1. Field of the Invention
The invention relates to a charge injection device (CID) for sensing infrared (IR), and more particularly to an improved readout circuit having plural readout channels optimized for a cryogenically operated IR sensor.
2. Prior Art
Infrared sensors, initially having smaller numbers of pixel sites, are now being developed with larger numbers of sites, comparable to visible light sensors. Two dimensional arrays of 128.times.128 and 256.times.256 sites are in development, as well as linear arrays having as many as 128.thrfore.12, or 128.times.24 sites. Both large two dimensional IR arrays and large linear IR arrays require parallel readout circuitry. Parallel readout circuitry is required both for greater speed in sampling the sites and for greater optimization of array sensitivity.
The problem presented in providing parallel readout circuitry is that of not interfering with the cryogenic environment, which is required for high performance infrared focal plane arrays, while at the same time avoiding crosstalk between parallelled channels. The focal plane array customarily combines an IR detector chip and readout circuitry. The readout circuitry is required to convert the signal obtained by sampling individual IR detector elements on the chip into a (usually) single serial signal. The signal must be amplified at each stage of the processing to a sufficiently high level to avoid noise problems in subsequent processing.
Interference with the cryogenic performance may occur if large numbers of high conductance electrical conductors leading from the cryogenic environment of the focal plane array to the warmer ambient environment are required for parallel readout. Any additional electrically conductive paths will provide additional thermally conductive paths which will add to the thermal load presented when the focal plane array is cooled. Added thermal bulk is undesirable for similar reasons.
The conventional solution to crosstalk between parallelled channels tends to compromise cryogenic efficiency. In particular, the requirements for thermal isolation make it difficult to provide Dewar penetrations of low electrical impedance that would reduce crosstalk. In addition, focal plane miniaturization prohibits adding the bulk required for effective electrical filtering on the focal plane that would reduce crosstalk. Consequently, crosstalk of the focal plane signals between channels due to voltage modulation in the power supply paths tend to be present if cryogenic efficiency is optimized.