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CORRELATED DOUBLE SAMPLER

OUT

IN1

OPA694

SOTA

HOLD1

27pF

402

Ω

100

Ω

100

Ω

300

Ω

300

Ω

402

Ω

402

Ω

402

Ω

O T A

IN2

SOTA

HOLD2

27pF

Ω

100

Ω

100

Ω

300

Ω

300

Ω

O T A

OPA615

SBOS299B – FEBRUARY 2004 – REVISED JULY 2005

The signal coming from the CCD is applied to the two

sample-and-hold amplifiers, with their outputs con-

Noise is the limiting factor for the resolution in a CCD

nected to the difference amplifier. The timing diagram

system, where the kT/C noise is dominant (see

clarifies the operation (see Figure 52). At time t1, the

Figure 51). To reduce this noise, imaging systems

sample and hold (S/H1) goes into the hold mode,

use a circuit called a Correlated Double Sampler

taking a sample of the reset level including the noise.

(CDS). The name comes from the double sampling

This voltage (VRESET) is applied to the noninverting

technique of the CCD charge signal. A CDS using

input of the difference amplifier. At time t2, the

two OPA615s and one OPA694 is shown in Fig-

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

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

video level, which is VRESET – VVIDEO. The output

reset period. When the reset switch opens again, the

voltage of the difference amplifier is defined by 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 resist-

voltage contains the kT/C noise, which is eliminated

ance. This difference causes the dominating kT/C

by the subtraction of the difference amplifier.

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

(

∆V).

from the instant of the first sample, the noise ampli-

tudes 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 ampli-

fier constitute the correlated double sampler.

Figure 50. Correlated Double Sampler

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