Electronic Design

Beyond Simple Photodiodes And Phototransistors

Historically, applications that must sense a light stimulus to execute a function have relied on a photodiode or a phototransistor. Neither of these technologies, however, can support the emerging needs for higher-performance, optical, sensor-based applications.

With that in mind, Texas Advanced Optoelectronics Solutions (TAOS) was formed about a decade ago. Its optoelectronic devices combine precision mixed-signal functionality with photodetectors on the same IC to produce products with performance and cost advantages over conventional solutions.

These sensors simplify the measurement and analog-to-digital conversion of light. They’re also designed to reduce the need for signal conditioning or pre-processing circuitry in light-centric systems. The result is improved system performance and a shortened design cycle.

“If you look at the trend over the last decade, you can see that the simple photodiode has gained a lot more functions that serve various applications like power savings and display color correction,” says Lloyd Hasley, vice president of engineering for TAOS. “We’ve been offloading more tasks from the microprocessor and making our photodetector product more intelligent.”

Take the company’s integrated light-to-voltage products, for example. These sensors combine a photodiode, color filter, and transimpedance amplifier on a single die. The output is then fed to an analog-to-digital converter (ADC) for digital processing. This approach, which improves noise immunity, is well-suited for applications such as colorimetry, printing process control, display color correction, and selectively ambient light detection or rejection.

Also, the firm’s integrated light-to-frequency offerings provide RGB color sensing using a photodiode grid consisting of 16 groups of four elements each. The output for each color is a square wave whose frequency is directly proportional to the intensity of the selected color. Each group consists of a red sensor, a green sensor, a blue sensor, and a clear sensor with no filter.

Since the sensor provides a digital output, the color information is input directly to a processor by sequentially selecting each color channel and then counting pulses or timing the period to obtain a value. This eliminates circuitry since transimpedance amplifiers and ADCs are no longer needed. It also allows the sensor to be located remotely from the processor with no loss of noise immunity. Both factors make it ideal for portable systems where space is critical.

“Our products are fabless designs that serve a wide variety of user needs in power savings for displays and color correction,” says Gene Dierschke, TAOS’s chief scientist. “This can be done at low cost by putting more functionality on the chip using standard semiconductor processes.”

“When you get into making wider-bandwidth optoelectronics for telecommunications applications, however, things become more materials-oriented. It’s not at all clear if this can be done cost-effectively on a standard CMOS process using a fabless design,” says Cecil Aswell, chief technology officer for TAOS.

TAGS: Components
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