AMIS-720341-A: Contact Image Sensor
Data Sheet
4.0 Output Circuit of the Image Sensor
The video signal from each photo-site is connected to a common video line on the sensor. Each photo-site is composed of a
phototransistor with a series MOS switch connecting its emitter to a common video line. The video line is connected to the pad labeled
IOUT. The photo-sites are read out upon the closure of the MOS switch, which is sequentially switched on and off by its internal
scanning shift register (see Figure 1). For the clock and timing operation image sensor see Figure 11. The photo-sensing element is the
base of the phototransistor where it detects and converts the light energy to proportional charges and stores them in its base and
collector capacitance. When the MOS switch is activated, the emitter is connected to the video line and acts as the source follower,
producing an impulse current proportional to the stored charges in the base. This current is a discrete-time analog signal output called
the video pixel. The charges in the video pixel are proportional to the light energy impinging in the neighborhood of its photo-sites.
Figure 4 shows an output structure of four photo-sites out of 128. The multiplexing MOS switch in each photo-site terminates into the
output pad, IOUT, through a common video line. As the shift register sequentially accesses each photo-site, the charges of the video
pixel is sent to the IOUT where they are processed with an external signal conversion circuit. See Section 5.0.
Figure 4: Video Pixel Output Structures
AMI Semiconductor – May 06, M-20570-001
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