Digital detection of visual and infrared (IR) images is a very widely used technology, ranging from consumer-oriented camera and video apparatus to law enforcement and military applications. For virtually all of these applications, there is a growing demand for higher image pixel counts, higher pixel density, increased sensitivity, improved dynamic range, and faster image processing.
In particular, many emerging thermal infrared (IR) sensing applications simultaneously demand high sensitivity, large dynamic range, large pixel count, and operation at fast data rates. Among these applications are day/night persistent surveillance, border patrol and protection, aerial search and rescue, and environmental remote sensing. Such applications require sensor systems capable of high-quality, large-pixel-count images. Organizations such as the US Army Night Vision and Electronic Sensors Directorate (NVESD) are demanding ever higher pixel counts and densities to meet the challenges brought about by such applications.
At the heart of all digital imaging systems is the Focal Plane Array (“FPA”), which is a two-dimensional array of elements upon which an image is focused, whereby each of the FPA elements or “pixels” develops an analog output “signal charge” that is proportional to the intensity of the light that is impinging on it. Traditionally, a readout integrated circuit (“ROIC”) uses an integration capacitor to store the signal charge at each pixel of the FPA, and then routes the analog signals onto output taps for readout and digitization by analog-to-digital converters (“ADC's”) external to the individual pixels. This approach requires storing a large signal charge at each pixel site, and further requires that an adequate signal-to-noise ratio and dynamic range be maintained as the analog signals are read out and digitized. Accordingly, this traditional approach suffers from sensitivity and dynamic range limitations.
Digital pixel readout integrated circuits have had a problem with getting sufficiently high gain for some applications. Known systems for analog to digital conversion have relied exclusively on counting the number of pulses over a given time for analog to digital conversion. This does not provide the requisite gain for these applications.
What is needed, therefore, are techniques for increased gain in the analog to digital conversion.