AD9927 Analog Devices, AD9927 Datasheet - Page 71

• Manufacturer Part Number: AD9927
• Description: 14-Bit CCD Signal Processor
• Manufacturer: Analog Devices

Variable Gain Amplifier
The VGA stage provides a gain range of approximately 6 dB to
42 dB, programmable with 10-bit resolution through the serial
digital interface. A gain of 6 dB is needed to match a 1 V input
signal with the ADC full-scale range of 2 V. When compared to
1 V full-scale systems, the equivalent gain range is 0 dB to 36 dB.
The VGA gain curve follows a linear-in-dB characteristic. The
exact VGA gain is calculated for any gain register value by
where Code is the range of 0 to 1023.
ADC
The AD9927 uses a high performance ADC architecture
optimized for high speed and low power. Differential non-
linearity (DNL) performance is typically better than 0.5 LSB.
The ADC uses a 2 V input range. See Figure 5 and Figure 7 for
typical linearity and noise performance plots for the AD9927.
Optical Black Clamp
The optical black clamp loop is used to remove residual offsets
in the signal chain and to track low frequency variations in the
CCD’s black level. During the optical black (shielded) pixel
interval on each line, the ADC output is compared with a fixed
black level reference, selected by the user in the clamp level
register. The value can be programmed between 0 LSB and
255 LSB in 1023 steps. The resulting error signal is filtered to
reduce noise, and the correction value is applied to the ADC
input through a DAC. Normally, the optical black clamp loop is
turned on once per horizontal line, but this loop can be updated
more slowly to suit a particular application. If external digital
clamping is used during postprocessing, the AD9927 optical
Gain
42
36
30
24
18
12
6
0
(dB)
127
=
. 0 (
0358
255
Figure 86. VGA Gain Curve
VGA GAIN REGISTER CODE
×
383
Code
)
511
+
. 5
75
639
dB
767
895
1023
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black clamping can be disabled using Bit D2 in the AFE Register
Address 0x00. When the loop is disabled, the clamp level
register can still be used to provide fixed offset adjustment.
Note that if the CLPOB loop is disabled, higher VGA gain
settings reduce the dynamic range because the uncorrected
offset in the signal path is gained up.
The CLPOB pulse should be aligned with the CCD’s optical
black pixels. It is recommended that the CLPOB pulse duration
be at least 20 pixels wide. Shorter pulse widths can be used, but
the ability for the loop to track low frequency variations in the
black level will be reduced. See the Horizontal Clamping and
Blanking section for timing examples.
Digital Data Outputs
The AD9927 digital output data is latched using the rising edge
of the DOUTPHASE register value, as shown in Figure 85.
Output data timing is shown in Figure 21 and Figure 22. It is
also possible to leave the output latches transparent, so that the
data outputs are valid immediately from the ADC. Programming
the AFE Register Address 0x01, Bit D1, to 1 sets the output latches
to transparent. The data outputs can also be disabled (three-stated)
by setting the AFE Register Address 0x01, Bit D0, to 1.
The DCLK output can be used for external latching of the
data outputs. By default, the DCLK output tracks the values
of the DOUTPHASE registers. By changing the DCLKMODE
register, the DCLK output can be held at a fixed phase, and the
DOUTPHASE register values are ignored. The DCLK output can
also be inverted with respect to DOUT, using the DCLKINV
register bit.
The switching of the data outputs can couple noise back into
the analog signal path. To minimize switching noise, it is
recommended that the DOUTPHASE registers be set to the
same edge as the SHP sampling location, or up to 15 edges after
the SHP sampling location. Other settings can produce good
results, but experimentation is necessary. It is recommended
that the DOUTPHASE location not occur between the SHD
sampling location and 15 edges after the SHD location. For
example, if SHDLOC = 0, DOUTPHASE should be set to an
edge location of 16 or greater. If adjustable phase is not required
for the data outputs, the output latch can be left transparent
using Address 0x01, Bit D1.
The data output coding is normally straight binary, but the
coding can be changed to gray coding by setting the AFE
Register Address 0x01, Bit D2, to 1.
AD9927