Photonic designs drive new application areas

Sensors are an essential interface that connects the world of electronics with external reality. Designing sensors to bridge that gap can be a daunting task, intertwining digital and analog electronics with physics and chemistry. A sensor element could be a fragment of DNA on an electrode, a potential well on a silicon chip, a pressure transducer or a MEMS-based accelerometer. Once the physical system has been designed, extracting information from the resulting signal becomes the task of the electronics engineer. Noise must be characterized and removed, and circuits or software that interpret the remaining information need to be designed. Often high-performance DSPs and complex expert-systems software are brought to bear on the task.

In this week's In Focus section, design engineers working with photons describe their latest concerns in such areas as signal drift, noise and timing. For example, image sensing is one of the most advanced and complex areas of sensor design due to the large role that optics plays in technology and product design. Electronic imaging systems are rapidly overtaking film in performance and resolution, and advanced silicon-processing capability is largely responsible for the revolution. However, it is still a challenge to handle noise issues on-chip, a subject Eric Fossum, Fellow at Micron Technology Inc. (Boise, Idaho), brings up in his contribution on CMOS active-pixel sensor design. Fossum pioneered the development of CMOS image sensors while a researcher at the Jet Propulsion Laboratory (Pasadena, Calif.) and was a founder of Photobit Corp., which has been developing imaging chips for professional camera designs. Photobit was recently acquired by Micron Technology.

Fossum points out that CMOS active-pixel sensors are highly integrated systems. In addition to hosting the basic photon detectors themselves, the chips include mixed-signal circuits, analog signal processing, analog-to-digital converters, bias generators, timing generators, digital logic and memory. All of this is put to use to create a variety of added capability ranging from basic noise control to zooming images or capturing fast motion.

CMOS sensors are currently in a race with the more established charge-coupled device (CCD) approach. Yuzo Shida, product-marketing manager at Analog Devices Inc. (Wilmington, Mass.), looks at a critical design component-the timing generator. CCD detectors store charge generated by photons in long rows that stretch across the chip. The charge packets are then clocked out of the row by being passed to their nearest neighbor. Thus, clocking becomes a significant design issue for this type of sensor. Shida shows how a redesign using programmable clock generation results in a highly flexible CCD system that can target different applications with essentially the same circuit.

Taking a broader look at optical-sensing technology, Carlo Strippoli at Texas Advanced Optoelectronic Solutions Inc. (Plano, Texas) describes how optical-sensing capability is now being built into Internet-enabled consumer products, offering not only instant image capture, but also other capabilities such as bar-code reading or blood-oxygen monitoring.

A fundamental problem every sensor system must address is signal drift due to ambient conditions-usually random temperature changes. Using a basic pressure sensor setup, Axel Kleinitz at Xicor Inc. (Milpitas, Calif.) shows how this problem can be tackled with a digitally programmable potentiometer. Response parameters over the temperature operating range are recorded in a small amount of EPROM on-chip.