Datasheet

ADA4410-6
Rev. B | Page 14 of 16
As previously discussed, the composite video output is
developed by passively summing the Y and C outputs that have
passed through their respective output gain stages, then multiplying
this sum by a factor of two to obtain the output (see
Figure 1).
The offset of this output is equal to 2× that of the other outputs.
Because of this, in many cases, it is necessary to ac-couple the
CV output or ensure that it is connected to an input that is ac-
coupled. This is generally not an issue because it is common
practice to employ ac coupling on composite video inputs.
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 tables 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 23 and Figure 24 illustrate several ways to use the
LEVEL1 and LEVEL2 inputs.
Figure 23 shows an example 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 24 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 24 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.
05265-048
DUAL SUPPLY
0.1µF
LEVEL1
9.53kΩ
1kΩ
9.53kΩ
+5V
–5V
0.1µF
LEVEL2
9.53kΩ
1kΩ
9.53kΩ
+5V
–5V
SINGLE SUPPLY
0.1µF
LEVEL1
1kΩ
9.09kΩ
+5V
0.1µF
LEVEL2
1kΩ
9.09kΩ
+5V
Figure 23. Generating Fully Adjustable Output Offsets
05265-049
0.1μF
LEVEL1
634Ω
10kΩ
+5V
DNP
LEVEL2
0Ω
DNP
+5V
Figure 24. Flexible Circuits to Set the LEVEL1 and LEVEL2 Inputs to Obtain
a 600 mV Output Offset on a Single Supply (G = ×2)
INPUT AND OUTPUT COUPLING
Inputs to the ADA4410-6 are normally dc-coupled. Ac coupling
the inputs is not recommended; however, if ac coupling is
necessary, suitable circuitry must be provided following the ac
coupling element to provide proper dc level and bias currents at
the ADA4410-6 input stages.
The ADA4410-6 outputs can be either ac- or dc-coupled. As
discussed in the
Output DC Offset Control section, the CV
output offset is different from the other outputs, and the CV
output is generally ac-coupled.
When driving single ac-coupled loads in standard 75 Ω video
distribution systems, 220 µF coupling capacitors are recommended
for use on all but the chrominance signal output. Because the
chrominance signal is a narrow-band modulated carrier, it has
no low frequency content and can therefore be coupled with a
0.1 µF capacitor.
There are two ac coupling options when driving two loads from
one output. One is to simply use the same value capacitor on
the second load, while the other is to use a common coupling
capacitor that is at least twice the value used for the single load
(see
Figure 25 and Figure 26).
0
5265-054
75Ω
75Ω
75Ω
CABLE
75Ω
CABLE
220µF
220µF
75Ω
75Ω
Figure 25. Driving Two AC-Coupled Loads with Two Coupling Capacitors
05265-055
75
Ω
CABLE
75Ω
CABLE
75Ω
75Ω
75Ω
75Ω
470µF
Figure 26. Driving Two AC-Coupled Loads with One Common Coupling Capacitor