Analog Devices AD9843A manual 42 to

AD9843A

DC RESTORE
CDS GAIN
REGISTER			INTERNAL
6			VREF
–2dB TO +10dB	2dB TO 36dB		2V FULL SCALE
0.1 F CCDIN			10-BIT	10
CDS	VGA				DOUT
			ADC
INPUT OFFSET
CLAMP					CLPOB
		8-BIT	OPTICAL BLACK
			CLAMP
	10	DAC
CLPDM			DIGITAL	0 TO 64 LSB
	VGA GAIN		FILTERING
	REGISTER			8
				CLAMP LEVEL
				REGISTER

Figure 11. CCD-Mode Block Diagram

CIRCUIT DESCRIPTION AND OPERATION

The AD9843A signal processing chain is shown in Figure 11. Each processing step is essential in achieving a high-quality image from the raw CCD pixel data.

DC Restore

To reduce the large dc offset of the CCD output signal, a dc-restore circuit is used with an external 0.1 ∝F series-coupling capacitor. This restores the dc level of the CCD signal to approxi- mately 1.5 V, to be compatible with the 3 V single supply of the AD9843A.

Correlated Double Sampler

Table VII. Example CDS Gain Settings

	Recommended
Max Input Signal	Gain Range	Register Code Range
250 mV p-p	8 to 10 dB	21 to 31
500 mV p-p	6 to 8 dB	10 to 21
800 mV p-p	4 to 6 dB	63 to 10
1 V p-p	2 to 4 dB	53 to 63
1.25 V p-p	0 to 2 dB	42 to 53
1.5 V p-p	–2 to 0 dB	32 to 42

The CDS circuit samples each CCD pixel twice to extract the video information and reject low-frequency noise. The timing shown in Figure 5 illustrates how the two CDS clocks, SHP and SHD, are used to sample the reference level and data level of the CCD signal respectively. The CCD signal is sampled on the rising edges of SHP and SHD. Placement of these two clock signals is critical in achieving the best performance from the CCD. An internal SHP/SHD delay (tID) of 3 ns is caused by internal propagation delays.

The CDS stage has a default gain of 4 dB, but uses a unique architecture that allows the CDS gain to be varied. Using the CDS Gain Register, the gain-of is programmable from –2 dB to +10 dB in 64 steps, using two’s complement coding. The CDS Gain curve is shown in Figure 12. To change the gain of the CDS using the CDS Gain Register, the Control Register Bit D3 must be set high (CDS Gain Enabled). The default gain setting when bit Control Register bit D3 is low (CDS Gain Disabled) is 4 dB. See Tables V and VI for more details.

	10
	8
– dB	6
CDS GAIN	4
	2
	0
	-2	40	48	56	0	8	16	24	31
	32
	(100000)								(011111)

CDS GAIN REGISTER CODE

Figure 12. CDS Gain Curve

Input Clamp

A CDS gain of 4 dB provides some front-end signal gain and improves the overall signal-to-noise ratio. This gain setting works very well in most applications, and the CCD-Mode Specifications use this default gain setting. However, the CDS gain may be varied to optimize the AD9843A operation in a particular application. Increased CDS gain can be useful with low output level CCDs, while decreased CDS gain allows the AD9843A to accept CCD signal swings greater than 1 V p-p. Table VII summarizes some example CDS gain settings for different maximum signal swings. The CDS Gain Register may also be used “on the fly” to provide a +6 dB boost or –6 dB attenuation when setting exposure levels. It is best to keep the CDS output level from exceeding 1.5 V~1.6 V.

A line-rate input clamping circuit is used to remove the CCD’s optical black offset. This offset exists in the CCD’s shielded black reference pixels. Unlike some AFE architectures, the AD9843A removes this offset in the input stage to minimize the effect of a gain change on the system black level, usually called the “gain step.” Another advantage of removing this offset at the input stage is to maximize system headroom. Some area CCDs have large black level offset voltages, which, if not corrected at the input stage, can significantly reduce the available headroom in the internal circuitry when higher VGA gain settings are used.

Horizontal timing is shown in Figure 6. It is recommended that the CLPDM pulse be used during valid CCD dark pixels. CLPDM may be used during the optical black pixels, either

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