ADV3221/ADV3222
Rev. 0 | Page 18 of 20
APPLICATIONS INFORMATION
The ADV3221 and ADV3222 are high speed multiplexers used
to switch video or RF signals. The low output impedance of the
ADV3221/ADV3222 allows the output environment to be
optimized for use in 75 Ω or 50 Ω systems by choosing the
appropriate series termination resistor. For composite video
applications, the ADV3222 (gain of +2) is typically used to
provide compensation for the loss of the output termination.
CK1
/
CK2
OPERATION
The ADV3221/ADV3222 provide a double latched architecture
for the A0, A1 (channel selection) and
CS
(output enable) logic.
This allows for simultaneous update of multiple devices in bank
switching applications or large multiplexer systems consisting of
multiple devices connected to common output busses.
Holding
CK1
and
CK2
low places the ADV3221/ADV3222 in a
transparent mode. In transparent mode, all logic changes to A0,
A1, and
CS
immediately affects the input selection and output
enable/disable.
CIRCUIT LAYOUT
Use of proper high speed design techniques is important to
ensure optimum performance. Use a low inductance ground
plane for power supply bypassing and to provide high quality
return paths for the input and output signals. For best performance,
it is recommended that power supplies be bypassed with 0.1 μF
ceramic capacitors as close to the body of the device as possible.
To provide stored energy for lower frequency, high current output
driving, place 10 μF tantalum capacitors farther from the device.
The input and output signal paths should be stripline or micro-
strip controlled impedance. Video systems typically use 75 Ω
characteristic impedance, whereas RF systems typically use
50 Ω. Various calculators are available to calculate the trace
geometry required to produce the proper characteristic
impedance.
TERMINATION
For a controlled impedance situation, termination resistors are
required at the inputs and output of the device. The input
termination should be a shunt resistor to ground with a value
matching the characteristic impedance of the input trace. To
reduce reflections, place the input termination resistor as close
to the device input pin as possible. To minimize the input-to-
input crosstalk, it is important to utilize a low inductance shield
between input traces to isolate each input. Consideration of
ground current paths must be taken to minimize loop currents
in the shields to prevent them from providing a coupling
medium for crosstalk.
For proper matching, the output series termination resistor
should be the same value as the characteristic impedance of the
output trace and placed as close to the output of the device as
possible. This placement reduces the high frequency effect of
series parasitic inductance, which can affect gain flatness and
−3 dB bandwidth.
CAPACITIVE LOAD
A high frequency output can have difficulties when driving a
large capacitive load, usually resulting in peaking in the frequency
domain or overshoot in the time domain. If these effects become
too large, oscillation can result.
The response of the device under various capacitive loads is
shown in Figure 6 through Figure 12, and in Figure 15. If a
condition arises where excessive load capacitance is encoun-
tered and the overshoot is too great or the device oscillates, a
small series resistor of a few tens of ohms can be used to improve
the performance.
