Introduction to Digital Image Sensors

Abstract

Data converters bridge the gap between the physical world of analog signals and the abstract world of mathematics and representational data. Digital-toanalog converters (DACs) convert a sequence of digital numbers into an analog signal, and analog-to-digital converters (ADCs) convert an analog signal into a sequence of digital numbers. In this book, the analog signals we will be discussing most will be derived from imagery collected through detecting elements. Data converters have been around longer than you might expect. Walt Kester of Analog Devices published the Analog-Digital Conversion Handbook online in 2004 and included a wonderfully extensive history of data conversion that extended back to the 18th century. Kester outlines an extremely early DAC developed in the early 18th century in the Ottoman Empire. This early data converter was an 8-bit binary-weighted hydraulic DAC used to control water distribution. Nozzle size was weighted for water flow in a binary fashion, allowing controllers to change the flow with 28 or 256 individual settings using only eight nozzles. In this case, the input is coming directly from human operators, and the output is water current, making this also the first documented current DAC with flowing water molecules instead of electrons. Analog-to-digital converters were developed much later. In a 1921 patent, Rainey disclosed what might be the first electro-mechanical ADC for his facsimile system. This converter utilized a light beam, photocells, a galvanometer, and 32 relays to reproduce data printed on a transparency into an “m-hot out of 32” digital word. This is a 5-bit thermometric output similar to a typical 5-bit flash ADC, which will be discussed in Chapter 4. It might be fascinating to image sensor professionals that this ADC utilized photonic components almost 100 years ago. In 1937, Reeves, at the International Telephone and Telegraph Corporation in Paris, is credited with the first all-electronic DAC and ADC design with his pulse code modulation patents. Electronic converters quickly became necessary components in every electronic component interfacing to real-world signals. Jumping forward a little more than 50 years to the late 1980s brings us to the beginning of the era of the low-cost, high-volume image sensor. Eric Fossum, the father of the modern CMOS active pixel sensor, published a very early report of image processing that included ADCs integrated on a focal plane array.2 In Smart CMOS Image Sensors and Applications, Jun Ohta suggests that it is possible that Eklund, Svensson, and Astrom published the first column-level ADC on an image-sensing chip in 1995. So began the revolution in digital imaging systems-on-chip, which has led to the prevalence of high-quality, low-cost image sensors in modern electronics.