ADA4412-3

Rev. 0 | Page 11 of 16

DUAL SUPPLY

0.1

μF

LEVEL1

9.53k

Ω

1k

Ω

9.53k

Ω

+5V

–5V

0.1

μF

LEVEL2

9.53k

Ω

1k

Ω

9.53k

Ω

+5V

–5V

SINGLE SUPPLY

0.1

μF

LEVEL1

1k

Ω

9.09k

Ω

+5V

0.1

μF

LEVEL2

1k

Ω

9.09k

Ω

+5V

Figure 16. Generating Fully Adjustable Output Offsets

0.1

μF

LEVEL1

634

Ω

10k

Ω

+5V

DNP

LEVEL2

0

Ω

DNP

+5V

Figure 17. Flexible Circuits to Set the LEVEL1 and LEVEL2 Inputs to

Obtain a 600 mV Output Offset on a Single Supply

INPUT AND OUTPUT COUPLING

Inputs to the ADA4412-3 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 ADA4412-3 input stages. The ADA4412-3 outputs can be

either ac- or dc-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.

Since 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 simply uses 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 18 and Figure 19).

When driving two parallel 150 Ω loads (75 Ω effective load),

the 3 dB bandwidth of the filters typically varies from that of

the filters with a single 150 Ω load. For the 9 MHz and 18 MHz

filters, the typical variation is within ±1.0%; for the 36 MHz

filters, the typical variation is within ±2.5%.

75

Ω

ADA4412-3

75

Ω

220

μF

220

μF

75

Ω

75

Ω

75

Ω

CABLE

75

Ω

CABLE

Figure 18. Driving Two AC-Coupled Loads with Two Coupling Capacitors

75

Ω

75

Ω

75

Ω

75

Ω

470

μF

75

Ω

CABLE

75

Ω

CABLE

ADA4412-3

Figure 19. Driving Two AC-Coupled Loads with One Common Coupling Capacitor

PRINTED CIRCUIT BOARD LAYOUT

As with all high speed applications, attention to printed

circuit board layout is of paramount importance. Standard high

speed layout practices should be adhered to when designing

with the ADA4412-3. A solid ground plane is recommended,

and surface-mount, ceramic power supply decoupling

capacitors should be placed as close as possible to the supply

pins. All of the ADA4412-3 GND pins should be connected to

the ground plane with traces that are as short as possible.

Controlled impedance traces of the shortest length possible

should be used to connect to the signal I/O pins and should not

pass over any voids in the ground plane. A 75 Ω impedance

level is typically used in video applications. All signal outputs of

the ADA4412-3 should include series termination resistors

when driving transmission lines.

When the ADA4412-3 receives its inputs from a device

with current outputs, the required load resistor value for

the output current is often different from the characteristic

impedance of the signal traces. In this case, if the interconnec-

tions are sufficiently short (<< 0.1 wavelength), the trace does

not have to be terminated in its characteristic impedance.

Traces of 75 Ω can be used in this instance, provided their

lengths are an inch or two at the most. This is easily achieved

because the ADA4412-3 and the device feeding it are usually

adjacent to each other, and connections can be made that are

less than one inch in length.

VIDEO ENCODER RECONSTRUCTION FILTER

The ADA4412-3 is easily applied as a reconstruction filter at the

DAC outputs of a video encoder. Figure 20 illustrates how to use

the ADA4412-3 in this type of application with an ADV7322 video

encoder in a single-supply application with ac-coupled outputs.