260 
2.2 Adaptation Processor 
A tiny analog processor is integrated with the photoreceptor. Its function is to realise a dynamic adaptation to the local 
ambiant light. This adaptation eliminates the slow temporal variation and reacts upon a fast temporal variation. Figure 1 
shows the bloc diagram of the pixel. The analog information is transformed in a digital one at the output. This digital 
output can be readout fastly without noise-speed problems such as in analog image sensors. The acquisition of one 
image is made with the following sequence: in a first time the photodiode is precharged to a value Vinit; in a second 
time it is insulated from polarisation, by blocking the precharge transistor, and receives the light information (exposition 
time); then to the end of a determined time laps, the photodiode voltage is thresholded. The threshold voltage is used as 
the adaptation variable. This voltage is stored in an analog memory composed of a high quality double-poly capacitor 
and a small size access transistor in order to reduce the leakage wich can be critical for low speed adaptation. This 
voltage is adjusted periodically by small steps according to the result of comparison between photoreceptor voltage 
and his own value. Figure 2 shows the details of one pixel. The output of the photoreceptor is compared with the value 
stored in the analog memory cell thresholded by a comparator (bloc delimited by full line in figure 2). The step generator 
is delimited by the dot line in figure 2. Steps are generated by sending pulses on Vinc and Vdec. To have a good 
precision, the steps have to be as small as possible. 
The binary output can inform on the pixel state. When it is adapted (its sensitivity is adjusted), the output has an 
interleaved "0" and "1". A lighting temporal variation locks the output to "1"or" 0" by the variation's direction. There is then 
a progressive return to the adapted state. Variations can thus be detected by examining the binary output. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Vinit 
Memorization 
Precharge 4 [ 
* | : Numerical Binary 
- memory cell output 
NIS Analog|memory cell 
L eer t 
Vphoto \ ; e dem Step li 
— € + 
ry ) 
Figure 1: Bloc diagram of a pixel. 
  
Memorization 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Vdd Memorization 71. 
Vinit p———4 
Vselect 
Precharge >—d[ i 7 Le Binary 
—À—— [output 
F RR T 
| IL. | vad 
» 
NI : 
mes ir Jr : 
| . 
voy TY : bI— Vinc 
*t re EEL LLL LLL EARL LLL RSS 
dz — Vdeci 
  
  
7 e Vmem - 
Figure 2: Pixel's details. 
  
ra a ar am an am am am am am am am am am am ar um 
  
  
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995