Printed Circuit Design & Fab - October 2008 - (Page 29) DESIGN baSicS Mixed SigNal design Considerations Designs with analog, digital and RF features challenge the PCB designer to reduce noise, improve EMI performance and lower distortion. by SyED W. ALI The layout of an efficient mixedsignal design can be both time consuming and challenging. However, diligently working on this type of layout can result in a system with less crosstalk, more noise immunity and better EMI performance. The PCB industry continues gearing toward higher speeds, miniaturized components and denser circuits, and consequently, the coexistence of digital and analog technologies in close proximity has become unavoidable. Add to this an element of RF circuitry, and the designer’s job has just become a bit more difficult. Digital and analog circuits are similar in the fact that they both rely on voltage variations to transmit information, but the difference lies between voltage amplitude levels and noise immunity. Digital circuits operate on distinct voltage levels, and a digital driver is designed to create a voltage level higher than that required by the receiver. As long as this threshold voltage is achieved, the digital circuit is functional. Hence, these circuits are less susceptible to noise and distortion. Digital circuits are also noisier than their analog counterparts. On the other hand, analog sigOCTOBER 2008 nals convey information in the form and the shape required by the receiver. Any amount of difference in this information can make the device function improperly. Analog signals usually have low amplitudes, as well as a high signal-to-noise ratio (SNR) that is more difficult to achieve. RF signals are similar to analog circuits with respect to signal propagation. However, these signals operate at specific frequencies that usually depend on odd-shaped circuits etched on the PCB. These circuits are the least resistant to outside noise, so it is important to isolate them from digital circuits for proper operation. The layout of a mixed-signal PCB poses many challenges to the OEM design engineer and the PCB designer. It is incumbent on both parties to keep in close coordination throughout the project because the more information that can be provided to the PCB designer, the better. It’s also in the best interest of the PCB designer to be aware of and educated on the different types of circuitry that will be present on the board layout. In other words, he or she should be able to identify the digital, analog and RF circuitry before the start of the layout. Component Placement Component placement is the most critical aspect in any mixed-signal design, and its effects will trickle down to all the steps following it. If ample time and thought are given at this stage, it will make life easier for the PCB designer. Prior to the start of the layout, the PCB should be divided into the subsections shown in FiGurE 1. High-speed digital circuits will have fast switching times and should be placed closer to the connectors and away from any analog circuitry because low-level analog signals are most sensitive to the switching noise FiGurE 1. typical placement in a mixed-signal PCb layout. printEd CirCuit dESign & fAB 29
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