ADA4420-6

Rev. 0 | Page 10 of 12

APPLICATIONS INFORMATION

OVERVIEW

With its high impedance inputs and high output drive, the

ADA4420-6 is ideally suited to video reconstruction and anti-

alias 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 ADA4420-6 can be loaded in whatever resistance provides

the best performance, and devices with voltage outputs can be

optimally terminated as well. The ADA4420-6 outputs can each

drive up to two source-terminated, 75 Ω loads and, therefore, can

directly drive the outputs from set-top boxes and DVDs without

the need for a separate output buffer.

DISABLE

The ADA4420-6 includes a disable feature that can be used to

save power when a particular device is not in use. When disabled,

the ADA4420-6 typically draws only 7 μA from the supply. The

disable feature is asserted by pulling the DIS pin low.

Table 5 summarizes the operation of the disable feature.

Table 5. Disable Function

DIS Pin Connection

Status

Enabled

GND

Disabled

INPUT AND OUTPUT COUPLING

Inputs to the ADA4420-6 can be ac- or dc-coupled. For dc-coupled

inputs, the signal must be completely contained within the input

range of 0 V to 2.1 V. When using ac-coupled inputs, the lowest

point of the signal is clamped to approximately 0 V. The ADA4420-6

outputs can be either ac- or dc-coupled.

When driving single ac-coupled loads in standard 75 Ω video

distribution systems, a minimum capacitance of 220 μF is

recommended to avoid line and field droop. There are two ac

coupling options when driving two loads from one output. One

option simply uses the same value capacitor on the second load,

while the other option uses a common coupling capacitor that

is at least twice the value used for the single load (see Figure 20

and Figure 21).

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 (see Figure 4).

75Ω

75Ω

75Ω

CABLE

75Ω

CABLE

220µF

ADA4420-6

220µF

75Ω

75Ω

Figure 20. Driving Two AC-Coupled Loads with Two Coupling Capacitors

ADA4420-6

75Ω

CABLE

75Ω

CABLE

75Ω

75Ω

75Ω

75Ω

470µF

Figure 21. Driving Two AC-Coupled Loads with One Common Coupling Capacitor

PRINTED CIRCUIT BOARD (PCB) LAYOUT

As with all high speed applications, attention to the PCB layout

is of paramount importance. When designing with the ADA4420-6,

adhere to standard high speed layout practices. 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. Connect all of the ADA4420-6 GND pins 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. When driving transmission lines,

include series termination resistors on the signal outputs of the

ADA4420-6.

When the ADA4420-6 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 interconnections are 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 most. This

is easily achieved because the ADA4420-6 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 ADA4420-6 is easily applied as a reconstruction filter at the

DAC outputs of a video encoder. Figure 22 illustrates how to use

the ADA4420-6 in this type of application following an ADV734x

series video encoder, with a single-supply and ac-coupled outputs.