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Linear pyroelectric arrays of up to 64 elements have been available for some time. Individual array elements are defined by photolithographic electrode patterning on the pyroelectric ceramic, and absorption of radiation is maximised across the wavelength range of interest by electrodeposition of 'platinum black on to the active element electrodes. Direct thermal conduction paths between adjacent elements in a monolithic array produce significant amounts of thermal crosstalk, decreasing the image resolution. Reticulation techniques have therefore been developed to isolate the elements thermally and hence decrease the crosstalk. Detector elements are individually compensated by large non-blacked areas connected in series opposition with the active element. This provides cancellation of common mode signals such as those produced by temperature drift and vibration. Discrete JFET chips have traditionally been used as buffer amplifiers, but MOSFET arrays have now been developed, giving a significant simplification in assembly. In its simplest form the MOSFET array consists of 32 transistors integrated onto a single chip; two of these chips are used to provide the 64 source followers for a 64 element linear pyroelectric array. A major advantage of MOSFET arrays is the lower gate leakage current than that obtainable with JFETs. This results in a lower current noise, which is manifested as an improvement in D* at low frequencies. The advantage of the MOSFET buffers is even more marked at elevated temperatures, because the leakage current of a JFET is strongly temperature dependent. As the element count of linear arrays increases so do the problems of interfacing the array to the following electronics; packages with more than 68 leads are very bulky. One solution to this problem is to incorporate multiplexers into the package, thus reducing the number of connections needed. To this end, multiplexed MOSFET arrays have been developed. The disadvantage of multiplexing immediately after the source follower is that no noise bandwidth limiting can be incorporated before the multiplexer, and the high frequency noise is aliased into the base band. This leads to a deterioration in the achievable signal to noise ratio. In order to improve on this, an integrated circuit has been developed which consists of 16 rising gain amplifiers and band limiting filters followed by a 16 way multiplexer. Eight of these integrated circuits have been incorporated into a single hybrid with a 128 element pyroelectric array.
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