Of course, a transmission line should always be properly terminated to prevent reflections and standing waves, as these lead to both signal radiation and waveform distortion. Waveform distortion, known in the digital world as signal-integrity issues, can lead to indeterminate timing (jitter) and noise.
Remember that pc-board vias (through holes) are primarily capacitive in nature and present signal-line impedance discontinuities that can lead to reflections and standing waves. This in turn can cause waveform distortion and energy radiation. For this reason, it’s best to avoid vias in signal lines that carry high-frequency signals. When a via is necessary, it’s best that it be near one end of the line or the other. Keep in mind that power-supply traces carry signal-frequency currents.
Having said that, the good news is that impedance discontinuities caused by vias are fairly small and usually not problematic. Even so, it’s best to minimize their use and to avoid them whenever possible.
All lines carrying high-speed signals, analog or digital, have some degree of sensitivity to layout and proper termination. Still, the ADC clock line is almost always the most sensitive, especially with high-speed ADCs. This is because the clock signal determines just when a sample is acquired. Sample-to-sample timing variations show up as noise in the ADC output spectrum.
Grounding Open Areas
Assuming an internal pc-board layer is the ground plane, it’s common practice to cover all unused areas on the top and bottom of the pc board with grounded copper, called copper fill or copper pour. However, grounding a copper area at a single point creates an antenna that will radiate energy at whatever frequencies are in the ground plane at that point.2 For this reason, you should ground these top and bottom copper areas at more than one point, or don’t cover these areas with copper at all. A look at antenna theory will tell us that we should also avoid floating (unconnected) areas of copper, especially strips.
Large copper areas should have many grounded vias. Small copper areas should be connected to the ground plane at a minimum of two points. If an area is so small that it’s not practical to ground it at two points, copper should be left off that area.
Bringing It All Together
Many things come into play when it comes to getting the best performance with high-speed ADCs and other mixed-signal components. Knowing the players on the field is the first step toward controlling the situation and getting good circuit performance.
Attention to component placement and power-supply routing makes it possible to achieve low noise performance with high-speed ADCs and other mixed-signal components while providing acceptable RFI/EMI levels. It’s also important to pay attention to the ADC’s clock signal, to the output bus, and to signal conditioning circuitry. Those considerations were discussed in previous articles.1,4,5 National Semiconductor’s ADC Web site (www.national.com/adc) and the National Semiconductor Analog University Web site (www.national.com/analogu) offer information that can help you achieve the performance you expect in your ADC and mixed signal designs.
References:
- "Attack The Noise Gremlins That Plague High-Speed ADCs," Electronic Design, Dec. 17, 1999, p. 107.
- Johnson, Howard and Graham, Martin High-Speed Digital Design: A Handbook of Black Magic (Prentice Hall, 1993), p. 151.
- Johnson, Howard and Graham, Martin High-Speed Digital Design: A Handbook of Black Magic (Prentice Hall, 1993), p. 158.
- "Maintaining Signal Integrity Enhances ADC Circuit Performance," Electronic Design, May 1, 2000, p. 115.
- "Pay Attention To The Clock And Output Bus To Improve High-Speed ADC Designs," Electronic Design, June 26, 2000, p. 137.