and is located at the end of the power supply. On a printed circuit board, according to the design example of the circuit function grounding layout, when different types of circuits such as analog, digital, and noisy circuits are on the same printed circuit board, each circuit must be Ground in the most suitable way for this type of circuit. Then connect the different ground circuits together.

Method 2: Use a local ground plane

Oscillator circuits, clock circuits, digital circuits, analog circuits, etc. can be installed on a single local ground plane.

This local ground plane is set on the top layer of the PCB. It is directly connected to the internal ground plane (0V reference plane) of the PCB through multiple through holes. The oscillator and clock circuit are mounted on a local ground plane, which can provide a mirror image layer to capture oscillations. The common mode RF current generated by the internal and related circuits of the device, so that RF radiation can be reduced. When using a local ground plane, be careful not to route through this layer, otherwise the function of the mirror layer will be destroyed. If a trace passes through a localized ground plane, there will be a small ground loop or discontinuous potential. These small ground loops can cause some problems at radio frequencies. If a device uses different digital grounding or different analog grounding, the device can be arranged on different local ground planes, and the device can be partitioned through insulating slots. The power supply voltage entering each component is filtered with ferrite, magnetic beads and capacitors

Method three, PCB adopts "noise-free" I/O ground and "noisy" digital ground to separate design

In order to use cable decoupling or shielding technology to suppress common mode noise, PCB design needs to consider providing "no noise" or "no noise" or "no noise" for the cable decoupling (to shunt current to the ground) and shielding that is not contaminated by the noise of digital logic circuits. "Clean" land. When designing the PCB layout, place all I/O lines in a certain area on the PCB, and provide a specially divided low-inductance I/O ground for this area, and connect the I/O ground at a single point To the ground of the digital logic circuit, the digital logic ground current cannot flow to the "noise-free" I/O ground.

The clock circuit and clock signal line should be far away from the I/O interface area.

Method four, two problems of PCB segmentation: isolation and interconnection

PCB segmentation needs to solve two problems: one is isolation and the other is interconnection. Isolation on the PCB can be achieved by using "ditches", as shown in the figure, that is, a blank area without copper coating is formed on all layers of the PCB. The minimum width of the "ditch" is 50 mil. "Hao" divides the entire PCB into individual "islands" according to their different functions. Obviously, the "moat" divides the mirror layer to form independent power and ground for each area, which can prevent RF energy from entering another area from one area through the power distribution system.

"Isolation" is not the purpose. As a system, each functional area needs to be connected to each other. Splitting is to better arrange layout and routing to achieve better interconnection. Therefore, it is necessary to provide channels for those lines that need to be connected to each sub-function area. There are two commonly used interconnection methods: one is to use an independent transformer, opto-isolator or common mode data line to cross the "trench", as shown in Figure 10.1.26 (a); the other is to " Only those signals with a “bridge pass” can enter (signal current) and exit (return current). It is difficult to design an optimized split layout. You can also use metal shielding and other methods to connect all signals. The generated, undesired RF energy is shielded, thereby controlling the radiation and enhancing the anti-interference ability of the PCB.

Method 5, adopt the form of "uniform ground plane"

In the ADC or DAC circuit, when the analog ground and digital ground pins of the ADC or DAC need to be connected together, the general recommendation is: connect the AGND and DGND pins to the same low impedance ground plane with the shortest lead .

If a digital system uses an ADC, as shown in Figure 10.1.29, the "ground plane" can be divided, and the analog ground and digital ground are connected together under the ADC chip. However, it is required to ensure that the width of the connecting bridge between the two grounds is the same as that of the IC, and any signal line cannot cross the division gap.

Most A/D converter chips do not connect the analog ground and digital ground together. The analog ground and digital ground must be connected by external pins. Any external impedance connected to DGND will be more digital by parasitic capacitance. The noise is coupled to the analog circuit inside the IC. To use a "uniform ground plane", you need to connect the AGND and DGND pins of the A/D converter to the analog ground. Simulate the problem of ground.

Method 6: Use digital power and analog power to split the power plane

In a digital-analog hybrid system, independent digital power and analog power are usually used to supply power separately. Use a split power plane on the mixed-signal PCB. It should be noted that the signal line close to the power layer cannot cross the gap between the power sources, and only the signal line on the signal layer close to the large area "ground" can cross the gap. The analog power supply can be designed in the form of PCB traces or fills instead of a power plane, which can avoid the problem of division of the power plane.