AD9804
VARIABLE GAIN AMPLIFIER (VGA) OPERATION
DETAILS
capacitor. The AD9804 performs the dc restoration, CDS, gain
adjustment, black level correction, and analog-to-digital con-
version. The AD9804’s digital output data is then processed by the
image processing ASIC. The internal registers of the AD9804
used to control gain, offset level, and other functions are pro-
grammed by the ASIC or microprocessor through a 3-wire serial
digital interface. A system timing generator provides the clock
signals for both the CCD and the AFE.
The VGA stage provides a gain range of 6 dB to 40 dB, pro-
grammable with 10-bit resolution through the serial digital
interface. The minimum gain of 6 dB is needed to match a 1 V
input signal with the ADC full-scale range of 2 V. When com-
pared to 1 V full-scale systems (such as ADI’s AD9803), the
equivalent gain range is 0 dB to 34 dB.
The VGA gain curve is divided into two separate regions. When
the VGA Gain Register code is between 0 and 511, the curve
follows a (1 + x)/(1 – x) shape, which is similar to a “linear-in-
dB” characteristic. From code 512 to code 1023, the curve follows
a “linear-in-dB” shape. The exact VGA gain can be calculated
for any Gain Register value by using the following two equations:
Generating the Reset (RSTB) Signal
After power-on, the AD9804 must be reset using Pin 43 (RSTB).
The reset pulse must be an active low signal, which goes low for
at least 100 ns after the power supplies have settled. After the
RSTB signal returns high, the AD9804 is internally reset to the
default VGA gain register value. If a system reset pulse is not
available, a simple RC network may be used, as shown in Figure
7. The time constant of this network should be comparable
to the power-on time of the AD9804’s power supplies. For
example, if the power supplies have a power-on time of 10 ms,
the RC network should have a time constant of 10 ms, giving
R = 10 kΩ and C = 1.0 µF.
Code Range Gain Equation (dB)
0–511
512–1023
Gain = 20 log10 ([658 + code]/[658 – code]) + 3.6
Gain = (0.0354)(code) + 3.6
As shown in the Analog Specifications, only the VGA gain range
from 2 dB to 36 dB has been specified. This corresponds to a
VGA gain code range of 95 to 1023.
Serial writes to the AD9804 internal registers must not be per-
formed until 20 µs after the reset pulse has occurred. This allows
enough time for internal calibration routines to be completed.
SDATA and SCK may be active before the reset sequence, but
SL should be held logic HIGH until 20 µs or more after the reset.
40
34
28
22
16
10
4
Alternatively, placing series resistors close to the digital out-
put pins may help reduce noise.
Grounding and Decoupling Recommendations
As shown in Figure 7, a single ground plane is recommended for
the AD9804. This ground plane should be as continuous as
possible, particularly around Pins 25 through 39. This will ensure
that all analog decoupling capacitors provide the lowest possible
impedance path between the power and bypass pins and their
respective ground pins. All decoupling capacitors should be
located as close as possible to the package pins. A single clean
power supply is recommended for the AD9804, but a separate
digital driver supply may be used for DRVDD (Pin 13). DRVDD
should always be decoupled to DRVSS (Pin 14), which should
be connected to the analog ground plane. Advantages of using a
separate digital driver supply include using a lower voltage (2.7 V)
to match levels with a 2.7 V ASIC, reducing digital power dissipa-
tion, and reducing potential noise coupling. If the digital outputs
(Pins 3–12) must drive a load larger than 20 pF, buffering is
recommended to reduce digital code transition noise.
0
127
255
383
511
639
767
895
1023
VGA GAIN REGISTER CODE
Figure 5. VGA Gain Curve
APPLICATIONS INFORMATION
The AD9804 is a complete Analog Front-End (AFE) product
for PC camera, digital still camera, and camcorder applications.
As shown in Figure 6, the CCD image (pixel) data is buffered
and sent to the AD9804 analog input through a series input
DIGITAL
OUTPUTS
AD9804
CCD
V
OUT
ADC
OUT
0.1F
DIGITAL IMAGE
PROCESSING
ASIC
SERIAL
INTERFACE
CCDIN
VGA GAIN
BUFFER
V-DRIVE
CDS/CLAMP
TIMING
CCD
TIMING
TIMING
GENERATOR
Figure 6. System Block Diagram
–7–
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