Applications Information
Functional Description
Typical Application Diagram
The circuit 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 offers slightly lower
power dissipation and high ESD protection ability.
SGM9117 operates from a single +2.5V to +5V supply. In
application, SGM9117 is a fully integrated solution for buffering
RGB or HDTV component signals in front of video decoder or
behind video encoder. For example, SGM9117 can replace
three amplifier drivers at R\G\B and Y\Pb\Pr output side in
set-top box, Monitor and LCD TV, this solution can help you
save PCB size and production cost, it also improves video
signal performance comparing with traditional design using
discrete components. SGM9117 features a DC-coupled input
buffer, 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 SGM9117 also can be
DC-coupled or AC-coupled.
The schematics in Figure 1 and Figure 2 are also popular in
design.
+5V
0.1µF
or 0.01µF
10µF
4
+VS
OUT1
SGM9117
1
8
IN1
R/Pr
G/Y
R/Pr
G/Y
75Ω
75Ω
75Ω
75Ω
2
3
7
6
IN2
OUT2
75Ω
75Ω
Input Considerations
IN3
OUT3
B/Pb
B/Pb
Besides AC coupling, the SGM9117 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
SGM9117 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 SGM9117 is driven by an unknown external
source or a SCART switch which has its own clamping circuit,
AC coupling is needed in such application.
GND
5
Figure 1. DC Coupling circuit
+5V
0.1µF
or 0.01µF
10µF
4
+VS
OUT1
SGM9117
0.1µF
0.1µF
0.1µF
1
8
R/Pr
G/Y
IN1
R/Pr
G/Y
75Ω
220µF
Output Considerations
75Ω
75Ω
The SGM9117 outputs can be DC-coupled or AC-coupled.
While input is 0V, the SGM9117 output voltage is 260mV
typically. In DC coupling design, one 75Ω resistor is used to
Connect SGM9117’s output pin with external load directly, this
serial back-termination resistor is used to match the
impedance of the transmission line between SGM9117 and
external load to cancel the signal reflection. The SGM9117
outputs can sink and source current allowing the device to be
7
6
2
3
IN2
OUT2
75Ω
75Ω
220µF
220µF
B/Pb
IN3
OUT3
B/Pb
GND
75Ω
5
AC-coupled with external load, in AC coupling, 220µF at least
capacitor will be used in order to cancel field tilt.
Figure 2. AC Coupling Application Schematic
Power-Supply Bypassing and Layout
+5V
0.1µF
or 0.01µF
10µF
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
SGM9117, please place these two capacitors as close to the
SGM9117 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 SGM9117 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 SGM9117 as short as possible to minimize the parasitic
stray capacitance and noise pickup, designer needs to cavern
out the ground layer under input and output trace in order to
reduce stray capacitance and avoid signal peaking.
4
+VS
OUT1
SGM9117
1
8
IN1
R/Pr
G/Y
R/Pr
G/Y
75Ω
220µF
75Ω
75Ω
75Ω
2
3
7
6
IN2
OUT2
75Ω
75Ω
220µF
220µF
IN3
OUT3
B/Pb
B/Pb
GND
5
Figure 3. Input DC and output AC Coupling circuit
７
SGM9117
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
