13MHz Rail-to-Rail Composite

SGM9147

Video Driver with 6dB Gain

SG Micro Corp

www.sg-micro.com

TYPICAL APPLICATION CIRCUIT

SGM9147

CVBS

75Ω

IN

GND

OUT

75Ω

0.1µF

or 0.01µF

10µF

+5V

CVBS

0.1µF

220µF

NOTE:

Figure 1. AC Coupling Application Schematic

APPLICATION INFORMATION

Output Considerations

 

Functional Description

SGM9147 operates from a single 3.0V to 5.5V supply.

In application, SGM9147 is a fully integrated solution

for filtering and buffering SDTV signals in front of video

decoder or behind video encoder. For example,

SGM9147 can replace a passive LC filter and an

amplifier driver at CVBS side 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. SGM9147 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 SGM9147 also can be

DC-coupled or AC-coupled.

The SGM9147 outputs can be DC-coupled or

AC-coupled. When input is 0V, the SGM9147 output

voltage is 290mV typically. In DC coupling design, one

75Ω resistor is used to connect SGM9147’s output pin

with external load directly, and this serial back-

termination resistor is used to match the impedance of

the transmission line between SGM9147 and external

load to cancel the signal reflection. The SGM9147

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

of SGM9147. Place these two capacitors as close to

the SGM9147 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 SGM9147 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 SGM9147 as short as possible to minimize the

parasitic stray capacitance and noise pickup.

Input Considerations

Besides AC coupling, the SGM9147 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 SGM9147 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 SGM9147 is driven by an

unknown external source or a SCART switch which has

its own clamping circuit, AC coupling is needed.

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