CCD191 Datasheet, PDF (2/8 Pages) Fairchild Semiconductor – 6000 Element Linear Image Sensor

CCD191
FUNCTIONAL DESCRIPTION
The CCD191 consists of the following functional elements illustrated
in the Block Diagram and Circuit Diagram.
Photosites — A row of 6000 image sensor elements separated by
a diffused channel stop and covered by a silicon dioxide surface
passivation layer. Image photons pass through the transparent sili-
con creating hole-electron pairs. The photon generated electrons
are accumulated in the photosites. The amount of charge accumu-
lated in each photosite is a linear function of the incident illumination
intensity and the integration period. The output signal will vary in an
analog manner from a thermally generated background level at zero
illumination to a maximum at saturation under bright illumination.
Two Transfer Gate — Gate structures adjacent to the row of im-
age sensor elements. The charge packets accumulated in the
photosites are transferred in parallel via the transfer gates (φX) to
the transport shift registers whenever the transfer gate voltages go
high. Alternate charge packets are transferred to the A and B trans-
port registers.
Two Analog Shift Registers — The transport shift registers
are used to move the light generated charge packets delivered by
the transfer gates. (φ1A, φ1B, φ2A, φ2B) serially to the charge detector/
amplifier. The parallel layout of the last elements of the two trans-
port registers provides for simultaneous delivery of charge packets
at the output amplifiers.
A Gated Charge Detector/Amplifier — Charge packets are
transported to a precharge capacitor whose potential changes lin-
early in response to the quantity of the signal charge delivered. This
potential is applied to the input gate of the two-stage NMOS amplifi-
ers producing a signal at the output “VOUT” pin. Before each charge
packet is sensed, a reset clock (φRA, φRB) recharges the input node
capacitor to a fixed voltage (VRDA, VRDB)
Integration and Anti-Blooming Controls — In many appli-
cations the dynamic range in parts of the image is larger than the
dynamic range of the CCD, which may cause more electrons to be
generated in the photosite area than can be stored in the CCD shift
register. This is particularly common in industrial inspection and
satellite applications. The excess electrons generated by bright illu-
mination tend to “bloom” or “spill over” to neighboring pixels along
the shift register, thus “smearing” the information. This smearing
can be eliminated using two methods:
Anti-Blooming Operation:
A DC voltage applied to the integration control gate (approximately
1 to 3 volts) will cause excess charge generated in the photosites to
be diverted to the anti-blooming sink (VSINK) instead of to the shift
registers. This acts as a “clipping circuit” for the CCD output.
Integration Control Operation:
Variable integration times which are less than the CCD exposure
time may be attained by supplying a clock to the integration control
gate. Clocking φIC reduces the integration time from tEXPOSURE to
tINT. Greater than 10:1 reduction in average photosite signal can
be achieved with integration control.
The integration-control and anti-blooming features can be imple-
mented simultaneously. This is done by setting the φIC, clock-low
level to approximately 1 to 3 volts.
Prescan Reference — Video output level generated from shift
register cells which provides a reference voltage equivalent to device
operation in the dark. This permits use of external DC restoration
circuitry.
Dynamic Range — The saturation exposure divided by the RMS
temporal noise equivalent exposure. Dynamic range is sometimes
defined in terms of peak-to-peak noise. To compare the two defini-
tions a factor of four to six is generally appropriate in that peak-to-
peak noise is approximately equal to four to six times RMS noise.
RMS Noise Equivalent Exposure — The exposure level that
gives an output signal to the RMS noise level at the output in the
dark.
Saturation Exposure — The minimum exposure level that will
provide a saturation output signal. Exposure is equal to the light
intensity times the photosites integration time.
Charge Transfer Efficiency — Percentage of valid charge infor-
mation that is transferred between each successive stage of the trans-
port registers.
Responsivity — The output signal voltage per unit exposure for a
specified spectral type of radiation. Responsivity equals output volt-
age divided by exposure.
Total Photoresponse Non-uniformity — The difference of the
response levels of the most and the least sensitive element under
uniform illumination. Measurement of PRNU excludes first and last
elements.
Dark Signal — The output signal in the dark caused by thermally
generated electrons that is a linear function of the integration time
and highly sensitive to temperature.
Saturation Output Voltage — The maximum usable signal out-
put voltage. Charge transfer efficiency decreases sharply when the
saturation output voltage is exceeded.
Integration Time — The time interval between the falling edge of
any two successive transfer pulses (φX). The integration is the time
allowed for the photosites to collect charge.
Exposure Time - The time interval between the falling edge of the
two transfer pulses (φX) shown in the timing diagram. The exposure
time is the time between transfers of signal charge from the photosites
into the transport registers.
Pixel - A picture element (photosite).
CCD191 CIRCUIT DIAGRAM
DEFINITION OF TERM
Charge-Coupled Device — A Charge-coupled device is a semi-
conductor device in which finite isolated charge-packets are trans-
ported from one position in the semiconductor to an adjacent posi-
tion by sequential clocking of an array of gates. The charge-packets
are minority carriers with respect to the semiconductor substrate.
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