AD9840A
–12–
REV. 0
together with CLPOB or separately. The CLPDM pulse should
be a minimum of four pixels wide.
Variable Gain Amplifier
The VGA stage provides a gain range of 2 dB to 36 dB, program-
mable with 10-bit resolution through the serial digital interface.
Combined with the typical 4 dB gain from the CDS
stage, the
total gain range for the AD9840A is 6 dB to 40 dB. A gain of 6 dB
will match a 1 V input signal with the ADC full-scale range of 2 V.
When compared 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:
Code Range Gain Equation (dB)
0–511 Gain = 20 log
10
([658 + code]/[658 – code]) – 0.35
512–1023 Gain = (0.0354)(code) – 0.35
Using these two equations, the actual gain of the AD9840A can
be accurately predicted to within 0.5 dB. As shown in the CCD-
Mode Specifications, only the VGA gain range from 2 dB to 36 dB
is specified. This corresponds to a VGA gain code range of 91 to
1023. The Gain Accuracy specifications also include a CDS gain
of 4 dB, for a total gain range of 6 dB to 40 dB.
VGA GAIN REGISTER CODE
36
0
VGA GAIN – dB
127 255 383 511 639 767 895 1023
30
24
18
12
6
0
Figure 13. VGA Gain Curve (Gain from CDS Not Included)
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. Any value between 0 LSB and 64 LSB may be pro-
grammed, with 8-bit resolution. The resulting error signal is
filtered to reduce noise, and the correction value is applied to
the ADC input through a D/A converter. 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 the post processing, the
AD9840A’s optical black clamping may be disabled using Bit D5
in the Operation Register (see Serial Interface Timing and
Internal Register Description section). When the loop is dis-
abled, the Clamp Level Register may still be used to provide
programmable offset adjustment.
Horizontal timing is shown in Figure 6. The CLPOB pulse
should be placed during the CCD’s optical black pixels. It is
recommended that the CLPOB pulse duration be at least 20
pixels wide to minimize clamp noise. Shorter pulsewidths may be
used, but clamp noise may increase, and the loop’s ability to
track low-frequency variations in the black level will be reduced.
A/D Converter
The AD9840A uses a high-performance ADC architecture,
optimized for high speed and low power. Differential nonlin-
earity (DNL) performance is typically better than 0.5 LSB.
Instead of the 1 V full-scale range used by the earlier AD9801 and
AD9803 products from Analog Devices, the AD9840A’s ADC
uses a 2 V input range. Better noise performance results from
using a larger ADC full-scale range.
AUX1-Mode
For applications that do not require CDS, the AD9840A can be
configured to sample ac-coupled waveforms. Figure 14 shows the
circuit configuration for using the AUX1 channel input (Pin
36). A single 0.1 µF ac-coupling capacitor is needed between the
input signal driver and the AUX1IN pin. An on-chip dc-bias
circuit sets the average value of the input signal to approxi-
mately 0.4 V, which is referenced to the midscale code of the ADC.
The VGA gain register provides a gain range of 0 dB to 36 dB
in this mode of operation (see VGA Gain Curve, Figure 13).
The VGA gains up the signal level with respect to the 0.4 V bias
level. Signal levels above the bias level will be further increased
to a higher ADC code, while signal levels below the bias level
will be further decreased to a lower ADC code.
AUX2-Mode
For sampling video-type waveforms, such as NTSC and PAL
signals, the AUX2 channel provides black level clamping, gain
adjustment, and A/D conversion. Figure 15 shows the circuit
configuration for using the AUX2 channel input (Pin 34). An
external 0.1 µF blocking capacitor is used with the on-chip
video clamp circuit, to level-shift the input signal to a desired
reference level. The clamp circuit automatically senses the most
negative portion of the input signal, and adjusts the voltage
across the input capacitor. This forces the black level of the input
signal to be equal to the value programmed into the Clamp Level
register (see Serial Interface Register Description). The VGA
provides gain adjustment from 0 dB to 18 dB. The same VGA
Gain register is used, but only the 9 MSBs of the gain register
are used (see Table VIII.)
