Dynamic Random Access Memories (DRAM)

Thursday, October 19th, 2017 - Light, Photoconductors, Photovoltaic

Dynamic Random Access Memories (DRAM)

These optical transducers make use of an ordinary DRAM chip whose ceramic package has had the lid removed so as to allow light to fall on to the memory cells, as shown in Figure 1 :

Dynamic Random Access Memories (DRAM)

Figure 1 DRAM camera

These devices therefore work by ‘memorizing’ the light pattern focused on the memory cells plane. The principle of operation includes 3 basic steps:

  1. the photosites array is ‘coated’ with electrical charges, that is all the memory cells are filled with logic ‘1’s
  2. the memory cells are then disconnected from the data bus whilst being exposed to light which, by creating electron-hole pairs (photo­ electric effect) beneath the Si02 gates, allows each cell to slowly discharge at a rate dependent on the amount of light falling on it.
  3. after a time interval, which by analogy with other cameras we shall call ‘exposure time’, the memory array is read, thus providing binary data corresponding to the light pattern falling on the array during the exposure time.

The features of this simple, yet very effective area scan camera, depend, predictably, on the MOS memory technology used. Memory cell design, in fact, aims to reduce the cell size, the stray capacitances and the signal path lengths in order to obtain the highest cell density with the lowest access time.

This may lead to a physical cell layout unsuitable for use as a light transducer and therefore not all DRAM chips available on the market are suitable for use as a solid state camera. Even those that have been commercially used require some image pre-processing in order to produce an acceptable image, such as the removal of the black line caused by the memory cells power supply rail. Two other drawbacks of these devices are the limited input dynamic range (and the relative long exposure times required) and the inherent binary nature of the image (since each cell output can only be a logic 1 or 0).

A grey scale image using this device can, however, be produced at the expense of image acquisition time by overlaying several binary images taken with different exposure times. DRAM cameras are inexpensive in comparison with other optical array transducers because the optical array transducers used (the DRAM memory chip) is manufactured in large numbers for the computing industry. This feature has produced wide acceptance of the DRAM camera in low cost vision systems, such as those used on educational robots and in light industrial automation (Lambert, 1984; Owen, 1982).

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