SGM9115

6

Applications Information

Functional Description

SGM9115 operates from a single +2.5V 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 you save PCB size and production cost, it also

improves video signal performance comparing with traditional

design using discrete components. SGM9115 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

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, then

SGM9115 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 SGM9115 is driven by an unknown external

source or a SCART switch which has its own clamping circuit,

AC coupling is needed in such applications.

Output Considerations

The SGM9115 outputs can be DC-coupled or AC-coupled.

When 0V is input, 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, 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

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

SGM9115, please place these two capacitors as close to the

SGM9115 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 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 cable connecting external load. In

design, please 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.

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

design. Figure 4 is a kind of special application in STB.

Y

C

75Ω

1

2

3

4

5

6

7

8

IN1

IN2

IN3

GND

OUT1

OUT2

OUT3

75Ω

75Ω

75Ω

SGM9115

0.1µF

or 0.01µF

10µF

+5V

75Ω

75Ω

Y

C

S-Video

S-Video

Figure 1. DC Coupling Application Schematic

Y

C

75Ω

1

2

3

4

5

6

7

8

IN1

IN2

IN3

GND

OUT1

OUT2

OUT3

75Ω

75Ω

75Ω

SGM9115

0.1µF

or 0.01µF

10µF

+5V

75Ω

75Ω

Y

C

0.1µF

0.1µF

0.1µF

220µF

220µF

220µF

S-Video

S-Video

Figure 2. AC Coupling Application Schematic

4

5

6

7

8

IN1

IN2

IN3

GND

OUT1

OUT2

OUT3

75Ω

75Ω

75Ω

SGM9115

0.1µF

or 0.01µF

10µF

+5V

Y

C

220µF

220µF

220µF

S-Video

Y

C

75Ω

1

2

3

75Ω

75Ω

S-Video

Figure 3. Input DC Coupling and Output AC

Coupling Application Schematic