AD8189ARUZ-R7 datasheet(17/24 Pages) AD

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AD8188/AD8189

Rev. 0 | Page 17 of 24

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).

VEE

DGND

RED

GRN

BLU

REDA

GRNA

BLUA

IN2B

IN1B

IN0B

IN0A

IN1A

IN2A

VREF

1.5V

1.5kΩ

3.48kΩ

BLUB

GRNB

REDB

DVCC

SEL A/B

×2

OUT0

OUT1

OUT2

VCC

×2

0.7V MAX

BLACK

LEVEL

BLACK

LEVEL

2.2V

1.5V

TYPICAL INPUT LEVELS

(ALL 6 OUTPUTS)

TYPICAL OUTPUT LEVELS

(ALL 3 OUTPUTS)

3.0V

1.4V

MAX

1.5V

AD8189

3V TO 5V 5V

Figure 53. AD8189 DC-Coupled (Bypassing and Logic Not Shown)

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