Datasheet

V
OUT
V
IN1
OPA694
SOTA
7
10
11
4
V
HOLD1
27pF
402
Ω
50
Ω
100
Ω
100
Ω
300
Ω
300
Ω
402
Ω
402
Ω
402
Ω
OTA
12
2
3
V
IN2
SOTA
7
10
11
4
V
HOLD2
27pF
50
Ω
100
Ω
100
Ω
300
Ω
300
Ω
OTA
12
2
3
OPA615
www.ti.com ...................................................................................................................................... SBOS299E –FEBRUARY 2004–REVISED SEPTEMBER 2009
CORRELATED DOUBLE SAMPLER The signal coming from the CCD is applied to the two
sample-and-hold amplifiers, with their outputs
Noise is the limiting factor for the resolution in a CCD
connected to the difference amplifier. The timing
system, where the kT/C noise is dominant (see
diagram clarifies the operation (see Figure 52). At
Figure 51). To reduce this noise, imaging systems
time t
1
, the sample and hold (S/H
1
) goes into the hold
use a circuit called a Correlated Double Sampler
mode, taking a sample of the reset level including the
(CDS). The name comes from the double sampling
noise. This voltage (V
RESET
) is applied to the
technique of the CCD charge signal. A CDS using
noninverting input of the difference amplifier. At time
two OPA615s and one OPA694 is shown in
t
2
, the sample-and-hold (S/H
2
) will take a sample of
Figure 50. The first sample (S
1
) is taken at the end of
the video level, which is V
RESET
– V
VIDEO
. The output
the reset period. When the reset switch opens again,
voltage of the difference amplifier is defined by the
the effective noise bandwidth changes because of the
equation V
OUT
= V
IN+
– V
IN–
. The sample of the reset
large difference in the switch R
ON
and R
OFF
voltage contains the kT/C noise, which is eliminated
resistance. This difference causes the dominating
by the subtraction of the difference amplifier.
kT/C noise essentially to freeze in its last point.
The double sampling technique also reduces the
The other sample (S
2
) is taken during the video
white noise. The white noise is part of the reset
portion of the signal. Ideally, the two samples differ
voltage (V
RESET
) as well as of the video amplitude
only by a voltage corresponding to the transferred
(V
RESET
– V
VIDEO
). With the assumption that the noise
charge signal. This is the video level minus the noise
of the noise of the second sample was unchanged
(ΔV).
from the instant of the first sample, the noise
amplitudes are the same and are correlated in time.
The CDS function will eliminate the kT/C noise as
Therefore, the noise can be reduced by the CDS
well as much of the 1/f and white noise.
function.
Figure 52 is a block diagram of a CDS circuit. Two
sample-and-hold amplifiers and one difference
amplifier constitute the correlated double sampler.
Figure 50. Correlated Double Sampler
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