AD8188 Просмотр технического описания (PDF) - Analog Devices
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AD8188 Datasheet PDF : 24 Pages
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AD8188/AD8189
APPLICATIONS
SINGLE-SUPPLY OPERATION
The AD8188/AD8189 are targeted mainly for use in single-
supply 5 V systems. For operating on these supplies, both VEE
and DGND should be tied to ground, and the control logic pins
should be referenced to ground. Normally, the DVCC supply
needs to 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 AD8188, the output dc level is the
same as the input, while for the gain-of-two AD8189, the VREF
input can be biased to obtain an appropriate output dc level.
Figure 53 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 voltage is applied
to VREF of the AD8189. The result is that when the video signal
is at 1.5 V, the output is also 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 changes by 1.4 V, making 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
AD8188
When a video signal is ac-coupled, the amount of dynamic
range required to handle the signal can potentially be double
the amount required for dc-coupled operation. For the unity-
gain AD8188, 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. Because the part is unity-gain, the outputs follow the
inputs, and there is adequate range at the output as well.
When the AD8188 is operated from a single supply of 5 V and
ground, ac-coupling is often useful. This is particularly true when
the input signals are a typical RGB source from a PC. These
signals go all the way to ground at the most negative, outside of
the AD8188 input range, when its negative supply is ground.
The closest that the input can go to ground is typically 1.3 V.
There are several basic methods for ac-coupling the inputs.
They all consist of a series capacitor followed by a circuit for
setting the dc operating point of the input and then the AD8188
input. If a termination is provided, it should be located before
the series coupling capacitor.
The different circuits vary in the means used to establish the dc
operating point after the coupling capacitor. A straightforward
way to do this is to use a voltage divider for each input.
However, because there are six inputs altogether, 12 resistors are
required to set all of the dc operating points. This means many
components in a small space, but the circuit has the advantage
of having the lowest crosstalk among any of the inputs. This
circuit is shown in Figure 54.
A circuit that uses the minimum number of resistors can be
designed. First, create a node, VMID, which serves as the bias
voltage for all of the inputs. Then, a single resistor is used to
connect from each input (inside the ac-coupling capacitor) and
VMID (see Figure 55).
3V TO 5V 5V
0.7V MAX
2.2V
REDA
GRNA
BLUA
5V
IN0A DVCC
IN1A
IN2A
1.5V
3.48kΩ
BLACK
LEVEL
TYPICAL INPUT LEVELS
(ALL 6 OUTPUTS)
1.5kΩ
1.5V
VREF
REDB
GRNB
BLUB
IN0B
IN1B
IN2B
DGND VEE
VCC
AD8189
OUT0
×2
OUT1
×2
OUT2
×2
SEL A/B OE
RED
3.0V
1.4V
MAX
GRN 1.5V
TYPICAL OUTPUT LEVELS
BLACK
(ALL 3 OUTPUTS)
LEVEL
BLU
Figure 53. AD8189 DC-Coupled (Bypassing and Logic Not Shown)
Rev. 0 | Page 17 of 24
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