ADA4412-3
Rev. 0 | Page 10 of 16
APPLICATIONS
OVERVIEW
With its high impedance inputs and high output drive, the
ADA4412-3 is ideally suited to video reconstruction and
antialias filtering applications. The high impedance inputs give
designers flexibility with regard to how the input signals are
terminated. Devices with DAC current source outputs that feed
the ADA4412-3 can be loaded in whatever resistance provides
the best performance, and devices with voltage outputs can be
optimally terminated as well. The ADA4412-3 outputs can each
drive up to two source-terminated 75 Ω loads and can therefore
directly drive the outputs from set-top boxes, DVD players, and
the like without the need for a separate output buffer.
Binary control inputs are provided to select the filter cutoff
frequency. These inputs are compatible with 3 V and 5 V TTL
and CMOS logic levels referenced to GND. The disable feature
is asserted by pulling the DISABLE pin to the positive supply.
The LEVEL1 and LEVEL2 inputs comprise a differential input
that controls the dc level at the output pins.
DISABLE
The ADA4412-3 includes a disable feature that can be used
to save power when a particular device is not in use. As
indicated in the
Overview section, the disable feature is
asserted by pulling the DISABLE pin to the positive supply.
The DISABLE pin also functions as a reference level for the
logic inputs and therefore must be connected to ground when
the device is not disabled.
Table 6 summarizes the disable feature operation.
Table 6. DISABLE Function
DISABLE Pin Connection Status
V
S+
Disabled
GND Enabled
CUTOFF FREQUENCY SELECTION
Four combinations of cutoff frequencies are provided for the
video signals. The cutoff frequencies have been selected to
correspond with the most commonly deployed component
video scanning systems. Selection between the cutoff frequency
combinations is controlled by the logic signals applied to the
F_SEL_A and F_SEL_B inputs.
Table 7 summarizes cutoff
frequency selection.
Table 7. Filter Cutoff Frequency Selection
F_SEL_A F_SEL_B Y/G Cutoff Pb/B Cutoff Pr/R Cutoff
0 0 36 MHz 36 MHz 36 MHz
0 1 36 MHz 18 MHz 18 MHz
1 0 18 MHz 18 MHz 18 MHz
1 1 9 MHz 9 MHz 9 MHz
OUTPUT DC OFFSET CONTROL
The LEVEL1 and LEVEL2 inputs work as a differential, input-
referred output offset control. In other words, the output offset
voltage of a given channel is equal to the difference in voltage
between the LEVEL1 and LEVEL2 inputs multiplied by the
overall filter gain. This relationship is expressed in Equation 1.
)
)(2)( LEVEL2LEVEL1OUTV
OS
−= (1)
LEVEL1 and LEVEL2 are the voltages applied to the respective
inputs, and the factor of 2 reflects the gain of ×2 in the output
stage.
For example, setting LEVEL1 to 300 mV and LEVEL2 to 0 V
shifts the offset voltages at the ADA4412-3 outputs to 600 mV.
This particular setting can be used in most single-supply
applications to keep the output swings safely above the negative
supply rail.
The maximum differential voltage that can be applied across the
LEVEL1 and LEVEL2 inputs is ±500 mV. From a single-ended
standpoint, the LEVEL1 and LEVEL2 inputs have the same
range as the filter inputs. See the
Specifications for the limits.
The LEVEL1 and LEVEL2 inputs must each be bypassed to
GND with a 0.1 µF ceramic capacitor.
In single-supply applications, a positive output offset must be
applied to keep the negative-most excursions of the output
signals above the specified minimum output swing limit.
Figure 16 and Figure 17 illustrate several ways to use the
LEVEL1 and LEVEL2 inputs.
Figure 16 shows examples of how
to generate fully adjustable LEVEL1 and LEVEL2 voltages from
±5 V and single +5 V supplies. These circuits show a general
case, but a more practical approach is to fix one voltage and
vary the other.
Figure 17 illustrates an effective way to produce
a 600 mV output offset voltage in a single-supply application.
Although the LEVEL2 input could simply be connected to
GND,
Figure 17 includes bypassed resistive voltage dividers for
each input so that the input levels can be changed, if necessary.
Additionally, many in-circuit testers require that I/O signals not
be tied directly to the supplies or GND. DNP indicates do not
populate.
