A common assumption is differential signals are “equal and opposite.” Is it true?

Periodically, questions about differential trace design rules come up. There is always confusion over whether it is necessary to route differential traces close together, whether a plane needs to be underneath differential traces, or whether to consider differential impedance design rules with differential traces. In one sense, the answers to these questions are difficult, but in another sense they are simple. In fact, if we are not concerned about signal integrity issues, there are no design rules at all. Here is my way of trying to clarify things.

First, what are differential signals, and why are they different? FIGURE 1 illustrates a single (sometimes referred to as a single-ended) trace connecting a driver and a receiver (a) and a differential trace pair (b). Let’s say the signal amplitude (with respect to the reference voltage) in Figure 1a is V = +1. In Figure 1b, there are two signals, V+ = +1 and V- = -1. What the receiver in Figure 2 sees is the difference between these two signals, V+ - V- = +1 – (-1) = +2. The first, and most obvious, difference between the two configurations is that differential signals offer twice the signal level to the receiver. Usually, this translates into twice the signal-to-noise (S/N) level. This is a clear advantage over the single-ended case, and is the primary advantage of differential signals, especially when signal levels are low (as with many sensors).

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A car’s Monocoque (a French term) refers to a type of vehicle construction in which the body is integral with the chassis; basically a “skin” that supports its load by distributing tension and compression across its surface.

Monocoque structure is a technical term used in designing cars and boats. Most cars no longer use frames for structural support and crash-protection.

Similar ideas have been introduced for wiring in electronic devices. More than half of flexible circuits are now designed with 3-D wiring for tight spaces in small electronic devices. Rigid printed circuits and wire harnesses were not adequate, so flexible circuits are options for 3-D wiring in smartphones and tablet PCs. The relatively high cost of flexible circuits is an issue for device manufacturers; they are considering alternatives for building electronic circuits on plastic housing with or without framing. Once a new 3-D wiring technology is available, they can significantly reduce wiring space and assembling costs.

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Updates in silicon and electronics technology.

Ed.: This is a special feature courtesy of Binghamton University.

Photovoltaic cell works at night. University of California researchers have developed a photovoltaic cell that can function at night. The cell can generate up to 50W/sq. m. at night, about 25% of what conventional solar cells generate in daytime. They currently are improving the output power and efficiency of the devices. The cell operates in reverse to a normal solar cell. An object that is hot compared to its surroundings will radiate heat as infrared light. The device can work during the day by blocking direct sunlight. Hence, this new solar cell could potentially operate around the clock. (IEEC file #11548, Science Daily, 1/29/20)

“Stretchy battery” for wearables. Researchers at Stanford University have developed a stretchy battery useful for wearable electronics. The battery can be stretched to twice its original length without any power loss. The polymers in lithium-ion batteries that conduct negative ions toward the battery’s positive pole are in the form of gels housed in a rigid casing. By providing a power source that could stretch and bend, wearable electronics can be more comfortable. (IEEC file #11547, Electronics Weekly, 1/29/20)

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In the previous newsletter, I wrote about the possibility of using our electronics advancements to create detection devices for the novel coronavirus.

I received many comments and ideas for these new medical devices. Several commented on current electronic projects intended to develop detection and diagnosis equipment.

The idea is to create a wearable electronic sensor that attaches to your body. The substrate requirements are different from those used in traditional materials (polyimide films or PET films). Device substrates have to be flexible and elastic to remain attached during body movements; urethane and silicone rubber could be an option. Larger-sized devices will require a permeable substrate to address moisture from sweating. One option for this basic material is to use adhesive bandages along with an appropriate coating material or glue. Copper foil, the standard conductor material for printed circuit boards, is not suitable for a wearable device because of its poor elasticity. Using meander patterns as conductors can improve the copper foil circuits’ elasticity, but it is not enough for general use. One alternative for wiring electronic devices is screen-printable conductive ink. The elasticity from the conductive ink can increase by adding a rubber component for the binder matrix.

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