CCD Basics

A CCD operates by accumulating charge on a semiconductor area.When photons

fall on a CCD pixel array, the energy from the photons is absorbed by the silicon,

causing an electron-hole pair to be formed. The number of electron-hole pairs is

directly related to the number of photons that were absorbed, and so is directly

related to the amount of light. The longer that charge is allowed to accumulate,

the more electron-hole pairs will be formed. The process of allowing light to fall

on a CCD array for a particular time to accumulate charge is called

and the amount of time that charge is allowed to accumulate is called the

time

The accumulated charge represents an electrostatic potential. It can be

moved by applying voltages to the clock pins of the CCD, creating changing

potential voltages that can push the electrostatic charge around. There are a

number of mechanisms to generate the needed voltages, all with different

numbers of clock inputs and timing requirements. The essential point is that

the CCD is configured as an analog shift register that passes the charges in one

direction, from one cell to the next. At the end of the shift register is a sense

node that converts the electrostatic charge to a voltage. Figure 3.16 illustrates

the CCD process.

The sense node is constructed using a floating gate. The output of the sense

node is directly proportional to the charge on this gate. To measure charge, the

gate must first be drained of existing charge, which is performed with a reset

transistor.

The functions that must be performed in any CCD-based system consist of the

following:

integration,integration.

.

register. This may require up to four input pins on the CCD, each with a clock

signal of a different phase.

Provide phase clocks to control movement of the charges along the CCD shift

.

Reset the output node prior to each measurement.

.

ADC.

Read the analog output voltage and convert it to a digital value using an