TC215

1024-

× 1024-PIXEL CCD IMAGE SENSOR

SOCS014B – AUGUST 1989 – REVISED DECEMBER 1991

2-4

POST OFFICE BOX 655303

detailed description

The TC215 consists of three basic functional blocks: (1) the image-sensing area, (2) the multiplexer block with

serial registers and transfer gates, and (3) the low-noise signal-processing amplifier block with charge-detection

nodes. The location of each of these blocks is identified in the functional block diagram.

image-sensing area

Figures 1 and 2 show cross sections with potential well diagrams and top views of image-sensing elements. As

light enters the silicon in the image-sensing area, free electrons are generated and collected in the potential

wells of the sensing elements. During this time, blooming protection is activated by applying a burst of pulses

to the antiblooming gate inputs every horizontal blanking interval. This prevents blooming caused by the spilling

of charge from overexposed elements into neighboring elements. After integration is complete, the signal

charge is transferred in the dark to the two serial registers, where it is read out line by line.

There are 24 full columns of elements at the left edge of the image-sensing area that are shielded from incident

light; these elements provide the dark reference used in subsequent video-processing circuits to restore the

video black level. There are also six dark lines at the bottom of the sensor.

multiplexer with transfer gates and serial registers

The multiplexer and transfer gates transfer charge line by line from the image-sensing columns into the

corresponding serial registers and prepare it for readout. Figure 3 illustrates the layout of the multiplexing gate

that vertically separates the pixels for input into the serial registers. Figure 4 shows the layout of the interface

region between the serial-register gates and the transfer gates. Multiplexing is activated during the horizontal

blanking interval by applying appropriate pulses to the transfer gates and serial registers; the required pulse

timing is shown in Figure 5. A drain is also included to provide the capability to clear the image-sensing area

of unwanted charge. Such charge can accumulate in the imager during the start-up of operation or under special

circumstances. The clearing timing is illustrated in Figure 6.

serial-register readout and video processing

After transfer into the serial registers, the pixels are normally read out 180

° out of phase (see Figure 7). Each

serial register must be reset to the reference level before the next pixel is read out. The timing for the resets and

their relationships to the serial-register pulses is shown in Figure 8. Figure 8 also shows the timing for the pixel

clamp and sample and hold needed for an off-chip double-correlated sampling circuit. These two output signals

can provide a data rate of 20 million pixels per second when combined off chip. After the charge is placed on

the detection node, it is buffered and amplified by a low-noise, dual-stage source follower. Each serial register

contains 12 dummy elements that are used to span the distance between the serial register and the output

amplifier. A schematic is shown in Figure 9. The location of the dummy elements, which are considered to be

part of the amplifiers, is shown in the functional block diagram. Figure 10 gives the timing for a single frame of

video. Operating the TC215 at the typical data rate of one field per frame generates video output at a continuous

15 frames per second.