REV. A
AD8186/AD8187
–15–
APPLICATION
Single-Supply Operation
The AD8186/AD8187 are targeted mainly for use in single-
supply 5 V systems. For operating on these supplies, both V
EE
and D
GND
should be tied to ground. The control logic pins will
be referenced to ground. Normally, the DV
CC
supply should be
set to the same positive supply as the driving logic.
For dc-coupled single-supply operation, it is necessary to set an
appropriate input dc level that is within the specified range of the
amplifier. For the unity-gain AD8186, the output dc level will
be the same as the input, while for the gain-of-two AD8187, the
V
REF
input can be biased to obtain an appropriate output dc level.
Figure 10 shows a circuit that provides a gain-of-two and is
dc-
coupled. The video input signals must have a dc bias
from their source of approximately 1.5 V. This same volt-
age is
applied to V
REF
of the AD8187. The result is that when
the
video signal is at 1.5 V, the output will also be at the
same
voltage. This is close to the lower dynamic range of
both the input and the output.
When the input goes most positive, which is 700 mV above the
black level for a standard video signal, it reaches a value of 2.2 V
and there is enough headroom for the signal. On the output
side, the magnitude of the signal will change by 1.4 V, which
will make the maximum output voltage 2.2 V + 1.4 V = 3.6 V.
This is just within the dynamic range of the output of the part.
AC Coupling
When a video signal is ac-coupled, the amount of dynamic range
required to handle the signal can potentially be double that
required for dc-coupled operation. For the unity-gain AD8186,
there is still enough dynamic range to handle an ac-coupled,
standard video signal with 700 mV p-p amplitude.
If the input is biased at 2.5 V dc, the input signal can potentially go
700 mV both above and below this point. The resulting 1.8 V and
2.2 V are within the input signal range for single 5 V operation.
Since the part is unity-gain, the outputs will follow the inputs,
and there will be adequate range at the output as well.
When using the gain-of-two AD8187 in a simple ac-coupled
application, there will be a dynamic range limitation at the output
caused by its higher gain. At the output, the gain-of-two will
produce a signal swing of 1.4 V, but the ac coupling will double
this required amount to 2.8 V. The AD8187 outputs can only
swing from 1.4 V to 3.6 V on a 5 V supply, so there are only
2.2 V of dynamic signal swing available at the output.
A standard means for reducing the dynamic range requirements
of an ac-coupled video signal is to use a dc restore. This circuit
works to limit the dynamic range requirements by clamping the
black level of the video signal to a fixed level at the input to the
amplifier. This prevents the video content of the signal from
varying the black level as happens in a simple ac-coupled circuit.
After ac coupling a video signal, it is always necessary to use a
dc restore to establish where the black level is. Usually, this
appears at the end of a video signal chain. This dc restore circuit
needs to have the required accuracy for the
system. It compen-
sates for all the offsets of the preceding
stages. Therefore, if a
dc restore circuit is to be used only for dynamic-range limiting,
it does not require great dc accuracy.
D
GND
V
EE
RED
GRN
BLU
IN2B
IN1B
IN0B
IN0A
REDA
IN1A
GRNA
IN2A
BLUA
V
REF
5V
1.5V
3.48k
1.5k
BLUB
GRNB
REDB
DV
CC
3V TO 5V
BLACK
LEVEL
TYPICAL OUTPUT LEVELS
(ALL 3 OUTPUTS)
3.0V
1.4V MAX
1.5V
BLACK
LEVEL
2.2V
1.5V
0.7V MAX
TYPICAL INPUT LEVELS
(ALL 6 OUTPUTS)
SEL A/B
OE
2
OUT0
OUT1
OUT2
V
CC
5V
AD8187
2
2
Figure 10. DC-Coupled (Bypassing and Logic Not Shown)
