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
V
S
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 V
S
, 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
V
S
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
06221-026
Figure 26. Driving Two AC-Coupled Loads with Two Coupling Capacitors
DA4417-3
75Ω
CABLE
75Ω
CABLE
75Ω
75Ω
75Ω
75Ω
470µF
06221-027
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
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.
