Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Patent
1994-08-04
1996-05-07
Williams, Howard L.
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
341141, H03M 300
Patent
active
055150462
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates generally to analog to digital (A/D) conversion and more particularly to a multiplexed oversampled analog to digital modulator suitable for processing an array of analog inputs, as for example, in a optical imager, to produce a digital output.
BACKGROUND OF THE INVENTION
Present approaches in focal-plane video imaging systems use some form of analog mutiplexing of the pixel data in order to read the image information. It is this multiplexing which defines the so called video data rates. In broadcast television, for example, the 30 hertz pixel data rate is multiplexed to the 4 megahertz video data rate. This same situation exists in industrial and military video systems where pixel rates are usually below 3 kilohertz and analog multiplexing is used with resulting megahertz video rates.
These multiplexing approaches have necessitated the use of analog to digital conversion processes employing high speed circuitry which, as a practical matter cannot be readily integrated with a focal-plane sensor. Moreover, the typical A/D converter in these applications comprises a high-speed video, flash converter which is generally considered too expensive for use in consumer applications.
As is discussed in Oversampling Delta-Sigma Data Converters, edited by James C. Candy and Gabor C. Temes, IEEE Press, 1992, New York, oversampled analog to digital (A/D) converters are known which use coarse quantization at a high sampling rate combined with negative feedback and digital filtering to achieve increased resolution at a lower sampling rate. Such converters may, therefore, exploit the speed and density advantages of modem very large scale integration (VLSI) while at the same time reducing the requirements for component accuracy.
In a type of oversampled A/D converter generally known as a delta-sigma modulator, the analog input is sampled at a rate well above the Nyquist frequency and fed to a quantizer via an integrator. The quantized output is fed back and subtracted from the input. This feedback forces the average value of the quantized output to track the average analog input value.
SUMMARY OF THE INVENTION
The present invention is directed to low cost analog to digital (A/D) conversion apparatus suitable for processing an array of analog inputs derived from an energy imager, such as an optical imager used in video cameras.
More specifically, the invention is directed to such A/D conversion apparatus which is capable of being located on or adjacent to the focal-plane sensor of an optical imager and which is characterized by the use of a time muliplexed oversampled conversion technique.
In accordance with a preferred embodiment, an array of analog inputs are time multiplexed into an oversampled A/D modulator which produces, for each input, a single-bit output that oscillates about the true value of the input at rates well above the Nyquist rate (the Nyquist rate being twice the highest signal frequency of interest). A plurality of analog storage elements are provided, each for storing the analog residue associated with the modulation of a different one of the analog inputs. Each analog storage element is periodically connected into the modulator circuit for modulation in a feedback loop to produce an updated residue value which is then stored in one of the analog storage elements.
The conversion apparatus thus produces at its output, a bit stream which is representative of the multiplexed analog inputs. This output may, subsequently, be processed through a digital filter that averages its coarse, single bit values and reduces the data rate to produce a high resolution output at the Nyquist rate.
In a preferred system embodiment, the analog inputs are generated by an array of light sensitive (including visible, infrared and ultraviolet) elements, e.g. charge control devices (CCD), mounted coincident with the focal plane of an image apparatus onto which an image is focused. The A/D conversion apparatus thus produces a bit stream output representative of the light en
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