XRD98L59 Datasheet, PDF (12/37 Pages) Exar Corporation – CCD Image Digitizers with CDS, PGA and 10-Bit A/D

XRD98L59
CORRELATED DOUBLE SAMPLE/HOLD (CDS)
The function of the CDS block is to sense the voltage
difference between the black level and video level for
each pixel. The PGA then amplifies this difference to the
desired level for the ADC. The CDS and PGA are fully
differential. The PGA output is converted to a single
ended signal and fed to the ADC. The CCDin pin (CDS
inverting input) should be connected, via a capacitor, to
the CCD output signal. The REFin pin (CDS non-
inverting input) should be connected, via a capacitor, to
the CCD “Common” voltage. This is typically the CCD
Reference output or ground.
At the beginning (or end) of every video line, the DC
restore switch forces one side of the external capacitors
to an internal Vbias1 level (approximately 0.8V). The DC
restore switch is controlled by the combination of the
CLAMP signal ANDed with the φ2 clock. (See Figure 5).
During the black reference phase of each CCD pixel the
φ1 (Sample Black Reference) switches are turned on,
shorting the PGA1 inputs to a second bias level. The
Coarse Offset DAC adds an adjustment to the Vbias2
level to cancel offset in the CCD signal. When the φ1
switches turn off, the pixel black reference(V_BLACK) is
sampled on the internal reference sample capacitors,
and the PGA is ready to gain up the CCD video signal.
During the video phase of each CCD pixel the difference
between the pixel black reference level and video level is
transmitted through the internal reference sample ca-
pacitors and converted to a fully differential signal by the
PGA1 amplifier. At this time the φ2 (Sample Pixel value)
switches turn on, and the internal video sample capaci-
tors track the amplified difference.
XRD98L59
External
DC blocking
capacitors
CCD
CCDin
CDS
Reset reject
switches
Vbias2
Coarse
φ1
Offset
DAC
AGND
REFin
r_ON
120Ω
CLAMP
φ2
DC restore
switches
Vbias1~0.8V
Internal
black sample
capacitors
(~5PF)
PGA
Fine
Offset
D A C φ2
PGA1
PGA2
To ADC
Internal
video sample
capacitors
(~5PF)
Figure 5. Block Diagram of CDS and PGAs
Rev. 2.00
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