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6

VSP2080

DIFFERENCE AMPLIFIER

The correlated double sampler function is completed when

the output of the data and reference channel are sent to the

difference amplifier where the signals are subtracted. In

addition to providing the difference function, the difference

amplifier amplifies the signal by a factor of 2 which helps

to improve the overall signal-to-noise ratio. The difference

amplifier also generates a differential signal to drive the

voltage-controlled attenuator.

INPUT CLAMP

The output from the CCD array is capacitively coupled to the

VSP2080. To prevent shifts in the DC level from taking place

due to varying input duty cycles, the input capacitor is

clamped during the dummy pixel interval by the REFCK

signal. A P-channel transistor is used for this input clamp

switch to be able to allow a 2V negative change at the input

that would bring the signal below ground by 1V. Under

typical conditions, the black level at the input to the VSP2080

is at 1V.

DUMMY PIXEL AUTO-ZERO LOOP

The output from the data and reference channel is processed

by the previously mentioned difference amplifier. The dif-

ferential output from the difference amplifier is sent to both

the voltage-controlled logarithmic attenuator and to an error

amplifier. The error amplifier amplifies and feeds a signal to

the difference amplifier to drive the offset measured at the

output of the difference amplifier to zero. A block diagram

of this circuit is shown in Figure 3. This error amplifier

serves the purpose of reducing the offset of the CDS to avoid

a large offset from being amplified by the output amplifier.

The effective time constant of this loop is given by:

where R is 10k

Ω, C is an external capacitor connected to

CCD R (pin 14), A is the gain of the error amplifier with a

value of 50, and D is the duty cycle of the time that the dummy

pixel auto-zero loop is in operation. The duty cycle (D) must

be considered as the loop operates in a sampled mode. Opera-

tion of the dummy auto-zero loop is activated by the DUMC

signal that happens once during each horizontal line interval.

TIMING

The REFCK and DATCK signals are used to operate the

CDS as previously explained. The input digital timing sig-

nals REFCK, DATCK, DUMC and OB are capable of being

driven from either 3V or 5V logic levels.

VOLTAGE-CONTROLLED ATTENUATOR

To maximize the dynamic range of the VSP2080, a voltage-

controlled attenuator is included with a control range from

0dB to –34dB. The gain control has a logarithmic relation-

ship between the control voltage and the attenuation. The

attenuator processes a differential signal from the difference

amplifier to improve linearity and to reject both power supply

and common-mode noise. The output from the attenuator is

amplified by 28dB prior to being applied to the A/D. A typical

gain control characteristic of the VSP2080 is shown in the

typical performance curve, “Gain Control Characteristics”.

BLACK LEVEL AUTO-ZERO LOOP

The black level auto-zero loop amplifies the difference

between the output of the output amplifier and a reference

signal during the dummy pixel interval. This difference

signal is amplified and fed back into the output amplifier to

correct the offset. In doing so, the output level of the entire

CCD channel can be controlled to be approximately –FS +

31mV under zero signal conditions. The black level auto-

zero loop is activated by the OB timing signal. Figure 4

shows a block diagram of the black level auto-zero loop. The

loop time constant is given by:

where C is the external filter capacitance applied to C (pin 4),

of the time that the black level auto-zero loop is in operation.

The duty cycle (D) must be considered as the loop operates

in a sampled mode. Operation of the black level auto-zero

loop is activated by the OB signal that happens once during

each horizontal line interval.

FIGURE 3. Simplified Block Diagram of Dummy Pixel

Loop.

FIGURE 4. Simplified Block Diagram of Optical Black

Level Auto-Zero Loop.

T

C

GD

M

=

•

DUMC

To VCA

CDS

Error

Amplifier

A

CCD D

CCD R

R

C

EXT

OUT

1.03 • REF IN

Error

Amplifier

Output Amplifier

From

VCA

C

C

EXT

OB

T

RC

AD

=

•

•