ADA4417-3

Rev. A | Page 13 of 16

APPLICATIONS

OVERVIEW

With its high impedance inputs and high output drive, the

ADA4417-3 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 ADA4417-3 can be loaded in whatever resistance provides

the best performance, and devices with voltage outputs can be

optimally terminated as well. The ADA4417-3 outputs can each

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

therefore directly drive the outputs from set-top boxes, DVDs,

and a like without the need for a separate output buffer.

DISABLE

The ADA4417-3 includes a disable feature that can be used to

save power when a particular device is not in use. The disable

feature is asserted by pulling the DISABLE pin to ground.

Table 6 summarizes the disable feature operation.

Table 6. DISABLE Function

DISABLE Pin Connection

Status

VS

Enabled

GND

Disabled

OUTPUT DC OFFSET CONTROL

The ADA4417-3 has a fixed, pin-selectable, input-referred dc

offset. When the DCO pin is tied to VS, the output is offset by

200 mV, preventing the video sync tips from hitting the ground

rail. When DCO is tied to GND, the dc level of the output

follows that of the input.

Table 7 summarizes the dc offset operation.

Table 7. DC Offset Function

DCO Pin Connection

Status

VS

Output offset = 200 mV

GND

No output offset

INPUT AND OUTPUT COUPLING

Inputs to the ADA4417-3 may be ac- or dc-coupled. AC

coupling requires suitable circuitry following the ac coupling

element to provide proper dc level and bias currents at the input

stages. The ADA4417-3 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 outputs.

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 26 and Figure 27).

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. Typical variation is within ±2.5%.

75Ω

75Ω

75Ω

CABLE

75Ω

CABLE

220µF

ADA4417-3

220µF

75Ω

75Ω

Figure 26. Driving Two AC-Coupled Loads with Two Coupling Capacitors

ADA4417-3

75Ω

CABLE

75Ω

CABLE

75Ω

75Ω

75Ω

75Ω

470µF

Figure 27. 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

ADA4417-3. 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 ADA4417-3 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 ADA4417-3

should include series termination resistors when driving

transmission lines.

When the ADA4417-3 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 interconnections 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 most. This is easily achieved because the ADA4417-3

and the device feeding it are usually adjacent to each other, and

connections can be made that are less than one inch in length.