Mixed-signal circuit design is very complex PCB layout , cabling, and power and ground handling components will directly affect the performance of the circuit and electromagnetic compatibility. This article will introduce the digital and analog circuit design partitioning to optimize performance mixed-signal circuits.
In the PCB board to reduce mutual interference between digital and analog signals , the design must be understood before the electromagnetic compatibility (EMC) of the two basic principles:
· Minimize the current loop area ;
· The system uses only one reference plane .
If there are two systems of reference plane , it may form a dipole antenna. If the signal does not return through the loop as small as possible , it is possible to form a large loop antenna , in the design to avoid this as much as possible .
The digital and analog mixed -signal circuit board split open, so to achieve isolation digital and analog ground. While this approach is feasible, but this method there are many potential problems in large, complex systems problem is particularly prominent , the most critical problems can not be split across the gap wiring , once crossed the dividing gap wiring , electromagnetic radiation and signal crosstalk will increase dramatically. PCB design in the most common problem is that the signal lines or power lines across the division to generate EMI problems.
1 . Split Mode 1
If the division method shown in Figure 1 using a signal line between two ground across a gap , the return path of the signal current is what Nick ? Assumed to be split in two connected together somewhere ( usually connected at a single point position ) In this case , the ground current will form a large loop , the flow through the large loop radiation and high-frequency current generates a high inductance.
Figure 1 split mode 1
If the flow is too low the analog loop current, which is easily affected by external interference. When dividing the power supply connected together , will form a very large current loops. In addition, analog and digital ground through a long wire connection from the ˉ would constitute a dipole antenna.
Understand the current return path to ground and the way is the key to mixed-signal circuit board design optimization. Many design considered only where the signal current from flowing , while ignoring the specific path of current. If the ground plane must be divided and must be divided by the gap between the wires can be divided between a single point ground connection , the two bridges are formed between the ground and the connection through the bridge wire . Thus , in the bottom of each signal line are able to provide a direct current return path , so that the formation of a small loop area .
Using optical isolation device or signal transformer can be achieved across the dividing gap . For the former, the gap across the divided optical signal ; the latter , is split across the magnetic gap . Another possible solution is to use a differential signal: A signal line from the inflow , the return signal line from another , in this case, no ground as the return path .
2 . Split Mode 2
Generally used in practical work uniformly , and the PCB partition analog and digital sections , Figure 2 shows the points
Cutting mode 2 . Analog signals in the analog area of the wiring board all layers , and the digital signal in the digital circuit wiring region . In this case, the return current does not flow into a digital signal to an analog signal ground .
Figure 2 -division 2
Only the digital signal on the analog section of the wiring board or the wiring when the analog signal on the digital part of the circuit board , only the digital signal to analog signal interference appears. This problem occurs not because there is no split , the real reason is improper wiring of the digital signal .
PCB board design uses a unified manner , through digital and analog circuits , and a suitable signal routing partitions , you can usually solve some of the more complex layout problems, but it does not produce a result divided bring some potential trouble. In this case , the layout and partition components on the merits of the key design decisions .
If the layout is reasonable layout , digital ground currents will be limited in the digital part of the circuit board , and will not interfere with the analog signals. For such wiring must be carefully checked and checked , to ensure compliance with the wiring rules , otherwise an improper signal line traces will completely destroy a circuit board design.
3 . A / D partition
When the A / D converter analog and digital ground pins are connected together, most of the A / D converter manufacturers will recommend the AGND and DGND pins are connected to the same low impedance ground via the shortest lead because most of the a / D converter chip does not connect the analog and digital ground together, must implement analog and digital ground connections through an external pin , DGND connection with any external impedance will be more through the parasitic capacitance digital noise coupling onto the IC internal analog circuitry. According to this proposal , you need the A / D converter AGND and DGND pins are connected to analog ground.
If the system has only one A / D converter , the above problem can easily be solved. As shown in Figure 3, split open , the A / D converter following the analog and digital ground portions together .
Figure 3 an A / D partition
If the system A / D converter is more, if both the analog and digital ground are connected together in each of the following A / D converter , the resulting multi-point connected to the isolation between the analog and digital ground on the milli meaningless , and if not so connected , in violation of the requirements of manufacturers .
The best approach is to start with a uniform , shown in Figure 4 , a uniformly divided into analog and digital sections .
Such a layout not only meets the requirements of IC device manufacturers to analog and digital ground pin low impedance connection , while not forming a loop antenna or dipole antenna.
FIG over 4 A / D partition
