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

O T A

12

2

3

V

IN2

SOTA

7

10

11

4

V

HOLD2

27pF

50

Ω

100

Ω

100

Ω

300

Ω

300

Ω

O T A

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 t1, the sample and hold (S/H1) 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 (VRESET) 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

t2, the sample-and-hold (S/H2) will take a sample of

Figure 50. The first sample (S1) is taken at the end of

the video level, which is VRESET – VVIDEO. 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 VOUT = VIN+ – VIN–. The sample of the reset

large

difference

in

the

switch

RON and ROFF

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 (S2) 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 (VRESET) as well as of the video amplitude

only by a voltage corresponding to the transferred

(VRESET – VVIDEO). 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

Copyright © 2004–2009, Texas Instruments Incorporated

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