Applications Information
Functional Description
Typical Application Diagram
The following schematic in Figure 3 is normally used for AC
coupled output and DC-coupled input with DAC which has an
output voltage range of 0V to 1.4V. AC coupled output offer
slightly lower power dissipation and high ESD protection ability.
The schematics in Figure 1 and Figure 2 are also popular in
SGM9123 operates from a single +2.5V to +5V supply. In
application, SGM9123 is a fully integrated solution for filtering
and buffering SDTV signals in front of video decoder or behind
video encoder. For example, SGM9123 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 you save PCB size and production cost, it also
improves video signal performance comparing with traditional
design using discrete components. SGM9123 features a
DC-coupled input buffer, 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
SGM9123 also can be DC-coupled or AC-coupled.
design. Figure 4 is a kind of special application in STB.
+5V
0.1µF
or 0.01µF
10µF
4
+VS
OUT1
SGM9123
1
8
CVBS1
IN1
CVBS1
75Ω
75Ω
75Ω
75Ω
7
6
2
3
Y
C
IN2
OUT2
Y
C
75Ω
75Ω
S-Video
S-Video
Input Considerations
IN3
OUT3
Besides AC coupling, the SGM9123 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, then
SGM9123 can be directly connected to the output of a
single-supply, current-output DAC without any external bias
network. Some time, if DAC’s output level exceeds the range of
0V to 1.4V, or SGM9123 is driven by an unknown external
source or a SCART switch which has its own clamping circuit,
AC coupling is needed in such applications.
GND
5
Figure 1. DC Coupling Application Schematic
Output Considerations
The SGM9123 outputs can be DC-coupled or AC-coupled.
When 0V is input, the SGM9123 output voltage is 260mV
typically. In DC coupling design, one 75Ω resistor is used to
Connect SGM9123’s output pin with external load directly, this
serial back-termination resistor is used to match the impedance
of the transmission line between SGM9123 and external load
to cancel the signal reflection. The SGM9123 outputs can sink
and source current allowing the device to be AC-coupled with
external load, in AC coupling, 220µF at least capacitor will be
used in order to eliminate field tilt.
Figure 2. AC Coupling Application Schematic
+5V
0.1µF
Power-Supply Bypassing and Layout
10µF
or 0.01µF
4
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 VCC pin of
SGM9123, please place these two capacitors as close to the
SGM9123 output 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
pin of SGM9123 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 cable connecting external load. In
design, please keep the board trace at the inputs and outputs
of the SGM9123 as short as possible to minimize the parasitic
stray capacitance and noise pickup.
+VS
1
8
CVBS1
IN1
OUT1
CVBS1
75Ω
220µF
75Ω
75Ω
75Ω
SGM9123
2
3
7
6
Y
IN2
OUT2
OUT3
Y
75Ω
75Ω
220µF
220µF
S-Video
S-Video
IN3
C
C
GND
5
Figure 3. Input DC Coupling and Output AC
Coupling Application Schematic
6
SGM9123
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
