Printed Circuit Design & Fab - October 2008 - (Page 30) DESIGN baSicS of digital circuits. Generally, lower speed digital circuits will include devices that interface between digital and analog circuits, including A/D convertors, DACs and MCUs. These circuits are placed between high-speed digital circuitry and analog circuitry. Analog circuits are most susceptible to EMI and crosstalk, therefore, as a group, their components should be placed close to each other in order to avoid long signal routes. Placing analog circuits away from digital circuits will also minimize the noise being injected from the digital returns through the ground planes. Noisy circuits include power supply, relays and high-current switches. Generally, these circuits should be placed as a block, separate from all other subsystems. avoid such problems is to provide a continuous ground plane to all digital and analog circuits. This will not only provide a low inductance return path to all the signals, but will also reduce ground bounce. Secondly, the analog and digital ground planes should be partitioned from each other, providing little chance of interference between the returns of the two subsystems. This may be crucial for very dense mixedsignal layouts where the digital and analog circuits are placed in close proximity due to space constraints. These two separate ground planes are connected at one point, typically below the device that interfaces the two systems, for example A/D. Grounds from the noisy power supply section should also be separated through ferrite beads to filter the high frequency switching noise. a substantial amount of transient current that flows through this path causing a wide range of problems– from slowing down the propagation time to false triggering. Decoupling capacitors are placed between the device and the stray inductance that can contribute to ground bounce. Usually, larger ones are used for a bulk charge and, smaller ones for a faster response that provides the switching requirements. That is why the smaller caps are placed closest to the power pin of any device. Power and ground planes can also be used to provide the small, but very fast, planar capacitance. Here, the small distance between the adjacent power and ground plane provides the capacitance. FiGurE 2a shows an incorrect layout of a decoupling capacitor, while FiGurE 2b shows a correct layout. Note that placing a via at the opposite side of the capacitor and closer to the device pin kills the real reason behind using the decoupling capacitor in the first place. The power supply area should also have sufficient copper to avoid any inductance build up. A generous use of ground vias in this area helps in localizing the switching noise of the regulator, and it also has heat-sinking applications. Grounding Scheme Suffice it to say that grounding is the heart of a mixed-signal layout. It can mean the difference between a product that meets all requirements and one that is subpar. The ground plane not only provides a return path to all the signals, whether they are digital or analog, but it also is a reference voltage to all the subcircuits in a PCB system. A poor ground layout will create problems such as reflections, crosstalk, EMI issues and distortion, but the best and simplest technique to Power Supply Decoupling A decoupling system provides a low impedance power system on the board, as well as meeting every device’s switching needs. This is true for both digital and analog circuits. All devices, whether digital or analog, have special decoupling requirements. If the rise time is short, a significant voltage drop can result across the inductance path between the power supply and devices. Moreover, if a high pin count BGA is used, there can be Routing Once the placement of the different subsections is finalized and a good grounding and power supply decoupling is achieved, routing is the final step of the signal layout. Laying out signal traces at the digital/analog junction is most critical. It is incumbent on the PCB layout designer to study the datasheet of an A/D converter device in order to isolate digital and analog signals, as shown in FiGurE 3. Digital traces should not run parallel with the analog traces in order to avoid any inductive crosstalk, and if these traces are routed in adjacent layers, the PCB designer should make sure that they run perpendicular to each other while passing. This will keep the capacitive crosstalk between the two to a minimum. OCTOBER 2008 FiGurE 2a. Incorrect layout of a decoupling capacitor. 30 FiGurE 2b. Correct layout of a decoupling capacitor. Note how the trace connects to the capacitor before switching to the plane. printEd CirCuit dESign & fAB
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