PI3042A Datasheet, PDF (5/18 Pages) AMI SEMICONDUCTOR – Contact Image Sensor Chip

Prelininary PI3042A datasheet
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 readout upon the closure of the MOS switch, which is sequentially switched on
and off by its internal scanning shift register, see Figure 1, PI3042A Block Diagram. For the
clock & timing operation of the image sensor see Figure 7, Timing Diagram Of The PI3042A
Sensor. 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 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, Video Pixel Output Structures, show a 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 the follow section, Signal Conversion Circuit.
Figure 4. Video Pixel Output Structures
Signal Conversion Circuit
Figure 5, Video Output Test and Application Circuit is an example of the charge conversion that
is used in the CIS modules. It is usually bonded on the same PCB on which the image sensors
are bonded. In applications where cost is an important factor, this simple circuit provides the
cleanest technique in processing the video output. It integrates all the currents from each pixel
element onto a capacitor, CAP. It, also, sums the switch edge’s energy along with the signal
current pulses, hence minimizes the switching patterns on the video pixels. The summed
charges stored on the CAP, produce a pixel voltage. Its voltage amplitude is proportional to the
charge from the current pulse and the value of the CAP.
Page 5 of 18 Date: 09/23/05