KAI-2001 datasheet(31/36 Pages) ONSEMI

31 / 36 page

KAI−2001

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Electronic Shutter Description

The voltage on the substrate (SUB) determines the charge

capacity of the photodiodes. When SUB is 8 V the

photodiodes will be at their maximum charge capacity.

Increasing VSUB above 8 V decreases the charge capacity

of the photodiodes until 48 V when the photodiodes have

a charge capacity of zero electrons. Therefore, a short pulse

on SUB, with a peak amplitude greater than 48 V, empties

all photodiodes and provides the electronic shuttering

action.

It may appear the optimal substrate voltage setting is 8 V

to obtain the maximum charge capacity and dynamic range.

While setting VSUB to 8 V will provide the maximum

dynamic range, it will also provide the minimum

anti-blooming protection.

The KAI−2001 VCCD has a charge capacity of

55,000 electrons (55 ke−). If the SUB voltage is set such that

the photodiode holds more than 55 ke−, then when the

charge is transferred from a full photodiode to VCCD,

the VCCD will overflow. This overflow condition manifests

itself in the image by making bright spots appear elongated

in the vertical direction. The size increase of a bright spot is

called

blooming

when

the

spot

doubles

size.

The blooming can be eliminated by increasing the voltage

on SUB to lower the charge capacity of the photodiode. This

ensures the VCCD charge capacity is greater than the

photodiode capacity. There are cases where an extremely

bright spot will still cause blooming in the VCCD. Normally,

when the photodiode is full, any additional electrons

generated by photons will spill out of the photodiode.

The excess electrons are drained harmlessly out to the

substrate. There is a maximum rate at which the electrons

can be drained to the substrate. If that maximum rate is

exceeded, (for example, by a very bright light source) then

it is possible for the total amount of charge in the photodiode

to exceed the VCCD capacity. This results in blooming.

The amount of anti-blooming protection also decreases

when the integration time is decreased. There is

a compromise

between

photodiode

dynamic

range

(controlled by VSUB) and the amount of anti-blooming

protection. A low VSUB voltage provides the maximum

dynamic range and minimum (or no) anti-blooming

protection. A high VSUB voltage provides lower dynamic

range and maximum anti-blooming protection. The optimal

setting of VSUB is written on the container in which each

KAI−2001 is shipped. The given VSUB voltage for each

sensor is selected to provide anti-blooming protection for

bright spots at least 100 times saturation, while maintaining

at least 40 ke− of dynamic range.

The electronic shutter provides a method of precisely

controlling

the

image

exposure

time

without

any

mechanical components. If an integration time of tINT is

desired, then the substrate voltage of the sensor is pulsed to

at least 40 V tINT seconds before the photodiode to VCCD

transfer pulse on V2. Use of the electronic shutter does not

have to wait until the previously acquired image has been

completely read out of the VCCD.

Large Signal Output

When the image sensor is operated in the binned or

summed interlaced modes there will be more than

40,000 electrons in the output signal. The image sensor is

designed with a 16

mV/e charge to voltage conversion on the

output. This means a full signal of 40,000 electrons will

produce a 640 mV change on the output amplifier.

The output amplifier was designed to handle an output

swing of 640 mV at a pixel rate of 40 MHz. If 80,000

electron charge packets are generated in the binned or

summed interlaced modes then the output amplifier output

will have to swing 1,280 mV. The output amplifier does not

have enough bandwidth (slew rate) to handle 1,280 mV at

40 MHz. Hence, the pixel rate will have to be reduced to

20 MHz if the full dynamic range of 80,000 electrons is

desired.

The charge handling capacity of the output amplifier is

also set by the reset clock voltage levels. The reset clock

driver circuit is very simple if an amplitude of 5 V is used.

But the 5 V amplitude restricts the output amplifier charge

capacity to 40,000 electrons. If the full dynamic range of

80,000 electrons is desired then the reset clock amplitude

will have to be increased to 7 V.

If you only want a maximum signal of 40,000 electrons in

binned or summed interlaced modes, then a 40 MHz pixel

rate with a 5 V reset clock may be used. The output of the

amplifier will be unpredictable above 40,000 electrons so be

sure to set the maximum input signal level of your analog to

digital converter to the equivalent of 40,000 electrons

(640 mV).

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