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14 DESIGN007 MAGAZINE I FEBRUARY 2019 Typically, for a digital design, a characteris- tic impedance of 40–60 ohms and differential impedance of 80–120 ohms are used. This be- comes more important as the edge rates be- come faster. Also, different technologies have their specific requirements. For instance, USB requires 90 ohms differential impedance, and DDR3/4 require 40/80 ohms single-ended/dif- ferential impedance. For perfect energy trans- fer, the impedance of the driver must match the impedance of the transmission line. A good transmission line is one that has constant im- pedance along the entire length of the line so that there are no mismatches resulting in re- flections. Technology is advancing rapidly in the field of medical electronics, providing doctors with more efficient ways to gather information. Fig- ure 1 shows an ultra-thin and wireless adhe- sive biosensor that is stuck to the skin. This device can monitor a person's heart rate and other vitals and transmit the data in real time to a smartphone or computer. In this case, there appears to be no reference plane, but the impedance of the critical RF traces can still be managed by the careful planning of coplanar structures on the surface of the flexible sub- strate. The coplanar impedance is determined by the ratio of trace width to clearance, so size reduction is possible without limit—the only penalty being higher losses. In addition, a virtual ground plane exists between any two adjacent traces, as there is no field at that point. Hence, crosstalk effects are very weak between adjacent traces. There is a common miscon- ception that digital signals are transferred in the copper con- ductors of a multilayer PCB substrate, which is flat-earth thinking. A transmission line does not carry the signal itself but guides electromagnetic en- ergy from one point to another through the substrate. Volt- age and current do exist in the conductor, but only as a conse- quence of the field being present as it moves past. The path should also control the charac- teristic impedance, so there are minimal reflec- tions. What we really need to do is to provide a smooth, consistent path for the flow of elec- tromagnetic energy. The speed of a computer does not depend intrinsically on the speed of electrons but on the speed of energy transfer between electron- ic components. The dielectric material deter- mines the velocity (v) of propagation of the electromagnetic (EM) energy (where the speed of light (c) is 3x10^8 m/s): With a typical Er (Dk) of four for FR-4 ma- terial, the signal will travel at approximately half the speed of light (c/2) through the sub- strate regardless of the clock frequency. The lower the Er, the faster the speed. With their relative timing requirements, the signals essen- tially ride the EM carrier wave. So, matching the propagation speed between signals on dif- ferent stackup layers is crucial to ensure the correct timing margin at the receiver. A stripline is any trace sandwiched between reference planes on both sides (Figure 2). The Figure 1: Biosensor patch worn on the skin. (Source: CBS News [1] )