SGM9115_16 Datasheet, PDF (6/11 Pages) SG Micro Corp – SDTV Video Filter Driver

SGM9115
APPLICATION INFORMATION
Functional Description
SGM9115 operates from a single 3.3V to 5V supply. In
application, SGM9115 is a fully integrated solution for
filtering and buffering SDTV signals in front of video
decoder or behind video encoder. For example,
SGM9115 can replace three passive LC filters and three
amplifier drivers at CVBS and S-video output side in
set-top box and DVD player. This solution can help
reduce PCB size and production cost, and it also
improves video signal performance comparing with
traditional design using discrete components. SGM9115
features a DC-coupled input buffer, a 3-pole low-pass
filter to eliminate out-of-band noise of video encoder, and
a gain of 6dB in the output amplifier to drive 75Ω load.
The AC- or DC-coupled input buffer eliminates sync
crush, droop, and field tilt. The output of SGM9115 also
can be DC-coupled or AC-coupled.
Input Considerations
Besides AC coupling, the SGM9115 inputs also can be
DC-coupled. In DC coupling application, no input
coupling capacitors are needed because the amplitude of
input video signal from DAC includes ground and extends
up to 1.4V, and SGM9115 can be directly connected to
the output of a single-supply, current-output DAC without
any external bias network. In applications where DAC’s
output level exceeds the range from 0V to 1.4V, or
SGM9115 is driven by an unknown external source or a
SCART switch which has its own clamping circuit, AC
coupling is needed.
Triple, 9MHz, 3rd-Order
SDTV Video Filter Driver
Output Considerations
The SGM9115 outputs can be DC-coupled or
AC-coupled. When input is 0V, the SGM9115 output
voltage is 260mV typically. In DC coupling design, one
75Ω resistor is used to connect SGM9115’s output pin
with external load directly, and this serial
back-termination resistor is used to match the impedance
of the transmission line between SGM9115 and external
load to cancel the signal reflection. The SGM9115
outputs can sink and source current allowing the device
to be AC-coupled with external load. In AC coupling, at
least 220µF capacitor will be used in order to eliminate
field tilt.
Power-Supply Bypassing and Layout
Correct power supply bypassing is very important for
optimizing video performance in design. One 0.1µF and
one 10µF capacitors are always used to bypass +V_S pin
of SGM9115. Place these two capacitors as close to the
SGM9115 supply pin as possible. A large ground plane is
also needed to ensure optimum performance. The input
and output termination resistors should be placed as
close to the related pins of SGM9115 as possible to avoid
performance degradation. The PCB traces at the output
side should have 75Ω characteristic impedance in order
to match the 75Ω characteristic impedance of the cable
connecting external load. In design, keep the board trace
at the inputs and outputs of the SGM9115 as short as
possible to minimize the parasitic stray capacitance and
noise pickup.
SG Micro Corp
6
www.sg-micro.com