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42 DESIGN007 MAGAZINE I MARCH 2020 • Multi-drop bus topologies require parallel or end termination, which prevents reflec- tions from being formed at the transmis- sion line ends • The end termination resistor values are twice that of the transmission line as they are in parallel from an AC perspective • A common method of reducing reflec- tions from differential pairs is to place a resistive load at the end of the pair, which matches the differential impedance of the transmission line • The single resistor differential termination only terminates the differential mode signal—not the common-mode signal • Depending on the impedance of the driver, any common-mode signal created will bounce back and forth down the transmission lines • The differential center tapped termination terminates both differential and common- mode signals. It is best used when you have a low impedance driver on a long transmission line • If the transmission line is short, reflections still occur but will be overwhelmed by the rising or falling edge of the signal and may not pose a problem DESIGN007 Further Reading • B. Olney, "Impedance Matching—Terminations," The PCB Design Magazine, October 2013. • B. Olney, "DDR3/4 Fly-by Topology Termination and Routing," Design007 Magazine, June 2018. • B. Olney, "Beyond Design: The Fundamental Rules of High-Speed PCB Design, Part 2," Design007 Magazine, October 2018. • E. Bogatin, Signal and Power Integrity: Simplified, Prentice Hall, 2008. • H. W. Johnson & M. Graham, High-Speed Digital Design: A Handbook of Black Magic, Prentice Hall, 1993. Barry Olney is managing director of In-Circuit Design Pty Ltd. (iCD), Australia, a PCB design service bureau that specializes in board-level simulation. The company developed the iCD Design Integrity software incorporat- ing the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at icd.com.au. To read past columns or contact Olney, click here. If someone asked you how stressed you are right now, what would you say? A little, a lot, or you don't know? Those are all valid responses, but they are not espe- cially useful to researchers and medical professionals be- cause they are subjective and not easily quantified. None- theless, in lieu of a better method of measuring stress, the common method for years has consisted of a stress questionnaire. The main alternative to the questionnaire, a blood test, can provide quantitative data but requires a trained professional to draw the blood, and the stress of the procedure itself—being poked with a large needle— can skew the results of a lot of people. But something better might be right around the corner. Wei Gao, assistant professor of medical engineering at Caltech, has produced a wireless sweat sensor that can detect levels of cortisol, a natural compound that is com- monly thought of as the body's stress hormone. In a new paper appearing in the journal Matter, Gao and his fellow researchers demonstrate how they designed and made the mass-producible device, how it works, how it is effec- tive at detecting cortisol levels in near real-time. The development of an inexpensive and accurate device for measuring cortisol could allow for more wide- spread and easier monitoring of stress but also of other conditions including anxiety, post-traumatic stress disorder, and depression—all of which are correlated with changes in cortisol levels. (Source: Caltech) Sweat Sensor Detects Stress Levels: May Find Use in Space Exploration