AD9974BBCZ datasheet(30/52 Pages) AD

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AD9974

Rev. A | Page 30 of 52

Input Configurations

The CDS circuit samples each CCD pixel twice to extract the video

information and reject low frequency noise (see Figure 39). There

are three possible configurations for the CDS: inverting CDS mode,

noninverting CDS mode, and SHA mode. The CDSMODE

SHA1

SHA2

SHD

SHP

DIFF

AMP

CDS_OUT

CCDINP

CCDINM

Figure 39. CDS Block Diagram (Conceptual)

Inverting CDS Mode

For this configuration, the signal from the CCD is applied to the

positive input of the (CCDINP) CDS system with the minus

side (CCDINM) grounded (see Figure 40). The CDSMODE

applications use this configuration with the reset level established

below the AVDD supply level by the AD9974 dc restore circuit,

at approximately 1.5 V. The maximum saturation level is 1.0 V

below the reset level, as shown in Figure 41 and Table 16. A maxi-

mum saturation voltage of 1.4 V is also possible when using the

minimum CDS gain setting.

IMAGE

SENSOR

SHA/

CDS

CCDINM

CCDINP

AD9974

NOTES

1. COUPLING CAPACITOR IS NOT REQUIRED FOR CERTAIN

BLACK LEVEL REFERENCE VOLTAGES.

Figure 40. Single Input CDS Configuration

(N) SIGNAL SAMPLE

(N) RESET SAMPLE

(N + 1) RESET SAMPLE

VDD

RESET LEVEL

(VRST)

SIGNAL LEVEL

(VFS)

Figure 41. Traditional Inverting CDS Signal

Table 16. Inverting Voltage Levels

Signal Level

Symbol

Min (mV)

Typ (mV)

Max (mV)

Saturation

VFS

1000

1400

Reset

VRST

VDD − 500

VDD − 300

VDD

Supply Voltage

VDD

1600

1800

2000

Noninverting Input

If the noninverting input is desired, the reset (or black) level

signal is established at a voltage above ground potential.

Saturation (or white) level is approximately 1 V. Samples are

taken at each signal level. See Figure 42 and Table 17.

(N) SIGNAL SAMPLE

(N) RESET SAMPLE

(N + 1) RESET SAMPLE

GND

RESET LEVEL

(VRST)

SIGNAL LEVEL

(VFS)

Figure 42. Noninverting CDS Signal

Table 17. Noninverting Voltage Levels

Signal Level

Symbol

Min (mV)

Typ (mV)

Max (mV)

Saturation

VFS

1000

1400

Reset

VRST

250

500

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