SGM9125 Datasheet, PDF (6/10 Pages) SG Micro Corp – Definition Video Filter Driver

SGM9125
APPLICATION INFORMATION
Functional Description
SGM9125 operates from a single 3.3V to 5.5V supply. In
application, SGM9125 is a fully integrated solution for
filtering and buffering SDTV signals in front of video
decoder or behind video encoder. For example,
SGM9125 can replace five passive LC filters and five
amplifier drivers 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.
SGM9125 features a DC-coupled input buffer, a 5-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 SGM9125
also can be DC-coupled or AC-coupled.
Input Considerations
Besides AC coupling, the SGM9125 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 SGM9125 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
SGM9125 is driven by an unknown external source or a
SCART switch which has its own clamping circuit, AC
coupling is needed.
5-Channel, 5th-Order, Standard
Definition Video Filter Driver
Output Considerations
The SGM9125 outputs can be DC-coupled or
AC-coupled. When input is 0V, the SGM9125 output
voltage is 396mV typically. In DC coupling design, one
75Ω resistor is used to connect SGM9125’s output pin
with external load directly, and this serial back-
termination resistor is used to match the impedance of
the transmission line between SGM9125 and external
load to cancel the signal reflection. The SGM9125
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_CC pin
of SGM9125. Place these two capacitors as close to the
SGM9125 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 SGM9125 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 SGM9125 as short as
possible to minimize the parasitic stray capacitance and
noise pickup.
SG Micro Corp
6
www.sg-micro.com