SGM9127 Datasheet, PDF (6/10 Pages) SG Micro Corp – 4-Channel, 5th-Order, Standard Definition Video Filter Driver

SGM9127
APPLICATION INFORMATION
Functional Description
SGM9127 operates from a single 3.3V to 5.5V supply. In
application, SGM9127 is a fully integrated solution for
filtering and buffering SDTV signals in front of video
decoder or behind video encoder. For example,
SGM9127 can replace four passive LC filters and four
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.
SGM9127 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 SGM9127
also can be DC-coupled or AC-coupled.
Input Considerations
Besides AC coupling, the SGM9127 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 SGM9127 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
SGM9127 is driven by an unknown external source or a
SCART switch which has its own clamping circuit, AC
coupling is needed.
4-Channel, 5th-Order, Standard
Definition Video Filter Driver
Output Considerations
The SGM9127 outputs can be DC-coupled or AC-
coupled. When input is 0V, the SGM9127 output voltage
is 458mV typically. In DC coupling design, one 75Ω
resistor is used to connect SGM9127’s output pin with
external load directly, and this serial back-termination
resistor is used to match the impedance of the
transmission line between SGM9127 and external load to
cancel the signal reflection. The SGM9127 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.
The 85mA/channel output current driving capability of the
SGM9127 is designed to drive two video lines
simultaneously - essentially a 75Ω load, while keeping
the output dynamic range as wide as possible.
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 SGM9127. Place these two capacitors as close to the
SGM9127 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 SGM9127 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 SGM9127 as short as
possible to minimize the parasitic stray capacitance and
noise pickup.
SG Micro Corp
6
www.sg-micro.com