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

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

)

(

LEVEL2

LEVEL1

OUT

−

=

(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.