AD9992_07 AD [Analog Devices], AD9992_07 Datasheet - Page 63

• Manufacturer Part Number: AD9992_07
• Description: 12-Bit CCD Signal Processor with Precision Timing Generator
• Manufacturer: AD [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 AD9992 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 AD9992.
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; 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 post-
processing, the AD9992 optical 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.
Gain (dB) = (0.0358 × Code) + 5.75
42
36
30
24
18
12
6
0
127
255
Figure 73. VGA Gain Curve
VGA GAIN REGISTER CODE
383
511
639
767
895
1023
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If the CLPOB loop is disabled, higher VGA gain settings reduce
the dynamic range because the uncorrected offset in the signal
path is amplified.
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 AD9992 digital output data is latched using the rising edge
of the DOUTPHASE register value, as shown in Figure 72.
Output data timing is shown in Figure 20 and Figure 21. 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 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 by 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.
AD9992