8MHz, 5th-Order Video Driver

with 6dB Gain

7

SG Micro Corp

www.sg-micro.com

SGM9121

APPLICATION INFORMATION

The SGM9121 low cost, integrated, 5th-order video filter

is intended to replace passive LC filters and drivers in low

voltage portable video applications.

The SGM9121 input must be AC-coupled because the

input capacitor stores the clamp voltage. It needs a

typical value of 0.1µF for the input clamp to meet the Line

Droop specification. The SGM9121 output can drive an

AC- or DC-coupled doubly terminated coax (150Ω) load

(see Figure 1). DC coupling the output removes the need

for an expensive and large output coupling capacitor (see

Figure 2). If an AC-coupled output is needed, the SAG

correction circuit can be used to reduce the AC output

coupling capacitor value.

Offering SAG correction, fixed gain of 6dB, and a 5th-

order low pass filter in a tiny space saving package

makes the SGM9121 well suited for space sensitive

applications such as digital cameras, cellular phones and

other portable devices.

Enable/Shutdown

The SGM9121 has a shutdown feature that disables the

output and reduces the quiescent current to 0.72μA. This

feature is particularly useful in portable applications, such

as video cameras, hand held gaming devices and cellular

phones, where video filtering and driving capability are

required.

Internal Sync Clamp

The typical embedded video DAC operates from a

ground referenced single supply. This becomes an issue

because the lower level of the sync pulse output may be

at a 0V reference level to some positive level. The

problem is that presenting a 0V input to most single

supply driven amplifiers will saturate the output stage of

the amplifier, resulting in a clipped sync tip and degrading

the video image. A larger positive reference may offset

the input above its positive range.

The SGM9121 features an internal sync clamp and offset

function to level shift the entire video signal to the best

level before it reaches the input of the amplifier stage.

These features are also helpful to avoid saturation of the

output stage of the amplifier by setting the signal closer to

the optimal voltage range.

The Typical Application Diagram of the SGM9121 in

Figure 1 is divided into four sections. The first, Section A

is the Sync Clamp. The AC-coupled video sync signal is

pulled negative by a current source at the input of the

comparator amplifier. When the sync tip goes below the

comparator threshold, the comparator output is driven

negative, and the PMOS device turns on, clamping sync

tip to near ground level. The network triggers on the sync

tip of video signal.

OUT

SAG

SGM9121

5th-Order

Filter

6dB

Level

Shifter

IN

GND

0.1µ

F

DISABLE

22µF

47µF

75

Ω

75

Ω

10k

Ω

To External Power

Supply or Control I/O

Figure 1. AC-Coupled Input/AC-Coupled Output