Preliminary PI3041A 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 7, Timing Diagram Of The PI3041A 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 important factor, this simply 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