DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.
There are many ways, and just as many opinions on the best method, to configure grounds for ADCs and other mixed-signal products. An earlier article written by the author ("Attack The Noise Gremlins That Plague High-Speed ADCs," Electronic Design, Dec. 17, 1999, p. 107) discussed the limitations of popular ADC grounding techniques and suggested a split ground plane. Though the method works for obtaining good noise performance for high-speed ADCs, a split ground plane isn't good for RFI/EMI performance.
Despite that grounding problem, there is a way to enhance noise performance while minimizing layout-related radiation. Three concepts dictate how to handle grounding for a combination of good dynamic performance and minimal RFI/EMI: the skin effect, the proximity effect, and a layout that considers these effects. The skin effect indicates that at high frequencies, the conductor's effective cross-sectional area is reduced below the actual full cross-sectional area. The proximity effect is when a flow of return current in the reference plane is restricted to a narrow area below or above the trace carrying the corresponding outgoing current.
This article discusses the difficulties surrounding skin and proximity effects and offers methods to deal with them. Among the topics are use of power traces instead of power planes, careful component placement, and the grounding of open areas. In particular, attention to component placement and power-supply routing makes it possible to achieve low noise performance while providing acceptable RFI/EMI levels.
HIGHLIGHTS:
The Skin Effect
When ac flows primarily near the outside surface of the conductor, or on its skin, that is the skin effect. It becomes more prominent as frequency increases. Therefore, the effective cross-sectional area of the conductor is smaller than the actual dimension.
The Proximity Effect
The magnetic field surrounding outgoing and return currents causes those currents to want to flow in close proximity to each other, ultimately restricting the return current to a narrow area below or above the trace.
Performance Needs
Three fundamental principles help determine what's necessary for best overall performance: all currents follow the path of least impedance; transmission-line impedance is proportional to the distance between conductors; and the amount of signal-path radiation increases with the loop area defined by the outgoing and return current paths.
Grounding Open Areas
It's common practice to cover all unused areas on the top and bottom of the pc board with grounded copper. Copper areas should be grounded at more than one point to avoid an antenna effect that would result in radiated energy at the frequencies contained in the ground plane. Obtaining the best performance from analog-to-digital converters (ADCs) has long eluded many circuit designers. One of the main obstacles involves proper grounding. Inadequate grounding techniques in high-speed ADC and mixed-signal circuits and systems can lead to excessive noise when digital return currents find their way into analog circuit areas.
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